review of the emissions, atmospheric chemistry, and gas/particle

REVIEW OF THE EMISSIONS, ATMOSPHERIC CHEMISTRY,

AND GAS/PARTICLE PARTITION OF BIOGENIC VOLATILE

ORGANIC COMPOUNDS AND REACTION PRODUCTS

Prepared for

Coordinating Research Council, Inc.219 Perimeter Center Parkway, Suite 400

Atlanta, GA 30346

CRC Project Number A-23

Prepared by

Brian LambDepartment of Civil and Environmental Engineering

Washington State UniversityPullman, WA 99164

Daniel GrosjeanDGA, Inc.

4526 Telephone Road, Suite 205Ventura, CA 93004

Betty Pun and Christian SeigneurAtmospheric & Environmental Research, Inc.

2682 Bishop Drive, Suite 120San Ramon, CA 94583

Document Number CP051-1b-99

November 1999

ACKNOWLEDGMENTS

This work was performed as part of Project A-23 under contract with the

Coordinating Research Council (CRC). Thanks are due to CRC for constructive

comments on the draft version.

LEGAL NOTICE

This report was prepared by Atmospheric and Environmental Research, Inc.

(AER) as an account of work sponsored by the Coordinating Research Council (CRC).

Neither the CRC, members of the CRC, AER nor any person acting on their behalf: (1)

makes any warranty, express or implied, with respect to the use of any information,

apparatus, method, or process disclosed in this report, or (2) assumes any liabilities with

respect to the use, inability to use, or damages resulting from the use or inability to use,

any information, apparatus, method, or process disclosed in this report.

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products i

TABLE OF CONTENTS

E. Executive Summary ......................................................................................... E-1

E.1 Emissions of Biogenic Volatile Organic Compounds ............................ E-1

E.2 Atmospheric Chemistry of BVOC and Products ................................... E-5

E.3 SOA Partition....................................................................................... E-8

E.4 Data Gaps and Recommendations......................................................... E-9

E.4.1 Data gaps............................................................................... E-9

E.4.2 Recommendations for future work ........................................E-10

1. Introduction......................................................................................................1-1

2. Biogenic VOC Emissions Related to Ozone and Aerosol Formation..................2-1

2.1 Introduction ..........................................................................................2-1

2.2 A Third Generation BVOC Emission Inventory .....................................2-4

2.2.1 BEIS3 compounds.....................................................................2-4

2.2.2 BEIS3 model algorithms ............................................................2-7

2.2.3 BEIS3 canopy modeling........................................................... 2-10

2.2.4 Summary ................................................................................. 2-11

2.3 Emissions of BVOC ............................................................................ 2-11

2.3.1 Isoprene................................................................................... 2-11

2.3.2 Monoterpenes.......................................................................... 2-15

2.3.3 Sesquiterpenes ......................................................................... 2-29

2.3.4 Oxygenated VOC .................................................................... 2-30

2.4. BVOC Emission Measurement Methods.............................................. 2-32

2.4.1 Enclosure methods................................................................... 2-33

2.4.2 Micrometeorological canopy flux methods ............................... 2-34

2.4.3 Mixed layer (landscape scale) flux methods .............................. 2-37

2.5 Reconciliation of BVOC Ambient Concentrations with Emission

Inventories .......................................................................................... 2-39

2.6 Summary and Conclusions................................................................... 2-42

3. Atmospheric Chemistry of Biogenic VOC .........................................................3-1

3.1 Introduction ..........................................................................................3-1

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products ii

TABLE OF CONTENTS (continued)

3.2 Kinetic Data ..........................................................................................3-6

3.2.1 Reaction rate constants ..............................................................3-6

3.2.2 Estimated rate constants ............................................................3-6

3.2.3 Reactivity considerations and atmospheric lifetimes.................. 3-12

3.3 First-Generation Reaction Products ..................................................... 3-16

3.3.1 Products of the reaction of OH with p-cymene and with

saturated aliphatic compounds ................................................. 3-34

3.3.2 Products of the reaction of OH with unsaturated compounds ... 3-34

3.3.3 Products of the reaction of O3 with unsaturated compounds..... 3-35

3.3.4 Products of the reaction of NO3 with unsaturated compounds .. 3-36

3.3.5 Formation of SOA ................................................................... 3-36

3.4. Atmospheric Reactions of First-Generation Products ........................... 3-45

3.4.1 Products discussed in this section............................................. 3-46

3.4.2 Kinetic data for first-generation products ................................. 3-47

3.4.3 Atmospheric lifetimes of first-generation products.................... 3-52

3.4.4 Second-generation oxidation products ..................................... 3-54

3.5. Reaction Mechanisms.......................................................................... 3-57

3.5.1 The reaction of OH with saturated compounds......................... 3-57

3.5.2 The reaction of OH with aldehydes: peroxyacyl nitrates ........... 3-61

3.5.3 The reaction of OH with unsaturated compounds..................... 3-63

3.5.4 The reaction of O3 with unsaturated compounds ...................... 3-67

3.5.5 The reaction of NO3 with unsaturated compounds.................... 3-70

4. Secondary Organic Aerosol Gas/Particle Partition.............................................4-1

4.1 Introduction ..........................................................................................4-1

4.2 SOA Partition Theories .........................................................................4-1

4.3 Review of Existing Secondary Organic Aerosol Modules.......................4-3

4.4 Description of Secondary Organic Aerosol Modules Under

Development .........................................................................................4-9

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products iii

TABLE OF CONTENTS (continued)

5. Knowledge Gaps and Recommendations for Future Work.................................5-1

5.1 BVOC Emissions ..................................................................................5-1

5.2 Atmospheric Chemistry .........................................................................5-4

5.2.1 Kinetic data ...............................................................................5-4

5.2.2 First-generation products ...........................................................5-5

5.2.3 Reactions of first-generation products ........................................5-7

5.2.4 Aerosol formation......................................................................5-7

5.2.5 Reaction mechanisms.................................................................5-8

5.3 Gas/Particle Partition of Organic Aerosols.............................................5-8

5.4 Recommendation for Future Work ...................................................... 5-10

6. References ........................................................................................................6-1

6.1 Biogenic Emissions References..............................................................6-1

6.2 Atmospheric Chemistry References...................................................... 6-17

6.3 SOA Partition References.................................................................... 6-34

Appendix A1. Chemical structures of biogenic compounds listed in Table 3-1 ............. A-1

Appendix A2. Chemical structures of first-generation products listed in Table 3-14..... A-6

Appendix A3. Structures of second-generation products listed Table 3-17 .................. A-9

Appendix A4. Structures of other compounds listed in Tables 3-8 to 3-10................. A-10

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products iv

LIST OF TABLES

Table E-1. Biogenic volatile organic compounds represented in BEIS3, their

emission mechanisms, and estimated annual emissions in North

America................................................................................................ E-2

Table E-2. Range of atmospheric half-lives ............................................................ E-6

Table 2-1. Emission factors and annual North American emission rates for

emissions from live vegetation. ..............................................................2-5

Table 2-2. Annual above canopy flux of NO (TgN), BVOC (TgC), and CO

(TgC) from natural sources in North America........................................2-6

Table 2-3. Preliminary analyses of terpene emissions composition (% of total

terpenes) for dominant conifer species at the Wind River Crane

Research Facility ................................................................................. 2-16

Table 2-4. List of compounds detected from enclosure sampling at three U.S.

Sites ................................................................................................... 2-18

Table 2-5. BVOC emission rates (µgC g-1 h-1) from branch enclosure

measurements at Fernbank forest in urban Atlanta ............................... 2-20

Table 2-6. Landscape emissions (µgC m-2 h-1) averaged over a one week

summer temperature and light record for three U.S. sites ..................... 2-23

Table 3-1. Biogenic compounds listed according to chemical functionality..............3-3

Table 3-2. OH reaction rate constants for p-cymene and saturated aliphatic

compounds............................................................................................3-7

Table 3-3. OH reaction rate constants for unsaturated aliphatic compounds............3-8

Table 3-4. Ozone reaction rate constants for unsaturated aliphatic compounds .......3-9

Table 3-5. NO3 reaction rate constants for unsaturated aliphatic compounds......... 3-10

Table 3-6. Range of reaction rate constants and atmospheric half-lives ................. 3-13

Table 3-7. Products of the reaction of OH with saturated compounds................... 3-17

Table 3-8. Products of the reaction of OH with unsaturated compounds............... 3-18

Table 3-9. Products of the reaction of ozone with unsaturated compounds ........... 3-24

Table 3-10. Products of the reaction of NO3 with unsaturated compounds.............. 3-31

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products v

LIST OF TABLES (continued)

Table 3-11. OH formation yields in the reaction of ozone with unsaturated

aliphatic compounds ............................................................................ 3-33

Table 3-12. Studies of aerosol formation from biogenic organic compounds........... 3-38

Table 3-13. Molecular composition of α-pinene aerosol ......................................... 3-43

Table 3-14. First-generation products listed according to chemical functionality ..... 3-48

Table 3-15. Kinetic data for the reactions of OH, O3 and NO3 with first-

generation products ............................................................................. 3-50

Table 3-16. Atmospheric lifetimes of first-generation products ............................... 3-53

Table 3-17. Second-generation oxidation products ................................................. 3-55

Table 4-1. Aerosol yields in terms of amount of precursor reacted..........................4-4

Table 4-2. Aerosol yield parameters used in Models-3............................................4-6

Table 4-3. Aerosol yield parameters and saturation concentrations in UAM-

AERO...................................................................................................4-8

Table 4-4. Aerosol yield parameters for the oxidation of aromatic compounds........4-8

Table 4-5. Aerosol yield parameters for the oxidation of biogenic organic

compounds.......................................................................................... 4-10

Table 4-6. Surrogate compounds in AER module................................................. 4-12

Table 5-1. Summary of knowledge gaps for products of the OH and ozone

reactions................................................................................................5-6

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products vi

LIST OF FIGURES

Figure 3-1. Reaction mechanism of OH with nopinone........................................... 3-60

Figure 3-2. Reaction mechanism of OH with pinonaldehyde................................... 3-64

Figure 3-3. Reaction mechanism of OH with cis-3-hexen-1-ol................................ 3-66

Figure 3-4. Reaction mechanism of O3 with trans-2-hexenyl acetate....................... 3-68

Figure 3-5. Reaction mechanism of NO3 with 2-methyl-3-buten-2-ol ..................... 3-71

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products E-1

EXECUTIVE SUMMARY

This document reviews the state of knowledge in several areas related to the

effects of biogenic volatile organic compounds (BVOC) in the ambient atmosphere and

provides a summary of the current knowledge gaps. Following a brief introduction,

Sections 2, 3, and 4 address the emissions of BVOC, the chemical fate of these

compounds and their secondary products, and the partition of condensable products

between the gas and particle phases, respectively. Knowledge gaps are summarized in

Section 5, which also provides the authors’ recommendations for future work. The

executive summary focuses on the overall content of the review document rather than on

specific details about the processes discussed in the document. The readers are

encouraged to turn to the appropriate sections for detailed information.

E.1 Emissions of Biogenic Volatile Organic Compounds

BVOC include isoprene, monoterpenes, sesquiterpenes, and oxygenated VOC,

which are released from a variety of ecosystems ranging from old growth forests to

grasslands and urban landscapes. The emissions of these compounds have been

represented in Biogenic Emissions Inventory System (BEIS) models. The first two

generations of BEIS models have been used to provide emissions estimates at a variety of

locations. BEIS3, the third-generation model, compiles the most up-to-date information

on biogenic emissions and provides the basis for much of this review. A list of BVOC

modeled in BEIS3 is provided in Table E-1. Biogenic emissions are dominated by

isoprene (31%), methyl butenol (5%), and monoterpenes (22%), although several other

oxygenated compounds have also been identified.

In BEIS3, the local BVOC flux from a vegetative canopy for a compound or a

class of compounds is given as:

F = εDγδρ (E-1)


Table E-1. Biogenic volatile organic compounds represented in BEIS3, their emission

mechanisms, and estimated annual emissions in North America in

Teragrams of carbon (TgC)(a).

Compounds Emission type(b) Annual emission (TgC)(c)

isoprene CHL 24.0

methyl butenol (MBO) CHL 4.1

α-pinene CHL 0.2

α-pinene DST 4.3

β-pinene DST 3.1

∆3-carene DST 1.9

sabinene, d-limonene, β-phellandrene, ρ-cymene, myrcene

DST 0.4 to 1.1

camphene, camphor, bornyl acetate, α-thujene,terpinolene, α-terpinene, γ-terpinene, ocimene,1,8-cineole, piperitone, α-phellandrene,tricyclene, other terpenoids

DST 0.1 to 0.4

methanol DUT, CDV or OTH 14

carbon monoxide OTH 4

ethene PGH 1.4

propene, ethanol, acetone, hexenyl-acetate,hexenal, hexenol, other reactive BVOC

DUT, CDV or OTH 1.4

acetaldehyde, formaldehyde, butene, hexanal,other BVOC

DUT, CDV or OTH 0.4

butanone, ethane, non-enzymatic isoprene,acetic acid, formic acid

DUT, CDV or OTH 0.1

(a) 1 Tg = 1012 g

(b) CHL: chloroplast; DST: defense specific tissue; DUT: defense unspecific tissue;

CDV: cut and drying vegetation; PGH: plant growth hormone; OTH: other.

(c) When several compounds are listed, emission refers to the group of compounds.


where ε is an area-average emission capacity (µg g-1 h-1), D is the foliar density (g m-2), γ

is an activity factor that accounts for light, temperature, and leaf age, δ is an activity

factor that accounts for other environmental effects, and ρ is a canopy escape efficiency

factor. ε is derived from measurements. D is a function of land cover. The dependence

of γ on light, temperature, and leaf age is derived from experimental data, and is different

for different emission mechanisms. δ is not currently used in the North American

calculations, but may be used to represent relative humidity effects. A canopy model is

also included for estimating microclimate conditions inside a canopy.

Of all the BVOC, isoprene has been the most thoroughly studied. Consequently,

the status of our current understanding of isoprene is used as a benchmark against which

the understanding of other BVOC emissions is measured. Isoprene is predominantly

emitted from deciduous species and also from spruce species. The emission capacities at

standard conditions (30°C, 1000 µmol photosynthetically active radiation (PAR)) are

well established for predominant emitters. Robust temperature and light correlation

algorithms are available for predominant deciduous and spruce species. The effect of

temperature history on emission capacities has been identified, as is the delayed onset of

isoprene emissions following budbreak in spring. Sampling and analytical systems are

available for isoprene, which allow for leaf, canopy, and mixed layer scale flux

measurements. Reasonable agreements are obtained when measurements at different

scales are compared. The agreement between the BEIS model estimates and observed

isoprene fluxes ranges from 30% to a factor of 2 for a range of sites, with errors in

landcover assignments being a major source of uncertainty in the BEIS model estimates.

Terpenes are emitted from all coniferous species and some deciduous species. As

a class, monoterpenes can constitute the largest emissions of BVOC on a regional basis.

The number of compounds in the class, however, is large, with α-pinene, β-pinene,

limonene, and ∆3-carene being the dominant compounds in terms of emissions. Actual

emission capacities for individual terpenes from specific vegetation types are poorly

known. Rapid chemical losses of terpenes within or above canopies may contribute to

the lack of observations in the ambient atmosphere of many terpene compounds. The

exponential temperature relationship of terpene emissions is well known. However, the

dependence of terpene emissions on humidity and rainfall is not defined. Because


terpenes are stored in resin ducts along the subsurface of the needles, herbivory and

mechanical breakage appear to increase terpene emissions. The impacts are not known

and not represented in regional inventories. Evaluations of terpene emissions have not

been addressed in comprehensive studies such as those conducted for isoprene emissions

(i.e., with measurements at different scales and subsequent model evaluation with those

measurements).

Sesquiterpenes are semi-volatile compounds that have relatively low emission

capacities compared to monoterpenes. There is very little information on individual

sesquiterpenes emitted from vegetation. Measurements have shown that they may

account for up to 16% of the total BVOC emissions from some landscape types.

Available information regarding sesquiterpenes is mostly derived from leaf or branch

enclosure measurements. Even so, data on individual sesquiterpenes have not been

reported in the literature beyond one or two instances, due to the difficulty in

measurements. No canopy-scale data are available, and regional emission estimates do

not exist.

BVOC include a large variety of oxygenated compounds, including 2-methyl-3-

buten-2-ol (MBO), the hexene family (hexenol, hexenal, hexenyl acetate, hexanal),

methanol, acetone, acetaldehyde, formaldehyde, butenone, and low molecular weight

organic acids. MBO is emitted from pines in a light-dependent mechanism similar to

isoprene. The hexene family of compounds is emitted primarily from deciduous

vegetation and agricultural crops via both temperature dependent and wounding or stress

mechanisms. Other oxygenated VOC are associated with the cutting, drying, and

decaying of plant matter. Regional emissions of oxygenated BVOC can be significant,

but intermittent emissions associated with wounding, cutting, or other stress events are

hard to determine. Essentially no canopy-scale information exists and no landscape or

larger scale evaluation has been conducted.

Measurements of BVOC can be made at a variety of scales, from leaf, branch,

canopy, to landscape scales. A leaf cuvette system can be used to collect emission

samples at the leaf scale, but they are difficult to apply to conifers. Branch enclosure

systems are simpler to use, but due to internal shading, only semi-quantitative emission

data can be obtained. The eddy covariance method measures fluxes directly by


measuring the concurrent fluctuations of concentrations and winds. This method requires

a fast response instrument, which at this point is only available for isoprene. The relaxed

eddy accumulation (REA) method derives fluxes from the difference in concentrations

related to updrafts and downdrafts. In addition to isoprene, this method has been used for

terpenes and other BVOC. In the modified Bowen ratio approach, the gradient of a

BVOC is scaled to that of a scalar (such as temperature, CO2, or H2O), whose flux is

directly measured using eddy covariance. Similarity theory is invoked, but it may not

apply within the full range of the canopy and large uncertainties are associated with the

small gradients that occur over the vertical range where similarity theory is valid. Mixed

layer or landscape-scale flux measurements are obtained using a tethered balloon or an

aircraft. These measurements provide useful information for evaluating emission

inventory systems. Advanced REA and analytical methods may be applied for canopy

and landscape scale measurements of terpenes, sesquiterpenes, and oxygenated VOC.

Several methods are available for the reconciliation of ambient concentrations

with emission inventories. Carbon isotope sampling may provide an overall test of the

fraction of biogenic carbon in the ambient air compared to the emission inventories

because fossil fuels and biogenic emissions have different carbon isotope fractions.

Ratios of specific BVOC in ambient air and emission inventory may be used to test

inventory accuracy. Inverse modeling is another tool to estimate biogenic emissions, but

is subject to inaccuracies in the model used and in its input parameters.

E.2 Atmospheric Chemistry of BVOC and Products

The atmospheric chemistry of BVOC contributes to the formation of ozone (O3)

and particulate matter (PM).

The reaction rate constants of many BVOC with hydroxyl radical (OH) and O3

have been measured. Their half-lives, listed in Table E-2, range from ca. 6 hours

(hexanal) to 36 days (acetone) for saturated compounds, and from ca. 30 minutes (α-

terpinene) to ca. 1 day (ethylene) for reaction with OH and from ca. 7 minutes (α-

terpinene) to ca. 12 days (camphene) for reaction with O3 for unsaturated compounds. α-

terpinene, α-humulene, and linalool are among the compounds with the shortest half-


Table E-2. Range of atmospheric half-lives.

Compound Atmospheric half-lives, hours (unless otherwise indicated)

OH = 1.0 x 106

molecule cm-3 O3 = 30 ppb

Saturated compoundsacetone 36 days —

hexanal 5.8 —

Alkenesethylene 22.6 6.6 days

trans-2-butene 3.0 1.3

Isoprene 1.9 20

Terpenescamphene 3.6 11.8 days

α-terpinene 0.53 0.01

Sesquiterpeneslongifolene 4.1 ≥ 21 days

α-humulene 0.65 0.02

Oxygenatestrans-2-hexenyl acetate 6.6 11.6

trans-2-hexenal 4.3 5.3 days

linalool 1.2 0.6


lives. Reaction rates that have not been measured, such as those for α-thujene and cis-3-

hexenal, can be estimated from structure-reactivity considerations with reasonable

accuracy. Kinetic information is also available for many identified first-generation

products, and reasonable estimates can be made of reaction rate constants that have not

been measured using structure-activity considerations.

The atmospheric reactions of BVOC lead to secondary products that contain,

typically, more functional groups than the parent compounds. Less information is

available for reaction products than for reaction rates. Isoprene and MBO are the only

compounds for which sufficient information exists to construct detailed mechanisms.

The atmospheric oxidation of α-pinene is also well documented, except for one

uncertainty related to the yield of pinonaldehyde. Major products have been identified

for a number of unsaturated compounds. However, no data are available for many

compounds, including several terpenes, sesquiterpenes, and unsaturated oxygenates.

First-generation products can undergo further gas-phase reactions in the

atmosphere. We reviewed twenty-six higher molecular weight first-generation products.

Some kinetic information is available for first-generation products. However, product

information exists only for pinonaldehyde, MPAN, and 4 of the carbonyl compounds; no

information is available for the majority of the first-generation products.

The formation of secondary organic aerosol (SOA) has been documented for ten

terpenes, two sesquiterpenes, and linalool. Aerosol yield and size distribution

information is more abundant than composition information, the latter being available

only for α-pinene, and, to a more limited extent, β-pinene, d-limonene, and ∆3-carene.

The overall features of the oxidation mechanisms initiated by the reactions of

BVOC with OH and O3 are reasonably well understood, but the mechanisms involving an

initial nitrate radical (NO3) attack are less certain. The mechanism of OH with saturated

compounds can be used to predict the atmospheric fates of saturated BVOC and ketone

products, such as nopinone. The reactions of OH with aldehydes provide the basis for

understanding reactions of peroxylacyl nitrates because they both lead to peroxyacyl

radicals. Numerous pathways are available for oxidation reactions of saturated

compounds and aldehydes, and many generations of products may be produced. The

reactions of OH, O3, and NO3 with unsaturated BVOC and products follow the


mechanisms of unsaturated hydrocarbons. Due to incomplete yield information for first-

and second-generation products, the relative importance of competing pathways, e.g.,

reaction pathways for alkoxy radicals, is not always clear.

E.3 SOA Partition

Condensable products may be formed from the atmospheric reactions of BVOC.

These compounds partition between the gas and particle phases and can constitute a

significant fraction of atmospheric PM. Currently, three partition theories (saturation,

organic-phase absorption, and aqueous dissolution) are used to explain the formation of

SOA from condensable gases. The saturation theory states that the formation of particles

occurs when the gas-phase concentrations reach saturation, no organic compound exists

in the particle phase when its gas-phase concentration is below saturation vapor pressure.

The fixed yield approach is a variation on the saturation theory, using zero saturation

vapor pressure. The organic-phase absorption theory assumes that the SOA is absorbed

into an organic particle phase. The equilibrium partition of the SOA between the gas

phase and the particle phase is characterized by a partition coefficient. This partition

coefficient can be determined experimentally or from theory. The aqueous dissolution of

soluble organic compounds results in the formation of SOA and is governed by Henry’s

law.

Many three-dimensional models provide options to simulate SOA. Models-3 and

DAQM2 use either the fixed yield approach or an absorption approach based on smog

chamber experiment data. The SOA module of SAQM-AERO is based on the fixed yield

approach. In UAM-AERO, the absorption theory is implemented using Raoult’s law to

determine the equilibrium partition constant based on the saturation vapor pressure of an

ideal gas dissolving into an ideal solution. AER is currently implementing an aerosol

module based on absorption theory into Models-3, this module uses the most recent

published information on experimental SOA parameters developed by Caltech for

biogenic and anthropogenic organic compounds. An aerosol module currently under

development combines SOA absorption into an organic particle phase and dissolution

into an aqueous particle phase.


E.4 Data Gaps and Recommendations

E.4.1 Data gaps

Major data gaps in BVOC emissions include: (1) isoprene emission factors for

vegetation types other than oaks, poplars, aspen, and spruce, quantitative description of

the effects of temperature and light history, and quantitative description of seasonal

dependence; (2) emission factors for specific monoterpene compounds for specific

vegetation species, and scale-up study of monoterpene emissions from leaf to canopy or

larger scales; (3) identity and emission factors of dominant sesquiterpene compounds for

dominant ecosystems, relationship of emissions to environmental factors (season,

temperature), and scale-up study; (4) emission factors for oxygenated compounds (MBO,

compounds of the hexene family, etc.) and their dependence on environmental factors,

estimates of intermittent emissions due to wounding, and experimental scale-up.

Progress in instrumentation is needed for the field measurements of BVOC

emissions, including: (1) REA methods for monoterpenes, sesquiterpenes, and

oxygenated BVOC; (2) fast analytical methods to be used for eddy covariance flux

measurements; and (3) landscape scale emission measurements using aircraft or balloons.

As data gaps are filled, the BEIS-type models need to be updated using improved

landcover data, emission factors, canopy escape factors, and correlations for

environmental factors. The list of BVOC should also be periodically reviewed. More

thorough evaluations of BEIS-type models against ambient data are needed for different

types of ecosystems and different BVOC.

The role of atmospheric multiphase chemistry is unknown (e.g., aqueous

chemistry of BVOC). For the gas-phase chemistry of BVOC, data gaps were identified

in the areas of kinetic data, first-generation products, second-generation products, aerosol

formation, and reaction mechanisms. Kinetic data are missing for a few BVOC,

including α-thujene and trans-2-hexenyl acetate, and for a larger number of secondary

species, although reasonable estimates can be made based on structure-activity

relationships. Product studies have not been conducted for many BVOC, including


terpenes, sesquiterpenes, and oxygenates, and significant uncertainties still exist for the

more familiar α-pinene. Even less is known about the reactions of the higher molecular

weight first-generation products and the formation of second-generation compounds.

While the formation of SOA is documented for 10 terpenes, 2 sesquiterpenes, and

linalool, there is little information on the molecular composition of SOA species. One

important data gap in both the gas-phase and particle-phase products is the formation and

yield of organic acids from BVOC. At present, the lack of identification of many first-

and second-generation products also limits the development of detailed chemical

mechanisms for use in computer models.

Several theories (saturation, organic-phase absorption, and aqueous dissolution)

are currently used to model the partition of condensable species between the gas phase

and the particle phase. The application of these theories to different environmental

conditions needs to be tested. Current SOA models rely on empirically determined

parameters, whose accuracy may be limited to the specific ranges of temperature, RH,

etc., used in environmental chambers. A stronger theoretical basis needs to be developed

before SOA formation models can have the same scientific credibility as inorganic

aerosol models. In addition, both theoretical development and measurement data are

needed to characterize the interactions between organic and inorganic compounds in the

particle phase.

Frequent communication among researchers is essential to integrate research

priorities among various areas of BVOC research and to prioritize the most pressing

research needs. The relevance of BVOC to atmospheric O3 and PM formation should be

used to guide the priorities of emission measurements. On the other hand, compounds

with significant emissions should be priorities for further research in chemistry.

Information on the BVOC products may be incorporated into chemistry and aerosol

partition models to provide more accurate model predictions of O3 and PM, which can

then confirm priorities for emission and chemistry research.

E.4.2 Recommendations for future work

Our recommendations for future work are summarized as follows.


1. Identify and determine the emission factors for specific terpenes,

sesquiterpenes, and oxygenates from specific vegetation species.

2. Reconcile emissions with ambient measurements for BVOC other than

isoprene.

3. Identify and quantify first- and second-generation products of several

terpenes, sesquiterpenes and unsaturated oxygenates.

4. Determine the role of multiphase, especially aqueous-phase, chemistry of

BVOC and products.

5. Investigate the composition and phase properties of atmospheric particles.

6. Develop theroretically rigorous approaches to model SOA from BVOC.

Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products 1-1

1. INTRODUCTION

The National Ambient Air Quality Standards (NAAQS) for ozone (O3) and fine

particulate matter (PM2.5) are likely to be exceeded in many parts of the United States. In

order to control ambient concentrations of O3 and PM2.5 effectively, we need a good

understanding of precursors emissions and chemical transformations responsible for the

formation of O3 and the secondary components of particulate matter (PM). One of the

key areas of uncertainties is the role of natural sources in the production of O3 and PM.

Natural sources include vegetation, soil microbes, lightning, and biomass burning. The

contribution of natural sources has significant implications towards the choice and extent

of controls needed on anthropogenic sources in order to attain NAAQS. This document

focuses on the air quality effects of biogenic emissions, the largest single natural emission

source.

Biogenic emissions consist of a variety of volatile organic compounds (VOC) that

are emitted from vegetation (e.g., isoprene, monoterpenes, sesquiterpenes, and

oxygenates) and inorganic compounds that are emitted from soils (nitrogen oxides, NOx)

and vegetation (CO). While anthropogenic emissions of O3 precursors (VOC and NOx)

dominate in many urban areas, non-negligible contributions of biogenic volatile organic

compounds (BVOC) have been identified in the emissions inventories of several urban

areas, including Nashville, TN (Guenther et al., 1994) and the Northeast. In addition, the

magnitudes of biogenic and anthropogenic emissions are thought to be comparable on a

nation-wide basis. The high reactivity of many BVOC, including isoprene, monoterpenes,

and sesquiterpenes, makes their contribution to O3 formation likely in many areas of the

country (e.g., Roselle et al., 1991; Roberts et al., 1998).

The relationship between biogenic emissions and the formation of PM was first

realized in the phenomenon of “Blue Haze” in the Smoky Mountains, TN, where tiny

particles are formed from biogenic terpene emissions that selectively scatter blue light

(Went, 1960). The reactions of biogenic compounds, such as monoterpenes and

sesquiterpenes, and the formation of biogenic secondary organic aerosols (SOA) have

been a subject of intense research interest for the past 20 years (e.g., Schuetzle and

Rasmussen, 1978; Pandis et al., 1990; Grosjean, 1995; Yu et al., 1998; Griffin et al.,


1999). However, the contribution of biogenic SOA to ambient PM2.5 is still largely

unknown.

Despite significant progress in the past decade, large uncertainties still remain in

the roles of biogenic emissions in the formations of O3 and PM2.5. Major processes

involved in the formation of O3 and SOA are emissions, atmospheric transformations

leading to O3 and condensable products, and the partition of condensable products leading

to the formation of SOA. The specific chemical compounds that constitute biogenic

emissions have not been fully elucidated. Except for isoprene and a limited number of

monoterpenes, the emission rates of other biogenic VOC are largely unknown, limiting

their representations in biogenic emissions inventories.

The importance of isoprene and monoterpenes chemistry is generally recognized,

and a reasonable understanding is available for isoprene and a few monoterpenes,

including a detailed computer mechanism to describe isoprene chemistry. Isoprene is

thought to be the key O3 contributor among the BVOC, and is fairly well represented in

current air quality models. Monoterpenes are of interest mainly from the stand point of

SOA. Since their reaction mechanisms have not been fully elucidated, a parametric

representation is typically used for the entire class. The treatment of the atmospheric

reactions of other biogenic VOC, including oxygenates and sesquiterpenes, remains quite

limited.

Several computer modules are implemented in air quality models to represent the

current state of knowledge in the gas/particle partition of condensable compounds, some

of which are formed from biogenic compounds. Due to the lack of detailed aerosol

composition information, SOA partition is treated for modeled surrogate species rather

than for explicit condensable compounds.

This document reviews the state of knowledge regarding the influence of biogenic

compounds on air quality. The emissions of BVOC are reviewed in Chapter 2. Chapter 3

reviews the atmospheric chemistry of BVOC leading to the production of O3 and

condensable compounds. In Chapter 4, the computer modules available for the

gas/particle partition of organic aerosols are summarized, focusing on the representations

of biogenic SOA. Literature on biogenic compounds has been appearing at a feverish

pace in the past couple of years. Major knowledge gaps as of mid-1999 are identified in


Chapter 5. Since the pace of research is expected to continue, additional information is

expected to become rapidly available.


2. BIOGENIC VOC EMISSIONS RELATED TO OZONE AND AEROSOL

FORMATION

2.1 Introduction

The metabolism of natural ecosystems leads to respiration of gaseous species

ranging from carbon dioxide and methane to a long list of biogenic volatile (and semi-

volatile) organic compounds (BVOC). The former play a role in the earth’s radiation

budget, while BVOC play a direct role in atmospheric photochemical cycles and form a

portion of the global carbon cycle. In particular, compounds including isoprene (C5H8),

monoterpenes (C10H16), sesquiterpenes (C15H24), and oxygenated VOC exhibit significant

reactivity with respect to hydroxyl radical (OH), ozone (O3), and other oxidants. The

photochemical oxidation of these species leads to the formation of tropospheric O3 and

secondary organic aerosols (SOA). For some ecosystems, the emission of these

compounds can account for several percent of the net carbon ecosystem exchange.

Concerns motivated by health effects of O3 and aerosols and the associated

regulations under the 1990 Clean Air Act (CAA) amendments require a clear

understanding of the role of BVOC in the formation of O3 and aerosols over urban and

regional scales. The impacts of O3 and secondary aerosol on regional radiation budgets

and climate change are further motivation for understanding BVOC emissions and fate.

BVOC are released from every ecosystem ranging from old growth forests to

managed forest plantations to grasslands and urban landscapes. However, the species

emitted and their emission rates depend in complex ways upon the type of ecosystem, the

health of the vegetation, time of year, ambient temperature, amount of sunlight, and a host

of other environmental factors. Information concerning these emissions are gained

through the application of measurement techniques that cover the scale of emissions from

the leaf scale (leaf enclosures) to the canopy scale (tower based methods) to the landscape

scale (balloon profiling or aircraft methods). From the canopy scale to the regional scale,

emission inversion methods can be used to reconcile emission inventories with ambient

observations.


In the early 1990’s, the state of the science related to emissions of BVOC was

presented in several reviews including 1) the National Research Council (1991) report

emphasizing the uncertainties in emission estimates related to O3 formation, 2) an

overview paper by Fehsenfeld et al. (1992), which addressed emissions, measurements,

and chemistry of BVOC, and 3) a report for the American Petroleum Institute (Indaco,

1992) which outlined research needs to reduce uncertainties related to BVOC. Details of

the leaf level mechanisms for BVOC emissions are addressed in a monograph edited by

Sharkey et al. (1991) (see also Lerdau et al., 1997). Subsequently, a critical review was

completed by Winer et al. (1995) for the application of emission inventory methods in

California. European findings from the Biogenic Emissions in the Mediterranean Area

(BEMA) program have been reported in a special issue of Atmospheric Environment

(Seufert, 1997). Proceedings of an international workshop on BVOC have been published

with support from the American Meteorological Society and the Air & Waste

Management Association (AMS, 1997). This volume provides a current summary of

many recent and ongoing studies of BVOC emissions, measurements, and chemical

modeling. Papers from this meeting are in publication (Fuentes et al., 1999a). Recent

results from the Southern Oxidants Study (SOS), including a number of papers on BVOC,

have been reported in a special section of the Journal of Geophysical Research (Cowling

et al., 1998). Very recently, Fuentes et al. (1999b) have prepared a new review of BVOC,

and a special session at the European Geophysical Union annual meeting on BVOC was

just completed (Seufert et al., 1999). Finally, a Gordon Conference on BVOC is

scheduled for spring 2000.

An important product of our understanding of BVOC emissions is the

development of regional BVOC emission inventories for use as input to urban and

regional-scale air quality modeling systems. In the U.S., the EPA Biogenic Emission

Inventory System (BEIS) has undergone continuous development beginning with BEIS1

(Lamb et al., 1993; Pierce and Waldruff, 1991; see also Zimmerman, 1979; Lamb et al.,

1987). BEIS1 provided emission estimates with county-scale spatial resolution and hourly

temporal resolution based upon a simple forest canopy model and leaf energy balance.

BEIS2 (Geron et al, 1994) provided better emission algorithms, more accurate land use

descriptions, and a more detailed treatment of tree genus and ecosystems. Now, BEIS3 is


nearing completion; it includes a much more extensive list of emitted compounds, further

improvements in land use data, and more general emission algorithms. BEIS3 also treats

the natural emission of NO from soils, biomass burning, and lightning, and CO production

from soils and biomass burning. Guenther et al. (1999) have described BEIS3 and given

preliminary emission estimates for the North American continent using BEIS3.

Regional emission inventories have been developed using methods similar to BEIS

coupled with region-specific landcover and biomass distribution data. These include

inventories for Los Angeles (Benjamin et al., 1997), the San Joaquin Valley of California

(Winer et al., 1992), the Seattle-Portland corridor of the western Cascades (Lamb et al.,

1997; Barna et al., 1999), and the Houston area of Texas (Wiedinmyer et al., 1999),

among others. These regional emission inventories have been developed in support of

regional photochemical air quality modeling studies. In most cases, intensive field

programs have also been conducted to obtain model development and evaluation data.

Beyond the U.S., the most current natural products emission inventory for Europe

has just been presented by Simpson et al. (1999). It follows the BEIS methods to a large

degree. A global model of biogenic emissions has also been developed (Guenther et al.,

1995a).

The purpose of this review of the emissions of BVOC is to document our current

level of understanding as a basis for identifying critical gaps in knowledge. These gaps

must be addressed to improve our understanding of the role BVOC play in the formation

of O3 and secondary aerosols. Natural emissions of CH4 (Khalil, 1993; Vose et al., 1997),

CO (Tarr et al., 1995; Vose et al., 1997), and NO (Yienger and Levy, 1995; Levy et al.,

1996; Price et al., 1997; Vose et al., 1997) are not addressed in this review. Because the

BEIS3 inventory represents the most current understanding of BVOC emissions, a brief

review of its structure, parameters, and data input are given in Section 2.2. Current

information on isoprene, monoterpenes, sesquiterpenes, and oxygenated VOC is reviewed

in Section 2.3, and the status of various measurement methods is covered in Section 2.4.

The use of emission inversion techniques is reviewed in Section 2.5, and summary and

conclusions are given in Section 2.6.


2.2 A Third Generation BVOC Emission Inventory

Information in this section is taken primarily from a draft manuscript by Guenther

and colleagues (1999) describing the development of a third generation BVOC emission

inventory system (BEIS3). This work was conducted as part of the NARSTO assessment

on BVOC. As part of the assessment, an annual average emission inventory for the North

American continent was developed using the methods outlined as BEIS3. However, a

finished BEIS3 modeling system does not yet exist in terms of a complete computer code,

required input data sets, and associated instruction manuals.

2.2.1 BEIS3 compounds

The BEIS3 approach addresses the emission of 36 BVOC that are emitted from

vegetation, NO from soils, biomass burning, and lightning, and CO from soils, vegetation,

and biomass burning. A list of the BEIS3 compounds and their assigned emission

capacities are given in Table 2-1. The annual average emission flux for North America for

the BEIS3 compounds is summarized in Table 2-2. BVOC emissions from vegetation

account for 98% of the total (BVOC) emissions. A small number of compounds or

compound classes dominate this total as follows: isoprene (31%), methyl butenol (MBO)

(5%), monoterpenes (22%). The remainder of emissions are due to other reactive BVOC

(16%, i.e., hexenol, ethene, formaldehyde, and acetic acid) and other (less-reactive)

BVOC (26%, methanol and acetone). Emissions of alkane and aromatic compounds are

very low and greatly overestimated in some earlier inventories (Guenther et al., 1999).

The emissions of BEIS3 compounds are dominated by mechanisms involving either

chloroplast production (CHL) or defense (specialized tissues, DST, and unspecialized

tissues, DUT). Other mechanisms, including plant growth hormone (PGH), plant

wounds/disease/harvesting (cut and drying vegetation, CDV) and floral scents (FS, Borg-

Karlson et al., 1994), may be important at specific times and locations, but do not

contribute substantially to the annual totals in Table 2-2.


Table 2-1. Emission factors and annual North American emission rates in Teragrams

of carbon (TgC)(a) for emissions from live vegetation (from Guenther et al.,

1999). Emission type corresponds to specific emission mechanisms: CHL

= chloroplast emissions; DST = defense specific tissue emissions; DUT =

defense unspecific tissue emissions, PGH = plant growth hormone, CDV =

cut and drying vegetation; FS = floral scents; and OTH = other.

Emission type Emission capacity(µµg g-1 h-1)

Annual emission(b)

(TgC)Compounds

CHL 0 to 100 24.0 isoprene

CHL 0 to 60 4.1 MBO

CHL 0 to 20 0.2 α-pinene

DST 0 to 2 4.3 α-pinene

DST 0 to 1.5 3.1 β-pinene

DST 0 to 1 1.9 ∆3-carene

DST 0 to 0.6 0.4 to 1.1 sabinene, d-limonene, β-phellandrene, ρ-cymene, myrcene

DST 0 to 1.5 0.1 to 0.4 camphene, camphor, bornyl acetate,α-thujene, terpinolene, α-terpinene, γ-terpinene, ocimene, 1,8-cineole,piperitone, α-phellandrene,tricyclene, other terpenoids

DUT, CDV or OTH 1 14 methanol

OTH 0.3 4 carbon monoxide

PGH 0.1 1.4 ethene

DUT, CDV or OTH 0.1 1.4 propene, ethanol, acetone, hexenyl-acetate, hexenal, hexenol, otherreactive BVOC

DUT, CDV or OTH 0.03 0.4 acetaldehyde, formaldehyde, butene,hexanal, other BVOC

DUT, CDV or OTH 0.01 0.1 butanone, ethane, non-enzymaticisoprene, acetic acid, formic acid

(a) 1 Tg = 1012 g

(b) When several compounds are listed, emission refers to the group of compounds.


Table 2-2. Annual above canopy flux of NO (Teragrams of nitrogen, TgN)(a), BVOC

(TgC), and CO (TgC) from natural sources in North America (from

Guenther et al., 1999). Terpenoid BVOC include hemiterpenes (isoprene

and MBO), monoterpenes (e.g. α-pinene), and sesquiterpenes. Other

reactive BVOC include all non-terpenoid VOC that have a lifetime of less

than one day for typical tropospheric conditions (e.g., hexenol, ethene,

formaldehyde, acetic acid). Other BVOC include all other BVOC (e.g.

methanol and acetone).

Compounds Vegetation Soils Lightning Biomassburning

Total

NO 0 0.9 0.9 0.3 2.1

CO 4 0.0 0 6 10

Total BVOC 77.3 1.1 0 0.6 79

Isoprene 24.0 0 0 0 24.0

MBO 4.1 0 0 0 4.1

Monoterpenes 17.3 0 0 0 17.3

Other reactive BVOC 12.4 0.2 0 0.2 12.8

Other BVOC 19.5 0.9 0 0.4 20.8

(a) 1 Tg = 1012 g


2.2.2 BEIS3 model algorithms

The BEIS series of emission models, including BEIS3, employ a linear

combination of terms where the local BVOC flux from a vegetative canopy for a given

compound or class of compounds is given as:

F = εDγδρ (2-1)

where ε is an area-average emission capacity (µg g-1 h-1), D is the foliar density (g m-2), γ

is an activity factor that accounts for the influence of light, temperature, and leaf age, δ is

an activity factor that accounts for other environmental effects, and ρ is a canopy escape

efficiency factor (the fraction of BVOC that escapes the canopy to the atmosphere).

Guenther et al. (1999) assigns a default value of 0.95 to the escape efficiency factor in the

absence of any definitive BVOC deposition or canopy loss data.

The emission capacity represents the emission rate of a compound, such as

isoprene, from a leaf under a standard set of conditions (usually a leaf temperature of 30oC and, for light dependent emissions, photosynthetically active radiation, (PAR) of 1000

µmol m-2 s-1). Emission capacities are listed in Table 2-1 for the BEIS3 compounds.

These emission capacities are derived from measurements reported in the literature

(Guenther et al., 1994; Guenther et al., 1995b; Guenther et al., 1996a). For chloroplast

and defense emission mechanisms, specific values of emission capacity are available for

isoprene, MBO, and monoterpenes emitted from different vegetation genus types.

However, for most oxygenated compounds and BVOC produced by other emission

mechanisms, measurement data are sparse and the emission capacity is assigned one of

five classes: 0.01, 0.03, 0.1, 0.3, and 1 µg g-1 h-1.

Foliar density (D) is required to convert the area-average emission capacity on a

per biomass basis to an emission flux on a per unit ground area basis. Values of D are

derived from biomass surveys in the literature and assigned to each class of vegetation

treated in BEIS3. The Seasonal Land Cover Regions (SLCR) Global Landcover

Characteristics database provides 1 km resolution information for 205 land cover


associations for North America (http://edcwww.cr.usgs.gov/landdaac/glcc/glcc.html)

covering the period April 1992 through March 1993. The plant species composition and

foliar density were assigned for each of the 205 landcover types using species distribution

data (Rzedowski, 1988; Geron et al., 1994). Changes in biomass density as a function of

time of year were estimated using the average monthly leaf area index (LAI) derived from

Advanced Very High Resolution Radiometer (AVHRR) measurements (Sellers et al.,

1994).

The γ activity factor for treating light, temperature, and leaf age effects consists of

three terms:

γ = γLγTγA (2-2)

where γL accounts for the effects of light on emissions, γT treats temperature effects, and

γA addresses leaf age effects. The first two terms have been slightly revised from the light

and temperature correction terms used in BEIS2 (Geron et al., 1994) and originally

developed by Guenther et al. (1991, 1993). The third term addresses new information that

emissions of isoprene depend upon leaf age. Early in the season, isoprene emissions do

not begin with budbreak, and late in the season, emissions decline near leaf senescence.

Results from Monson et al. (1994), Geron et al. (1997), and Goldstein et al. (1998)

suggest that isoprene emission onset is initiated after 650 heating degree days (defined as

number of days with mean temperature above 65 F) which occurs several weeks after

budbreak. Peak isoprene emissions occur after 1050 heating degree days and continue

until nighttime temperatures fall below some minimum temperature.

The light correction term for isoprene chloroplast emissions is given as:

γL = (α CL L)/ [(1+α2 L2)0.5] (2-3)

where L is current PAR (µmol m-2 s-1), and α and CL are empirical coefficients. Guenther

et al. (1993) assumed constant values, α = 0.0027 and CL = 1.066, for these coefficients

but recent studies have shown that these coefficients vary with past PAR levels (Harley et

al., 1996, 1997).


The temperature correction term for isoprene emissions associated with the

chloroplast mechanism is given as:

γT = Eopt CT2 exp(CT1 x)/[CT2-CT1{1-exp(CT2 x)}] (2-4)

where x = [(1/Topt)-(1/T)]/R, T is current leaf temperature (K), R is the gas constant (=

0.00831 kJ K-1 mol-1), Eopt is the maximum normalized emission capacity, Topt is the

temperature at which Eopt occurs (K), and CT1 (= 95 kJ mol-1) and CT2 (= 230 kJ mol-1) are

empirical coefficients that determine the rate of emission change with temperature. This

expression is nearly equivalent to the algorithm of Guenther et al. (1993) for Eopt = 1.9 and

Topt = 312.5 K. Recent studies have shown that the coefficients Eopt and Topt depend on

the mean temperature of the past 18 hours (C. Geron, unpublished data) and the mean

temperature of the past one to three weeks (Sharkey, 1997; Hopkins et al., 1999).

For monoterpene emissions via defense related mechanisms, the temperature

dependence is given as an exponential function of temperature:

γT = Esexp[β(T – Ts)] (2-5)

where β is the temperature coefficient, usually taken to equal 0.09 K-1, and Es is the

normalized emission capacity at temperature Ts.

Guenther et al. (1999) suggest a simple relationship for the leaf age parameter that

depends on the change in biomass density from month to month (∆Df), normalized by the

maximum foliar density:

γA = [A1 ∆Df] + [A2 (1-∆Df)] (2-6)

where A1 (= 0.5) is the average emission activity of young and old leaves, and A2 (= 0.95)

is the fraction of mature foliage present during the month of peak foliar density. Guenther

et al. also note that these coefficients may vary, but no data are available to describe these

changes.


2.2.3 BEIS3 canopy modeling

The microclimate in a forest canopy must be modeled in order to provide leaf-level

estimates of temperature and PAR for use in the emission algorithms. A variety of

approaches have been used to account for changes in temperature and light as a function

of height and vertical biomass distribution within a canopy. Lamb et al. (1993) introduced

a simple scaling model to adjust above canopy observations of temperature, PAR, wind

speed, and humidity as a function of height in the canopy. These authors then solved a

leaf energy balance to predict leaf temperature as a function of height within the canopy.

This approach provided the basis for BEIS1 (Pierce and Waldruff, 1991). Geron et al.

accounted for the exponential decay of light downward through the canopy, but assumed

leaf temperature equaled the above-canopy temperature in the development of BEIS2. In

BEIS3, a revised version of the BEIS1 leaf energy balance has been introduced, and PAR

levels are adjusted for sun and shaded leaves as a function of height in the canopy

(Guenther et al., 1999).

Treatment of leaf temperature appears necessary to account for upper level heating

of the canopy (Singaas and Sharkey, 1997; Hall et al., 1997). This is particularly true for

deciduous canopies; its importance is less apparent for coniferous canopies where the thin

needle structure promotes better thermal equilibrium with the local air temperature. Lamb

et al. (1996) compared the use of the BEIS1 simple canopy model with a numerical,

dynamic canopy model (Baldocchi and Harley, 1995) and found that use of the more

complex model did not make a significant improvement in emission predictions.

A potentially significant improvement in treatment of canopies may derive from

very strong correlations between measured canopy heat flux and isoprene emissions

(Westberg et al., 1999a). It appears that canopy heat flux may serve as a surrogate for an

integrated canopy temperature. This is valuable since current mesoscale meteorological

models, as well as current climate models, incorporate heat flux predictions as part of the

land/air energy balance.


2.2.4 Summary

In summary, BEIS3 provides an improved framework for estimating the emissions

of BVOC from vegetation. Specific changes include a detailed listing of specific

compound emissions for over thirty different compounds, slight modification to the light

and temperature algorithms for estimating leaf-level chloroplast and defense specific tissue

emissions, incorporation of a leaf age emission adjustment term, revision of a simple

canopy model for estimating microclimate conditions, specification of emissions due to

other emission mechanisms, and compilation of detailed plant species and biomass density

estimates for 205 unique landcover classes in North America.

2.3 Emissions of BVOC

2.3.1 Isoprene

On a global scale and most regional scales, isoprene is the dominant individual

VOC emitted from vegetation (Rasmussen and Khalil, 1997; Geron et al., 1994; Guenther

et al., 1995a). Sampling and analytical methods for measuring isoprene are well

developed and relatively straightforward. Similarly, methods for measuring the flux of

isoprene from ecosystems are well developed, including a fast response instrument for use

as an eddy covariance sensor (Hills and Zimmerman, 1990). As a result, the science

describing isoprene emissions from vegetation is more mature than for other BVOC, and

the uncertainties in isoprene emission inventories are less than for other compounds.

Consequently, our approach is to describe the status of our understanding of isoprene

emissions and to use that status as a benchmark against which we can measure our

understanding of other BVOC emissions.

It is well known that isoprene is emitted only during daylight hours from specific

deciduous species, such as oak, willow, aspen, and poplar, and only from spruce (Kempf

et al., 1996) among coniferous species. Isoprene emissions exhibit a pronounced

dependence upon leaf temperature and PAR. The emission algorithms developed by

Guenther et al. (1993) that describe these dependencies appear to be relatively robust and


applicable to both deciduous and spruce emissions. Refinements to these algorithms have

mainly been to improve the emission factor at standard conditions (typically 30 oC and

1000 µmol m-2 s-1 PAR) for various tree species and to account for changes in emission

factor as a function of position in a canopy (sunlit leaves versus shaded leaves, Harley et

al., 1997; Sharkey et al., 1991a), time of year, and recent (1 to 3 weeks) temperature

history.

The reason why isoprene is emitted is not completely understood, but it has been

suggested that it may be emitted as a thermal stress relief mechanism (Sharkey and

Singaas, 1995). The biochemical pathway is well known (Sharkey et al., 1991b; Fall and

Wildermuth, 1998), and has been described most recently in a review by Fuentes et al.

(1999a). A critical aspect of isoprene emissions from vegetation is that isoprene emissions

are very vegetation species specific so that the distribution of isoprene emissions in an

ecosystem may be very heterogeneous. In order to predict which species may emit

isoprene, Benjamin et al. (1996) developed a taxonomic scheme for assigning emission

capacities to a long list of vegetation as a basis for determining low emitting vegetation

suitable for large scale planting in urban areas. In their work, they found good

correlations for species within the same genera, but less correlation among emissions for

species within the same family.

Maximum isoprene emission rates (at standard conditions of 30 oC and 1000 µmol

m-2 s-1 PAR) are of the order of 100 µg g-1 h-1 which translates to isoprene fluxes from

deciduous forests of about 4000 µg m-2 h-1. Under very warm and clear conditions, actual

fluxes can be much higher. Hopkins et al. (1999) recently reported an isoprene flux

exceeding 30,000 µg m-2 h-1 from a managed poplar plantation during extremely warm (35oC) August mid-afternoon conditions. Above canopy, isoprene concentrations closely

follow the isoprene fluxes and exhibit very low concentrations at night, rapid increase in

the morning, a large maximum during mid to late afternoon, and rapid decrease in the

evening. There is an ongoing debate concerning the mechanisms responsible for the rapid

loss of isoprene in the early evening (Starn et al., 1998).

Several evaluations of the BEIS canopy emission models for isoprene have been

reported that generally show that for a specific site, with on-site meteorological input data,

agreement between measured and predicted isoprene fluxes can be within 30% (Lamb et


al., 1996; Guenther et al., 1996b; Fuentes et al., 1996, 1997; Baldocchi et al., 1995). Hall

et al. (1997) reported that while the overall level of agreement was reasonably good, it

was not possible to predict both the average isoprene emissions and the peak afternoon

maximum emission correctly. Very recently, Makar et al. (1999) described results of a

very detailed numerical model of isoprene emission, transport, and chemistry within and

above a forest canopy. Their results suggested that peak isoprene emissions measured

above a canopy would underestimate the actual leaf-level emissions by as much as 40%

due to isoprene chemical loss within the canopy. This chemical loss could explain the

difficulties reported by Lamb et al. in matching average and peak emission fluxes with the

BEIS emission model. This suggested chemical loss was not addressed by Guenther et al.

(1999) in the assignment of the canopy escape factor in BEIS3 described above.

Goldstein et al. (1998) measured isoprene fluxes over a northeastern mixed

deciduous forest continuously through a growing season using a micrometeorological

gradient method. Their results were consistent with the onset of isoprene emissions

approximately 2 weeks after budbreak, maximum emission capacity during approximately

2 months through the summer, and steady decrease in emission capacity during September

and October. The authors pointed out that BEIS2 does not consider this change in

emission capacity and hence overestimated emissions in the spring and fall. Presumably,

the incorporation of leaf age in the BEIS3 model will eliminate this problem. Predictions

with BEIS2 underestimated the observed mid-day fluxes, but this was attributed to the use

of a low oak emission factor (70 µg g-1 h-1) in BEIS2 compared to site-specific

measurements that were closer to 100 µg g-1 h-1. There was also an indication that the

BEIS canopy model failed to capture the actual diurnal pattern of emissions.

Recent compilation of regional emission inventories have demonstrated how

critical accurate landcover information is. In the Pacific Northwest, Lamb et al. (1997)

employed U.S. Forest Service tree inventory data provided by Geron (personal

communication) to derive new landcover distributions for a gridded emission inventory.

These data were used with chemical compound-specific emission capacities (an early

version of the BEIS3 compound list) to estimate emissions on a 5 km gridded basis.

Comparison with results from BEIS2 showed that the new estimates for isoprene were as

much as a factor six less than in BEIS2 due to changes in landcover, while estimates for


terpenes were in relatively good agreement with BEIS2. Wiedinmyer et al. (1999)

conducted a detailed biomass survey for the Houston, TX region and developed a regional

emission inventory. These authors found that the revised inventory showed substantial

differences from BEIS2 in terms of the distribution and magnitude of isoprene emissions in

the region. Total BVOC emissions were a factor of two higher with the new landcover

data compared to that used in BEIS2. In both of these cases, improved landcover/biomass

distribution data resulted in very significant changes in the final emission inventory

estimates.

The following statements summarize our current understanding of isoprene emissions:

• The emission capacities at standard conditions for isoprene emitted from the

predominant deciduous species and several species of spruce are relatively well

established;

• Temperature and light correction algorithms are available which appear to be

robust for the predominant deciduous species and for spruce;

• The effect of temperature history upon emission capacity has been identified

and a method for treating the effect has been proposed, but the actual timescale

for the effect has not been clearly defined (18 hours and one to three weeks are

suggested);

• The delayed onset of isoprene emissions in spring following budbreak (by

several weeks dependent upon heating degree days) and the decrease of

isoprene emission capacity late in the season have been identified;

• The effect of leaf position in the canopy (sunlit and shaded leaves) has been

shown to affect the isoprene emission capacity;

• Sampling and analytical systems are available for isoprene, including a fast-

response analyzer, which allows leaf, canopy, and mixed layer scale flux

estimates to be obtained;

• Recent modeling results suggest that rapid chemical loss within a canopy may

cause as much as a 40% difference between canopy-scale flux measurements

and leaf-level emission estimates


• Evaluation of BEIS canopy models indicates that the models agree with

observed fluxes to within 30% for site-specific situations.

• Recent analyses of BEIS2 inventories compared to specialized regional

inventories indicate that errors in landcover assignments can significantly

change isoprene emission estimates; similar evaluations of BEIS3 isoprene

emissions have not been conducted.

2.3.2 Monoterpenes

Monoterpene emissions occur for conifer species (Guenther et al., 1994) and some

deciduous species (Keiser, 1997). Monoterpene emissions, as a class of compounds, can

be the largest emissions on a regional basis. However, the actual members of the class are

not necessarily clearly defined. In almost all studies, terpene emissions are given in terms

of a small number of predominant compounds, typically α-pinene, β-pinene, limonene, and

∆3-carene, along with a larger number of terpenes present only in trace amounts.

Consequently, there is a long list of identified terpene emissions from vegetation (Isidorov

et al., 1985), but the emission capacities for individual terpene compounds are not well

established.

As an example, Pressley et al. (1998) have identified terpene emissions from

branchlet cuvette measurements on the predominant conifer species in an old growth

forest. Their results are shown in Table 2-3. While there are a number of terpenes

identified, the emissions of α-pinene, β-pinene, limonene and ∆3-carene account for 95%

of the emissions from these conifers. Similarly, Lamanna and Goldstein (1999) used an

automated gas chromatography (GC) system to measure ambient concentrations and

fluxes above a ponderosa pine plantation in California and reported data only for α-pinene,

∆3-carene, and d-limonene. Helmig et al. (1999a) employed a branch enclosure system

with adsorbent cartridge sampling and gas chromatography/mass spectrometry (GC/MS)

analysis to measure BVOC emissions at three US sites. Sixty-three vegetation


Table 2-3. Preliminary analyses of terpene emissions composition (% of total

terpenes) for dominant conifer species at the Wind River Crane Research

Facility (Pressley et al., 1998).

Compound Douglas-Fir(PSME)

W. Red Cedar(THPL)

P. Silver Fir(ABAM)

W. Hemlock(TSHE)

α-pinene 28.6% 9.8% 22.8% 30.3%

∆3-carene 22.5% 10.1% 36.8% 7.6%

β-pinene 14.2% 6.6% 13.7% 17.6%

limonene 13.4% 17.1% 15.2% 22.5%

sabinene 6.8% 7.1% 0.4% 0.2%

myrcene 5.7% 3.4% 4.0% 8.6%

other 3.7% 17.4% 2.8% 2.8%

α-terpinolene 3.0% 4.1% 1.7% 0.9%

camphene 0.8% 0.2% 0.3%

tricyclene 0.8% 0.3% 0.3%

α-phellandrene 0.3% 1.7% 4.7%

thujene 0.1%

2-carene 0.1% 2.2% 0.4% 2.4%

β−phellandrene 1.8%

thujone 22.2%


species were sampled and 114 BVOC were detected with 69 compounds structurally

identified and 30 tentatively identified (Table 2-4). A number of monoterpenes were

included in the tentatively identified list. Emission rates were assigned to a number of

identified monoterpenes (for example, see Table 2-5 and Table 2-6) at the three sites.

In the BEIS3 North American annual emissions estimates, Guenther et al. (1999)

list 16 individual terpenoid compounds (Table 2-1). However, many of these are

estimated using the same range of emission factors which suggests that actual emission

capacities for specific terpenes from specific vegetation types are poorly known.

Croteau (1987) has described the biochemical pathway for terpene production in

needles. Monoterpenes are produced and stored in speciallized locations along the

subsurface of the needle. Tingey et al. (1991) have developed a needle diffusion model to

predict α-pinene emissions as a function of needle structure and environmental conditions.

Because the terpenes are stored in resin ducts, the emission rate is controlled by the vapor

pressure of the terpene pool and the diffusion characteristics of the needle. Thus, terpene

emissions do not show a direct dependence upon photosynthesis rates, but rather are

emitted continually as an exponential function of temperature which can be described with

Equation 2-5. Further, because of the storage pools, wounding or breaking of the needles

due to mechanical stress (Juuti et al., 1990) or herbivory can short-circuit the diffusion

path and lead to large increases in the emission rate. Litvak (1997) has shown that

wounding to mimic herbivory also leads to the secretion of non-volatile materials to seal

the wound so that the effect is short-lived.

Due to the temperature dependence, terpene emission fluxes from vegetation

follow a diurnal pattern with low emissions at night, an increase in emissions during the

day to a mid-afternoon maximum, and a return to low emission fluxes in the late evening.

Very few canopy-scale emission flux measurements have been reported. Most of what is

known about terprene emissions is based upon branch enclosure or laboratory

measurements. Ambient concentrations of terpenes immediately above a canopy exhibit

the highest concentrations at night when atmospheric mixing is limited.

Rapid chemical losses of terpenes within the canopy may contribute to the low

levels of terpenes observed during the daytime. Ciccioli et al. (1999) measured emission

rates of terpenes and sesquiterpenes from orange trees using a branch enclosure system


Table 2-4. List of compounds detected from enclosure sampling at three U.S. sites

(Helmig et al., 1999a).

Compound Compound Compound

Ethanol Camphene Trans-verbenol

Acetone Phenol Ni

Pentane 6-methyl-5-hepten-2-one Borneol

Isoprene Mt Mt

Methacrolein Artemiseole 1-heptanol

Methylvinylketone 5-methyl-3-heptanone α-fenchene

3-methylfuran Sabinene Cis-3-hexenyl n-butyrate

2-methyl-3-butyl-2-ol Ni 4-terpineol

Acetic acid 6-methyl-5-heptanene-2-ol Methyl salicylate

2-ethylfuran β-pinene Decanal

Ni 1-methylethenylbenzene Cis-3-hexenyl iso-valerate

3-methyl-2-butenal isomer Mt Ni

3-methyl-2-butenal isomer Octanal Ni

Ni β-myrcene Thymol

2-penten-1-ol Cis-3-hyxenyl acetate Isobornyl acetate

Ni Trans-3-hexenyl acetate Dodeceneol isomer

2-methyl-butenoic acid methylester

α-phellandrene Hexene-ol-hexanoate isomer

Ni ∆3-carene Ni

2-methyl-4-pentanal Ni

Hexanal α-terpinene Ni = not identified

1-octene p-cymene Mt = monoterpene

C7H10O isomer 1,8-cineole

C7H10O isomer Ni

2-hexyn-1-ol Ni

2-hexenal d-limonene

Ni Cis-ocimene

Cis-3-hexen-1-ol Acetophenone

Trans-3-hexen-1-ol Trans-ocimene

2-hexen-1-ol γ-terpinene


Table 2-4. List of compounds detected from enclosure sampling at three U.S. sites

(Helmig et al., 1999a) (continued).

1-hexanol 1-octanol

C10H16 Mt

2,4-hexadrenal Artemisia alcohol

Mt Fenchone

1-nonene p-cymenene

Methoxybenzene (anisole) Mt

Cis-3-hexanyl formate Terpinolene

Santolana triene Mt

Mt Nonanal

Dimethyl-3(5H)-furanoneisomer

Ni

Trans-3-hexanyl-formate α-thujone

Dimethyl-3(5H)-furanoneisomer

β-thujone

5-ethyl-2(5H)-furanone β-fenchol

Tricyclene Ni

α-thujene Ni

Mt Ni

Benzaldehyde C10H14 isomer

α-pinene C10H14 isomer

Mt Camphor


Table 2-5. BVOC emission rates (µgC g-1 h-1) from branch enclosure measurements at Fernbank forest in urban Atlanta (from

Helmig et al., 1999a).

Compound N. RedOak

DawaRed-wood

Bass-wood

Whitemul-berry

Easternhem-lock

Iron-wood

Amer.beech

PostOak

Slip-peryelm

Tulippoplar

Glaluge Sweetgum

loblollypine

Redmul-berry

Blackoak

Whiteoak

Sth.Redoak

Blackgum

Ethanol 0.5 0.2

Acetone 0.4 0.2 0.0 0.6 0.1

Pentane 1.7 0.2 0.2

Isoprene 140 3.1 0.1 29 0.9 1.4 2.9 77 1.4 0.1 0.0 9.3 0.8 0.5 67 77 29 4.3

Methacrolein 1.0

Methyvinyl ketone 0.2

2-methylfuran 0.3

Acetic acid 0.3 0.1 0.5 0.3 0.3 0.2

2-ethyfuran 0.6 0.3

Ni 0.4 0.0

Butanoic acidmethyl ester

0.2

Hexanal 0.5 0.9 0.8 0.3

C7H10 isomer 0.3

C7H10 isomer 0.3

2-hexenal 1.6 0.3 1.3 0.8

Ni 0.2

Cis-3-hexen-1-ol 3.7 0.2 2.0 0.0 0.4 2.5 0.6 0.1

Trans-2-hexen-1-ol 0.7 1.0

1-hexanol 0.1

Mt 0.7

Methoxybenzene 0.7

α-thujene 0.4 0.2



Helmig et al., 1999a) (continued).

Compound N. RedOak

DawaRed-wood

Bass-wood

Whitemul-berry

Easternhem-lock

Iron-wood

Amer.beech

PostOak

Slip-peryelm

Tulippoplar

Glaluge Sweetgum

loblollypine

Redmul-berry

Blackoak

Whiteoak

Sth.Redoak

Blackgum

Mt 0.2 0.6

Benzaldehyde 0.6 0.5 0.3 0.3

α-pinene 5.7 0.2 0.3 9.9 0.3 2.1 1.6 0.2

1-heptanol 0.1

α-fenchene 0.7 0.9

Phenol 0.1

Camphene 2.1 0.2 0.2 5.3 1.4 0.9 0.2

Mt 0.4 0.4

Ni 1.5

Sabinene 0.3 0.4

β-pinene 0.1 0.2 1.6 0.2 0.4

1-methylethenylbenzene

0.1

β-myrcene 0.9 2.3 0.3 0.3

Octanol 0.8 0.2 1.2 0.2 0.2 0.2 0.2 0.6

Cis-3-hexenylacetate

2.6 2.3 0.0 1.2 13.3 0.7 2.4 0.3 1.5 0.2

Trans-2-hexenylacetate

0.1 0.2

α-phellandrene 0.1 3.7 1.8 0.5

∆3-carene 0.6

Ni 0.5

α-terpinene 0.1 12 8.3 0.3



Helmig et al., 1999a) (continued).

Compound N. RedOak

DawaRed-wood

Bass-wood

Whitemul-berry

Easternhem-lock

Iron-wood

Amer.beech

PostOak

Slip-peryelm

Tulippoplar

Glaluge Sweetgum

loblollypine

Redmul-berry

Blackoak

Whiteoak

Sth.Redoak

Blackgum

p-cymene 1.3 0.9 0.2 0.2 17 10 0.7 0.3

Ni 0.2

d-limonene 1.4 0.1 0.2 23 0.2 2.9 0.7 0.2

cis-ocimene 2.2

Acetophenone 0.1

trans-ocimene 3.3

1-octanol 0.1

γ-terpenene 0.2 9.2 5.8 0.5

Mt 0.2 0.2

p-cymenene 0.5 6.5 0.9 0.2 0.8

Mt 0.1 0.1

Terpinolene 0.2 1.3

Mt 5.0 1.5

Nonanal 1.8 0.3 13 0.3 0.3 0.7 0.9 0.2 0.2 0.1 0.8

Ni 0.2

Cis-3-hexenyl n-butyrate

5.2

Methyl salicylate 0.5 0.3

Decanal 0.3 0.2 0.3 0.5

Dodecene-1-ol 0.6

Ni 0.7 0.1 0.1

Sesquiterpenes 0.8 78 8.5 2.6

Total VOC 150 20 12 2.9 4.4 5.4 120 160 5.9 8.6 0.0 46 5.9 1.1 76 82 30 5.6


Table 2-6. Landscape emissions (µgC m-2 h-1) averaged over a one week summer

temperature and light record for three U.S. sites (from Helmig et al.,

1999b).

Compound Fernbank Forest(urban Atlanta

forest)

Willow Springs(mixed deciduous &

coniferous WI forest)

Temple Ridge(mixed shrub oakwoodland in CO)

Total % of total Total Total % of total

Ethanol 0.1 0.6

6 4 14

Pentane 0.1

890 1700 1300

Methacrolein 0.5 5 0.2

0.5 0.0

1 0.1 0.0

Acetic acid 0.2 1

2-ethyfuran 9 19 0.4

2 0.1

4 0.2

0.1 0.0

methyl ester5

Ni 0.1

2-methyl-4-pentanal 7 0.1 0.0

10 0.5 0.2 4

1-octene 1

C7H isomer 0.0

7 10 0.4

2-hexyn-1-ol 1.5

190

Ni 0.0

34 1200 190

Trans-2-hexen-1-ol 0.3 0.4

0.1 0.1

Mt 0.0



temperature and light record for three U.S. sites (from Helmig et al., 1999b)

(continued).


forest)




Total % of total Total % of total Total % of total

1-nonene 3 0.1

Methoxybenzene 0.4 0.0

Cis-3-hexen-1-ol-formate 12 0.3

Santolina trieme 2 0.1

Mt 2 0.0

Dimethyl-3(5H)-fluoroneisomer

9 0.4

Dimethyl-3(5H)-fluoroneisomer

17 0.7

5-ethyl-2(5H)-fluorone 15 0.3 15 0.6

Tricyclene 5 0.1 33 1.4

α-thujene 1 0.1 6 0.1 4 0.2

Mt 1 0.0

Benzaldehyde 0.5 0.0 6 0.1 0.1 0.0

α-pinene 57 2.8 150 3.3 39 1.6

Mt 2 0.0

1-heptanol 0.1 0.0

α-fenchene 2 0.1 10 0.2 0.2 0.0

Camphene 32 1.6 130 2.9 12 0.5

Phenol 0.1 0.0 0.1 0.0

6-methyl-5-hepten-2-one 0.1 0.0

Mt 2 0.1

Artemiseole 4 0.2

Sabinene 3 0.1 0.3 0.0

Ni 2 0.1 1 0.0




1999b) (continued).


forest)





β-pinene 9 0.4 55 1.2 3 0.1

1-methylethenylbenzene 0.1 0.0

Mt 0.5 0.0

Octanal 7 0.4 4 0.1 10 0.4

b-myrcene 14 0.7 14 0.3 0.4 0.0

Cis-3-hexenyl acetate 50 2.5 540 12 440 18

Trans-2-hexenyl acetate 9 0.4 6 0.1

α-phellandrene 16 0.8 12 0.3 2 0.1

∆3-carene 0.7 0.0 26 0.6

Ni 0.5 0.0 5 0.1

α-terpinene 59 2.9 16 0.4 2 0.1

p-cymene 88 4.4 54 1.2 8 0.3

1,8 cineole 5 0.2

Ni 0.2 0.0

d-limonene 71 3.6 120 2.7 15 0.6

cis-ocimene 48 2.4

Acetophenone 0.1 0.0

trans-ocimene 73 3.7 33 1.4

γ-terpinene 43 2.2 9 0.2 1 0.1

1-octanol 0.1 0.0

Mt 1 0.1

Artemisia alcohol 9 0.4

Fenchone 2 0.0

p-cymenene 21 1.1 21 0.5 1 0.0

Mt 0.3 0.0




1999b) (continued).


forest)





Terpinolene 5 0.3 15 0.3 2 0.1

Mt 17 0.8

Nonanal 15 0.8 33 0.7 21 0.9

α-thujone 1 0.0

β-fenchol 1 0.0

Ni 0.2 0.0

Ni 11 0.5

C10H14 0.3 0.0

Camphor 1 0.0 7 0.3

Ni 26 1.1

Borneol 1 0.0 1 0.0

Cis-3-hexenyl n-butyrate 11 0.6 13 0.3 6 0.2

4-terpineol 3 0.1

Methyl salicylate 1 0.1 3 0.1

Decanal 0.7 0.0

Cis-3-hexen-1-ol-iso-valerate

7 0.2 2 0.1

Thymol 1 0.0

Isobornyl acetate 29 0.7

Dodecene-1-ol 0.7 0.0

Hexene-ol-hexanoateisomer

1 0.1

Ni 4 0.2

Sesquiterpenes 320 16 4 0.1 103 4

Total VOC 2000 100 4500 100 2400 99


and compared those results with ambient concentrations and fluxes measured above the

canopy with a relaxed eddy accumulation (REA) system. They found that Citrus species

emit large amounts of the sesquiterpene β-caryophyllene during summer and the

oxygenated terpene, linalool during the flowering season (see also Arey et al., 1991a).

However, the comparison of the branch enclosure results with the above-canopy flux data

showed that both compounds have very short atmospheric lifetimes so that only small

fractions reach the atmospheric boundary layer. Consequently, d-limonene, emitted from

decomposing orange peels on/in the soil, was the dominant terpene emitted from the

canopy to the atmosphere. Further, these measurements showed fluxes of acetone and

acetaldehyde that were suggested to be due to heterogeneous chemical production of via

terpene ozononolysis on leaf surfaces (Ciccioli et al., 1999).

Recent work has shown that certain Mediterranean oaks emit terpenes, but not

isoprene, and the terpene emissions exhibit the light and temperature dependence

associated with isoprene (Kesselmeier et al., 1996; Ciccioli et al., 1997). This light-

dependent terpene emission may exist for a limited distribution of North American

vegetation types (Keiser, 1997; Kesselmeier et al., 1996), but the importance of light-

dependent terpene emissions is not certain. For locations where this emission occurs, it

may dominate the local BVOC flux since the chloroplast mechanism is often an order of

magnitude larger in emissions than other types of emissions (Guenther et al., 1999).

An early suggestion that terpene emissions increase with humidity or rainfall by

Lamb et al. (1985) has recently been confirmed by Schade et al. (1999), who

demonstrated relationships between humidity and terpene fluxes over a ponderosa pine

forest. Elevated emissions immediately following rainfall events were also observed.

Schade et al. (1999) proposed an algorithm for treating the humidity effects upon terpene

emissions. The humidity effect and the impact of rainfall events upon terpene emissions

are not addressed in BEIS3 at present.

Seasonal effects upon terpene emissions have not been established. Lerdau et al.

(1994) measured terpene emissions from ponderosa pine over the course of a growing

season and found that while photosynthesis rates sharply decreased in late summer,

terpene emissions exhibited no change. Pressley et al. (1998) measured terpene emissions

from Douglas fir and hemlock in an old growth forest using branchlet enclosure methods


and found no difference in emissions normalized to standard conditions (30 oC) during the

course of a growing season. These authors employed the Wind River Crane Research

Facility to access branches throughout the 60 m high canopy and also found no significant

differences in emissions as a function of height through the canopy. Preliminary analyses

of these data do suggest an effect of humidity and rainfall on elevated emissions in line

with the discussion above. Lerdau et al. (1995) have shown that terpene emissions from

Douglas fir are affected by nitrogen availability and that there is also a linear correlation

between monoterpene concentration in needle oil and monoterpene emissions.

Because there have been few canopy-scale flux studies of terpene emissions, there

appears to be very little information for direct evaluation of the BEIS models with respect

to terpene emissions at the canopy scale. Early studies by Arnts et al. (1978) and Lamb et

al. (1985) reported α-pinene fluxes from micrometeorological gradient methods over

loblolly pine and Douglas fir canopies, respectively. More recently, the work by Lamanna

and Goldstein (1999) over a ponderosa pine canopy and by Ciccioli et al. (1999) over an

orange grove provide canopy scale fluxes for comparison to BEIS type emission models.

The following statements summarize our current understanding of terpene

emissions from vegetation:

• Terpenes are emitted from all coniferous species and some deciduous species;

• Emission capacities for individual terpene compounds are not very well

specified, but it appears that a small number of terpenes dominate emissions;

• Rapid chemical losses of terpenes within or above canopies may contribute to

the appearance of just a few compounds in ambient samples;

• Terpene emissions follow an exponential relationship with temperature; the

temperature coefficient is reasonably well known (β = 0.09);

• Terpene emissions also appear to depend upon humidity and elevated

emissions have been observed following rainfall events; a model of the

humidity effect has been proposed, but not tested independently; no model

exists for treating rainfall effects;

• Herbivory appears to increase terpene emissions, but the impact of herbivory is

not known in terms of regional inventories;


• Evaluation of BEIS model terpene emissions has not been addressed in any

comprehensive study at the canopy or landscape scales.

2.3.3 Sesquiterpenes

Sesquiterpenes are semi-volatile compounds and thus have relatively low emission

capacities from vegetation. There is very little information on individual sesquiterpenes

emitted from vegetation (Isidorov et al., 1985). In the work by Helmig et al. (1999a),

sesquiterpenes were not identified individually in the branch enclosure samples because the

GC elution times were outside the linear programmed temperature range, no n-alkane

reference compounds were available, and detailed mass spectrometric reference data are

very limited. Thus, the sesquiterpenes were treated as a single compound class. At the

three U.S. sites, sesquiterpene emissions were identified for 17 tree species: eastern

hemlock, labrador tea, subalpine fir, aspen, big sagebrush, lodgepole pine, Gambal oak,

rabbit brush, salt bush, ironwood, post oak, black oak, white oak, speckled alder, black

cherry, red raspberry, and white spruce. When the estimated sesquiterpene emissions

were combined with landuse and vegetation biomass data to yield landscape fluxes, the

contribution of sesquiterpenes equaled 16%, 0.1%, and 4% of the total landscape flux at

the three U.S. sites: an Atlanta urban forest, a northern Wisconsin mixed deciduous forest,

and a Colorado mixed shrub oak woodland (Helmig et al., 1999b). It was assumed in this

case that sesquiterpenes follow the same temperature dependence as monoterpenes.

As indicated previously, Ciccioli et al. (1999) found substantial emissions of β-

caryphyollene from orange trees measured with branch enclosures, but very little flux from

the top of the canopy measured with REA systems.

We can summarize our understanding of sesquiterpene emissions in the following:

• Emission capacities are not known, but are less than for monoterpenes;

• Individual sesquiterpene emissions are difficult to determine and have not been

reported in the literature beyond one or two instances;

• Essentially all of the available information is based upon leaf or branch

enclosure measurements, no canopy-scale data are available;


• Regional estimates of emissions do not exist.

2.3.4 Oxygenated VOC

It is now well known that a wide variety of oxygenated VOC are emitted from

vegetation. These include MBO (2-methyl-3-buten-2-ol), the hexene family of

compounds, aldehydes, alcohols, ketones, and organic acids (see Table 2-1; Kesselmeier et

al., 1997; Kirstine et al., 1998; MacDonald and Fall, 1993; Winer et al., 1992; Bode et al.,

1997; Gabriel et al., 1999). While MBO is released via the chloroplast mechanism, most

of the oxygenated compounds are emitted via defense mechanisms. It appears that MBO

is released from pines with a light-dependent mechanism similar to isoprene (Goldan et al.,

1993; Harley et al., 1998). Lamanna and Goldstein (1999) employed a factor analysis and

found that isoprene and MBO occurred in the same factor group for data collected above

a ponderosa pine canopy, where isoprene was due to a small percentage of oak biomass in

the area. It should be noted that both isoprene and MBO occurred in greater abundance

than any individual monoterpene at this site, which is a reflection of the greater emission

rates associated with chloroplast releases compared to releases from leaf/needle tissue.

Generally, however, the magnitude and distribution of MBO emissions have not been well

documented. In the BEIS3 estimate, the assigned emission capacity ranges up to 60 µg g-

1 h-1, and the total estimated for North America equals 4.1 TgC annually, i.e., 5% of the

total BVOC emission rate.

The hexene family of compounds includes 2-hexenal, 3-hexenal, 3-hexenyl acetate,

hexanal, and hexenol. These compounds have antibiotic properties (Croft et al., 1993)

and can be emitted at relatively high rates from some vegetation (Helmig et al., 1999a;

Arey et al., 1991b; Kirstine et al., 1998). The release of these compounds is associated

with wounding and damage of cell membranes due to mechanical breakage (de Gouw et

al., 1999) or due to the presence of pathogens. This implies that herbivory will be an

important aspect of emissions, but quantitative estimates are difficult to determine.

Helmig et al. (1998) found elevated ambient concentrations of hexenyl acetate in a rural

area immediately after the passage of an intense storm, which they attributed to

mechanical damage to foliage. This type of elevated, intermittent emission will also be


very difficult to incorporate into regional models. Fall et al. (1999) employed fast

response (< 3 s) proton-transfer-mass spectrometry (PTR-MS) to monitor the evolution of

a family of hexene compounds immediately following wounding of aspen, beech, and

clover leaves. They found that the emission of (Z)-3-hexenal occurred within 1 to 2 s of

wounding, and that metabolites, including (E)-2-hexenal, hexenols, and hexenyl acetates

appeared as the parent compound disappeared. The emission was proportional to the

degree of wounding and was not dependent upon light. Emissions from aspen averaged

500 µg C g-1 of drying leaf biomass. Further work of this type is needed to develop

quantitative methods for incorporating the effects of wounding or harvesting into regional

emission inventories.

At the same time, it appears that emissions can also occur from vegetation where

wounding is not present, but the rates may be lower. Helmig et al. (1999a) found that the

hexene class of compounds were emitted at the highest rates (up to 25 µg g-1 h-1 for cis-3-

hexenyl acetate) from deciduous vegetation and often concurrently with isoprene

emissions. Lamanna and Goldstein (1999) reported ambient concentrations of hexenal (a

few ppt) above the ponderosa pine forest and found that hexenal occurred in the same

factor group as the monoterpenes (temperature dependent emissions). Winer et al. (1992)

found that the hexenal family was a dominant component of emissions from agricultural

crops in the San Joaquin Valley of California.

Other oxygenated BVOC released from vegetation include methanol,

acetaldehyde, acetone, and butanone. These appear to be released during cutting and

drying of vegetation (Fall, 1999; Kirstine et al., 1998; de Gouw et al., 1999). Thus,

significant emissions of these compounds may occur in rural agricultural areas or in

relation to suburban lawn mowing. Estimating the regional emissions due to cutting and

drying is very difficult due to the intermittent nature of the process. Emissions of these

compounds can also occur as plant matter decays over long periods (Warneke et al.,

1999). In BEIS3, the emission capacity for this mechanism is assigned values of 1 µg g-1

h-1 for methanol, 0.1 µg g-1 h-1 for acetone and the hexene family of compounds, 0.03 µg

g-1 h-1 for acetaldehyde and formaldehyde, and 0.01 µg g-1 h-1 for butanone. Total

emissions for these compounds are estimated to be approximately 1.9 TgC annually, i.e.,

2% of the total BVOC emission rate for North America.


To summarize our understanding of oxygenated BVOC emissions, the following

statements can be made:

• Oxygenated VOC emitted from vegetation cover a wide range of compounds,

including MBO, the hexene family of compounds, methanol, acetone,

acetaldehyde, formaldehyde, butenone, and low molecular weight organic

acids;

• MBO is emitted from pines in a light-dependent mechanism similar to isoprene;

• The hexene family of compounds is emitted primarily from deciduous

vegetation and agricultural crops via both temperature dependent and

wounding or stress mechanisms;

• Oxygenated VOC emissions are associated with vegetation cutting and drying

and also with the decay of plant matter;

• Regional emissions can be significant, but very difficult to determine due to the

intermittent nature associated with wounding, cutting or other stress events;

• Essentially no canopy-scale information exists and no evaluations at landscape

or larger scales have been attempted.

2.4 BVOC Emission Measurement Methods

As indicated above, emission data for BVOC can be obtained at the leaf scale

using leaf cuvette systems (Harley et al., 1996, 1997; Pressley et al, 1998), at the branch

scale using branch enclosure systems (Lerdau et al, 1994; Helmig et al., 1999a), and at the

canopy scale using micrometeorological methods, including tracer methods (Arnts et al.,

1982; Lamb et al., 1986), modified Bowen ratio gradient techniques (Knoerr and Lowry,

1981; Lamb et al., 1985; Fuentes et al., 1996, 1997; Pattey et al., 1999; Goldstein et al.,

1998), eddy covariance methods (Guenther and Hills, 1998), and relaxed eddy

accumulation (REA) methods (Lamb et al., 1996; Pattey et al., 1999; Ciccioli et al., 1999).

At landscape scales, tethered balloon or aircraft data can be used via mixed layer methods

to calculate fluxes (Davis et al., 1994; Guenther et al., 1996a,b). There are also very

recent reports of direct flux measurements from aircraft using REA systems (Davis et al.,


1996; Zhu et al., 1999). Details of many of these methods have been reviewed by Winer et

al. (1995), Fuentes et al. (1999b), and Guenther et al. (1999).

With the exception of the eddy covariance method, where a fast-response analyzer

is required, all of the methods listed above involve collection of an air sample and

subsequent analysis of the sample for the compounds of interest. Sample collection

methods generally involve whole air sampling into electropolished stainless steel canisters,

Teflon bags, or adsorbent cartridge sampling using multi-component adsorbents. Analyses

are typically achieved using high resolution gas chromatographic (GC) methods coupled

with either flame ionization detection (FID) or mass spectrometry (MS). Westberg and

Zimmerman (1993) have reviewed analytical methods for hydrocarbons. Helmig et al.

(1999a) and Ciccioli et al. (1999) are excellent examples of recent work using adsorbent

cartridges and GC/MS analytical methods. Nie et al. (1995) and Geron et al. (1997)

present details concerning an REA system based upon the use of adsorbent sampling

systems. Lindinger et al. (1998) have reported the development of a proton-transfer mass

spectrometer instrument which can provide part per trillion sensitivities with time

resolutions of less than one minute. This may become a very powerful analytical system

for application to BVOC emission studies.

2.4.1 Enclosure methods

Leaf cuvette systems, typically based upon the LICOR 6200 or 6400

photosynthesis systems, allow control of temperature, light, and humidity levels while

sampling individual leaves for BVOC emissions. This approach has been used to

determine emission capacities as a function of leaf location in the canopy by Harley et al.

(1996, 1997). For deciduous leaves, it is straightforward to use. For conifers, it is more

difficult to apply because of the difficulty in sampling individual needles (branch ends are

sometimes enclosed) and because of the critical need to avoid mechanical damage of the

needles during sampling. The latter effect is a problem with all enclosure systems. There

is no simple way to determine whether needle breakage occurs, except by careful visual

inspection, or by the appearance of an obvious outlier in the emission data. Juuti et al.


(1990) found that terpene emissions increased by factors of 10 to 50 due to rough

handling of branches for Monterey pine.

The advantage to the cuvette system is that there is no internal shading of the

leaves by other leaves so that the emission capacity obtained is a true measure of the

emission from the leaf in the controlled environment. Branch enclosures inherently include

internal shading and as a result can yield significantly lower emission capacities compared

to leaf cuvette measurements. Guenther et al. (1996b) assigned a factor of 75% to the

effect of internal shading in branch enclosure studies for isoprene emissions. As a result,

leaf cuvettes are the preferred method for determining emission capacities. The tradeoff in

cuvettes versus branch enclosures is that less biomass is enclosed in the cuvette so the

emission levels may be low and difficult to detect. However, this factor can be offset

somewhat by differences in the flow rate of sweep air through the cuvette versus the

enclosure. Because a branch enclosure system is less expensive and simpler to use,

enclosures are very useful for screening studies to obtain semi-quantitative emissions data

rapidly from a large number of vegetation types.

2.4.2 Micrometeorological canopy flux methods

Canopy flux methods have the inherent advantages of sampling a forest ecosystem

without disturbing the vegetation and that the flux measurement is a direct measure of the

contribution of the ecosystem to the atmospheric boundary layer. However, there are

uncertainties in associating the measured flux with the 'source footprint' upwind of the

tower that is contributing to the flux (Finn et al., 1996). This can cause difficulties in

comparing canopy flux measurements to canopy model predictions at sites with

heterogeneous distribution of species types (Lamb et al., 1996; Hall et al., 1997). There

can also be difficulties in comparing flux measurements to leaf level emissions for cases

where chemical or deposition loss in the canopy is significant (Ciccioli et al., 1999).

Nonetheless, canopy-scale flux methods provide a powerful tool for assessing emission

inventory models and for measuring the impact of ecosystems on the atmosphere. It is

significant with respect to the purpose of this review that the experience gained from


recent isoprene flux studies is now beginning to be applied to terpene and other compound

emissions.

The eddy covariance method is the most direct way to measure BVOC fluxes from

a forest:

,c,wF= (2-7)

where w’ is the fluctuation of vertical velocity measured typically measured with a sonic

anemometer, c’ is the fluctuation of BVOC concentration measured with a suitable fast

response continuous analyzer, and the , notation indicates a temporal average of the

product. However, the only fast response analyzer which has been applied for this

purpose is specific for isoprene. Guenther and Hills (1998) have given a good description

of its application. It should be noted that even with this analyzer, corrections for loss of

high frequency eddy motions are required. The corrections can be derived from

concurrent heat flux measurements as described by Guenther and Hills (1998), but the

result is an increased level of uncertainty in the measured fluxes. Application of this

method requires relatively sophisticated instrumentation and associated eddy flux

software. However, the method can be used to obtain continuous flux measurements over

long periods of time. For night time measurements (in the case of emissions other than

isoprene), the eddy flux system must be supplemented by a canopy profiling method to

account for nighttime canopy storage (Hollinger et al., 1994) in order to obtain full diurnal

sampling.

There is the possibility that new mass spectrometric methods may be suited for

eddy covariance applications (Lindinger et al., 1998). In this case it may be possible to

obtain eddy covariance flux measurements for a wide range of compounds including

terpenes and oxygenated VOC. However, the difficulty in this case may be relatively

weak fluxes and associated low concentrations. The instrumentation must be sufficiently

sensitive to detect the ambient concentration fluctuations for the selected species and there

must be sufficient resolution to detect differences in concentration on the order of 10% of

the mean concentration value.


REA has been demonstrated as a method that circumvents the fast response issue

in eddy covariance and also allows ultra-sensitive analytical systems to be employed for

detection of target compounds (Businger and Oncley, 1990; Oncley et al., 1993; Pattey et

al., 1993; Bowling et al., 1999).

F = bσw(Cup – Cdn) (2-8)

where b is an empirical coefficient which is weakly dependent upon stability, σw is the

standard deviation of vertical velocity fluctuations, Cup is the BVOC concentration

measured in a reservoir sampling updrafts, and Cdn is the BVOC concentration measured

in a reservoir sampling downdrafts.

REA systems have been deployed for measuring isoprene fluxes over mixed

deciduous canopies (Lamb et al., 1996), over boreal spruce forests (Pattey et al., 1999),

and over managed poplar plantations (Hopkins et al., 1999). The REA system is typically

a batch process requiring on-site operation which yields 30 min average fluxes. In the

studies cited above, REA samples were collected approximately 30 min out of each hour

to yield a relatively complete set of diurnal data for each sampling day. However, this is a

very labor intensive operation and is not very well suited for long-term automated

operations. More recently, Ciccioli et al. (1999) have reported successful operation of an

REA adsorbent cartridge sampling system applied to terpene and other VOC compounds

over an orange orchard. This is an excellent description of an advanced REA system

which could be used for a wide variety of BVOC over a wide range of ecosystems.

Application to different compounds and ecosystems will be restricted by instrument

sensitivity and resolution as noted above.

Micrometeorological gradient methods generally follow a modified Bowen ratio

approach where the gradients of both BVOC (dBOVC) and another scalar (dS, temperature,

CO2, or H2O) are measured above a canopy, the flux of the scalar (FS) is measured directly

using eddy covariance, and the flux of BVOC is estimated assuming similar transport and

diffusion mechanisms:

FBVOC = FS(dBVOC/dS) (2-9)


This method is more difficult to apply and has larger uncertainties compared to

either eddy covariance or REA methods due to the need to make measurements at two

heights (which compounds the uncertainty in the contributing footprint), and due to the

difficulty in resolving the concentration gradient of the BVOC. Goldstein et al. (1998)

and Lamanna and Goldstein (1999) have employed this method to measure isoprene flux

over a northeastern deciduous forest and ponderosa pine forest, respectively. Pattey et al.

(1999) found that the gradient method yielded significantly lower fluxes compared to the

REA method for isoprene fluxes over a spruce forest. Similar results were obtained for

isoprene fluxes over a mixed deciduous forest by Hall et al. (1997). It was suggested that

the position of the lower gradient sampling inlet was within the roughness sublayer so that

the assumptions of similarity theory were violated. However, increasing the height of the

lower inlet would have decreased the gradient significantly and made resolution of the

gradient extremely difficult.

2.4.3 Mixed layer (landscape scale) flux methods

Guenther and colleagues (1996a,b; Lamb et al., 1996; Hall et al., 1997; Isebrands

et al., 1999) have demonstrated the value of the experimental scale-up of emissions as a

basis for thorough evaluation of BEIS type emission inventories. This scale-up involves

leaf-level measurements of the emission capacities for dominant vegetation at a specific

site; canopy-scale flux measurements to confirm consistency among leaf emission capacity,

local biomass density, and canopy-scale fluxes, and finally, landscape-scale flux estimates

to demonstrate consistency of estimates over larger scales. The latter is obtained from

BVOC concentration profiles through the mixed layer from either tethered balloon

sampling or aircraft sampling. Generally, these mixed layer methods are only applied

during daytime, convective conditions. In this case, the scale of the flux estimate is

determined by the footprint associated with the vertical profile and mixing rates in the

mixed layer. It is typically estimated to be of the order of ten km during mid-day

convective conditions. A mixed layer gradient method (Davis et al., 1994) and a mass


balance method (Guenther et al, 1996b) have been employed for use with upper air BVOC

measurements.

In the mass balance method, the flux is estimated as

F = ziLC (2-10)

where zi is the height of the mixed layer, L is the loss rate of BVOC due to chemical

oxidation, and C is the mixed layer average concentration. While uncertainties in zi and C

are relatively small, uncertainties in L can be large (50%) since OH concentrations must

normally be estimated. Nonetheless, there is considerable benefit in making these

landscape-scale flux measurements since the results provide a basis for direct evaluation of

the BEIS type emission inventory at a location. This type of evaluation has been

completed for BEIS2 estimates of isoprene emissions at a number of locations (Guenther

et al., 1995b; Lamb et al., 1997; Isebrands et al., 1999) and the general result has shown a

relatively good level of consistency between the landscape flux estimate and the BEIS

isoprene emission estimate. For example, Greenberg et al. (1999) show flux estimates

which agree with BEIS2 predictions to within a factor of two at a number of different

locations. Similarly, Isebrands et al. (1999) report good agreement between tethered

balloon flux estimates and BEIS calculations for northern mixed hardwood forests.

A promising approach for making aircraft flux measurements over landscape scales

is the use of the REA method. Zhu et al. (1999) developed a REA canister sampling

system for aircraft use and measured isoprene fluxes over the boreal forests of Canada as

part of the NASA BOREAS project. In this application, flights were conducted at very

low levels (31 to 47 m) so the footprint extended between 270 to 1400 m upwind.

Measurements were made over separate black spruce, jack pine, and mature aspen forests.

Mean fluxes over spruce and aspen equaled 0.36 ± 0.21 µg m-2 s-1 and 0.92 ± 0.33 µg m-2

s-1. For comparison, Pattey et al. (1999) reported that mean tower based REA fluxes from

the same black spruce forest equaled 0.64 µg m-2 s-1, which converted to 0.33 µg m-2 s-1 at

standard temperatures of 20 oC. Gradient flux measurements over the aspen forest

equaled 0.64 µg m-2 s-1 at 20 oC and 1000 µmol m-2 s-1 PAR (Westberg et al., 1999b). The


relatively good agreement among the aircraft and tower based systems is further evidence

for the benefits of experimental scale-up of emissions.

To summarize the current capabilities with respect to emission measurement

methods:

• Leaf cuvette methods are well established and provide the best basis for

determining leaf emission capacities;

• Branch enclosure methods are widely used and provide a good way to screen

large numbers of species for emissions, but internal shading limits the utility of

branch enclosure data for establishing emission capacities;

• Tower based methods are becoming more widely used and provide a key way

to evaluate emission inventory models at a local site; evaluation at the canopy

scale requires accurate information regarding local biomass density, vegetation

distribution, and the spatial flux footprint;

• Experimental scale-up of emissions to the landscape scale using tethered

balloon or aircraft systems has been demonstrated as a powerful tool for

evaluation of emission inventory systems and it appears that current inventories

exhibit consistency within a factor of two at a variety of sites;

• There is a lack of data for terpenes, sesquiterpenes, and oxygenated VOC at

canopy and landscape scales, but advances in REA and analytical methods

provide the basis for collecting these data for emission model evaluation.

2.5 Reconciliation of BVOC Ambient Concentrations with Emission Inventories

The methods described above provide ways to link measured concentrations

directly to emissions at the canopy and landscape scales. There are additional ways to

evaluate emission estimates using measured ambient concentrations. Lewis et al. (1999)

describe the use of 14C isotope measurements (see also Larsen et al., 1998), analysis of

detailed VOC ambient measurements, and the use of BEIS2 emissions to reconcile

ambient observations with predicted emissions. The results from this work showed

consistency between the fraction of BVOC observed in the isotopic measurements, the


sum of BVOC relative to total observed VOC, and the predicted ratio of BVOC to

anthropogenic VOC emissions from inventories.

Emission inversion methods provide another tool for evaluating emission

inventories. These methods involve the use of a transport model with measured BVOC

concentrations to derive a best-fit emission estimate for the scenario of interest. At the

canopy scale, Gu and Fuentes (1999) have investigated the possibility of using vertical

profiles of isoprene concentration within the canopy to infer the vertical distribution and

source strength of isoprene. The method employs a combination of near field and far field

dispersion models to account for transport between source and receptor and then invokes

a regression analysis to determine the best-fit vertical profile of isoprene emissions. This

provides another tool for estimating emissions at a site. While it may be redundant for

isoprene, given the availability of both REA and eddy covariance methods, it may prove to

be quite valuable for obtaining emission estimates for other BVOC.

At the urban to regional scale, McRae and colleagues (1998; see also Tatang et al.,

1997) have employed advanced statistical uncertainty methods to investigate the accuracy

of anthropogenic emission inventories in Los Angeles basin. By using ambient

observations of pollutant concentrations with a detailed urban photochemical grid model,

they were able to show that the existing emission inventory required an adjustment

spatially and temporally in order to fit the observed pattern of concentrations. This type

of urban-scale emission inversion is needed for BVOC in order to investigate the

consistency of BEIS emissions with ambient concentrations. However, it must be noted

that using emission inversion to reconcile BVOC concentration and emissions inherently

includes all of the uncertainties associated with measurements and emissions as well as the

uncertainties associated with advection, diffusion, deposition, and, perhaps most

importantly, chemical processing in the atmosphere. Chang et al. (1996) completed an

emission inversion calculation using ambient surface measurements for isoprene in the

Atlanta urban area together with the Urban Airshed Model (UAM-IV) and the BEIS2

emission inventory for Atlanta. They found that isoprene emissions had to be adjusted

upward by a factor of 2 in order to match the model predictions with isoprene

observations. However, this adjustment changed to a factor of 1.2 to 1.5 when the

vertical profile of isoprene was taken into account. It should also be noted that the BEIS2


emission inventory was based upon a county scale of resolution while UAM-IV was

applied with a 4 x 4 km grid resolution.

In summer 1998, the U.S. EPA along with state agencies and university

researchers conducted an intensive isoprene sampling program to investigate isoprene in

the Ozarks of Missouri where the highest density of oak in the U.S. occurs. One objective

of the Ozarks Isoprene Experiment (OZIE) was to examine the consistency between

surface layer isoprene measurements and upper air balloon and aircraft isoprene

observations. In densely forested areas, such as the Ozarks or Atlanta, surface

measurements can strongly reflect very nearby sources and may not be representative of

regional emission inventories. Results from OZIE will be useful for determining the extent

to which surface layer measurements can be used for reconciliation of ambient data with

emission inventories.

We can summarize attempts to reconcile ambient observations with emission

inventories as follows:

• Reconciliation of ambient concentrations with emission inventories is an

essential part of assessing inventory accuracy, but there is no single method

which best accomplishes this reconciliation;

• Carbon isotopic sampling can provide an overall test of the fraction of biogenic

carbon in ambient air compared to the fraction in emission inventories;

• Ratios of specific compounds in ambient air and in emission inventories

provide a second test of inventory accuracy, but the use of this technique is

limited by the fact that emission inventories are typically lumped

representations of the actual emissions;

• Application of a mathematical emission inversion method with gridded air

quality models for isoprene indicate agreement to within approximately 50% is

possible when the vertical profile of isoprene is taken into account;

• Emission inversion methods based upon airshed air quality models inherently

include all of the uncertainties in the model associated with measurements,

emissions, advection, deposition, and chemistry.


2.6 Summary and Conclusions

Our state of understanding of BVOC emissions has advanced considerably in the

past decade. We have a relatively mature understanding of isoprene emissions and the

capability to develop relatively accurate regional inventories of isoprene emissions. For

terpenes, we have a relatively good understanding of emissions at the needle/leaf scale,

but, compared to isoprene, only a modest level of understanding of terpene emissions at

the canopy and larger scales. Details concerning the emissions of individual terpenes are

sparse for many vegetation types. Evaluations of terpene emission estimates are relatively

rare. For sesquiterpenes, we are aware of the emissions, but have only very sparse

estimates of emission rates, and essentially no information at canopy or larger scales. For

oxygenated BVOC, we have learned a great deal recently concerning emissions, types of

compounds emitted, and the mechanisms for the emissions. We are still lacking canopy

and larger scale verification studies for oxygenated compounds. The tools primarily

developed to measure isoprene fluxes are advancing to the point where they can be used

to obtain canopy scale fluxes of terpenes and other BVOC. Emission inversion methods

for reconciling ambient levels with emission inventories are still in development and have

not been widely applied to BVOC emissions. This is also true for other reconciliation

methods including isotopic analyses.


3. ATMOSPHERIC CHEMISTRY OF BIOGENIC VOC

3.1 Introduction

Compounds emitted by biogenic sources play an important role in the chemistry of

the troposphere, where they contribute to the formation of ozone (O3) and of secondary

organic aerosols (SOA). Motivated in part by new regulations, including the recently

issued U.S. ambient air quality standards for O3 and for fine particles (PM2.5), research on

biogenic volatile organic compounds (BVOC) continues to gain momentum, and this in

several areas including the identification of new compounds emitted by biogenic sources,

the measurements of their emission rates, the construction and update of regional and

global emission inventories, the determination of rate constants for the gas-phase reactions

of biogenic compounds, the identification of reaction products, the determination of

product formation yields, the elucidation of atmospheric oxidation mechanisms, and, for

those compounds whose oxidation leads to condensable species, the characterization of

aerosols including formation yields and molecular composition.

To estimate the contribution of biogenic emissions to O3 and PM2.5 air quality, it is

important to review critically the atmospheric chemistry of biogenic compounds and to

address the corresponding uncertainties and knowledge gaps. Multiphase chemistry, e.g.,

aqueous oxidation of VOC in cloud and fog droplets (e.g., Graedel and Goldberg, 1983;

Faust, 1994) contribute to the production of water-soluble organic compounds in the

atmosphere. Experimental information on atmospheric multiphase chemistry remain quite

scarce, and this review focuses on gas-phase processes. The information that needs to be

examined can be divided into five categories:

• kinetic data, i.e., rate constants for the gas-phase reactions that are important

in the atmosphere. These reactions include the reaction with the hydroxyl


radical (OH) for all compounds, the reactions with O3 and with the nitrate

radical (NO3) for unsaturated compounds, and photolysis for carbonyls.

• product data, i.e., the nature of the gas-phase reaction products and their

formation yields.

• reaction mechanisms that describe the formation of the products under

atmospheric conditions.

• aerosol formation, including molecular composition and aerosol yield.

• atmospheric oxidation of the “first-generation” products (kinetics, products,

mechanisms and aerosol formation), e.g., the oxidation of the carbonyls

pinonaldehyde and nopinone which are major oxidation products of α-pinene

and β-pinene, respectively.

Many volatile organic compounds have been identified in biogenic emissions.

While isoprene and terpenes have been recognized as major components of biogenic

emissions for many years (e.g., Went, 1960; Rasmussen, 1970, 1972; Khalil and

Rasmussen, 1992; Lamb et al., 1993), research carried out in the past several years has led

to the identification of many other organic compounds including sesquiterpenes, alcohols,

esters, aldehydes, ketones, and acids (Ohta, 1984; Isidorov et al., 1985; Nondek et al.,

1992; Koenig et al., 1995; Ciccioli et al., 1993, 1999; Kesselmeier et al., 1996, 1997;

Fruekilde et al., 1998; Hakola et al., 1998; Harley et al., 1998; Fukui and Dorskey, 1998;

Kirstine et al., 1998; Helmig et al., 1998a, 1998b, 1999a, 1999b; DeGouw et al., 1999;

Isebrands et al., 1999; Martin et al., 1999; Fall et al., 1999). Emissions of BVOC have

been recently reviewed by Guenther et al. (1999). A list of the compounds deemed to be

important (from literature data on emissions as of mid-1999) is given in Table 3-1. More

compounds are likely to be identified in the next several years as more sensitive


Table 3-1. Biogenic compounds listed according to chemical functionality.

Saturated Aliphatic:

alkanes: ethane CH3CH3

alcohols: methanol CH3OH

ethanol CH3CH2OH

aldehydes: formaldehyde HCHO

acetaldehyde CH3CHO

hexanal CH3(CH2)4CHO

ketones: acetone CH3C(O)CH3

2-butanone CH3CH2C(O)CH3

camphor (a)

carboxylic acids: formic acid HCOOH

acetic acid CH3COOH

esters: bornyl acetate (a)

ethers: cineole (a)

Aromatic:

p-cymene (a)

Unsaturated aliphatic:

hydrocarbons with one C=C bond:

alkenes: ethylene CH2=CH2

propene CH3CH=CH2

1-butene CH3CH2CH=CH2

cis-2-butene cis-CH3CH=CHCH3

trans-2-butene trans-CH3CH=CHCH3

terpenes: α-pinene (a)

β-pinene (a)

∆3-carene (a)

camphene (a)

sabinene (a)

α-thujene (a)

sesquiterpenes α-cedrene (a)

α-copaene (a)

longifolene (a)


Table 3-1. Biogenic compounds listed according to chemical functionality (continued).

Unsaturated aliphatic (continued):

Hydrocarbons with 2 C=C bonds:

dienes: isoprene CH2=CHC(CH3)=CH2

terpenes: d-limonene (a)

terpinolene (a)

α-terpinene (a)

γ-terpinene (a)

β-phellandrene (a)

sesquiterpenes β-caryophyllene (a)

Hydrocarbons with 3 C=C bonds:

terpenes: myrcene CH2=CHC(=CH2)CH2CH2CH=C(CH3)2 (a)

ocimene CH2=CHC(CH3)=CHCH2CH=C(CH3)2 (a)

sesquiterpenes α-humulene (a)

(a) structures are shown in Appendix A.

Unsaturated oxygenates:

alcohols: 2-methyl-3-buten-2-ol CH2=CHC(CH3)2OHcis-3-hexen-1-ol CH3CH2CH=CHCH2CH2OHlinalool (CH3)2C=CHCH2CH2C(OH)

(CH3)CH=CH2

esters: cis-3-hexenyl acetate CH3C(O)OCH2CH2CH=CHCH2

CH3

trans-2-hexenyl acetate CH3C(O)OCH2CH=CHCH2CH2

CH3

aldehydes: cis-3-hexenal CH3CH2CH=CHCH2CHO

trans-2-hexenal CH3CH2CH2CH=CHCHO

ketones: piperitone (a)

6-methyl-5-hepten-2-one CH3C(O)CH2CH2CH=C(CH3)2


analytical methods and more sophisticated emission measurement protocols become

available.

Table 3-1 includes 47 organic compounds, which are listed according to chemical

functionality: thirteen saturated aliphatics, one aromatic (the terpene p-cymene), and

thirty three unsaturated aliphatics. The chemical structures of these compounds are given

in Table 3-1 or in Appendix A. The saturated aliphatics include one alkane (ethane), two

alcohols (methanol and ethanol), one ether (cineole), one ester (bornyl acetate), two

carboxylic acids (formic acid and acetic acid), and six carbonyls, i.e., three aldehydes

(formaldehyde, acetaldehyde, and hexanal) and three ketones (acetone, 2-butanone, and

camphor). The unsaturated aliphatics also include a variety of functional groups: three

alcohols (2-methyl-3-buten-2-ol, cis-3-hexen-1-ol and linalool), two esters (cis-3-hexenyl

acetate and trans-2-hexenyl acetate), two aldehydes (cis-3-hexenal and trans-2-hexenal),

two ketones (piperitone and 6-methyl-5-hepten-2-one), and twenty four unsaturated

hydrocarbons including 5 alkenes, one diene (isoprene), 13 terpenes (C10), and 5

sesquiterpenes (C15). The 24 hydrocarbons are listed in Table 3-1 according to the

number of unsaturated carbon-carbon bonds they contain. The first group contains one

C=C bond: the alkenes ethylene, propene, 1-butene, cis-2-butene, and trans-2-butene; the

terpenes α-pinene, β-pinene, ∆3-carene, camphene, sabinene, and α-thujene; and the

sesquiterpenes α-cedrene, α-copaene, and longifolene. The second group contains two

C=C bonds: isoprene, the terpenes d-limonene, terpinolene, α-terpinene, γ-terpinene, and

β-phellandrene; and the sesquiterpene β-caryophyllene. The last group contains three

C=C bonds: the terpenes myrcene and ocimene and the sesquiterpene α-humulene.

To review the atmospheric chemistry of the 47 organic compounds listed in Table

3-1, we have organized this report into four major sections that discuss kinetic data

(reaction rate constants and atmospheric lifetimes), first-generation reaction products (gas

phase and aerosol products identified and their formation yields), atmospheric oxidation

of the first-generation products (kinetics and second-generation products), and reaction


mechanisms. Knowledge gaps and recommendations for future research are discussed in

Section 5.

3.2 Kinetic Data

3.2.1 Reaction rate constants

We summarize in this section the reaction rate constants that are relevant to the

atmospheric oxidation of the compounds listed in Table 3-1. For saturated compounds,

reactions with O3 and with the NO3 radical are too slow to be important, and atmospheric

removal is initiated by reactions with the OH radical. Rate constants for the OH-

saturated compounds reactions are listed in Table 3-2. For the aromatic compound p-

cymene, reaction with O3 is negligibly slow, reaction with NO3 may constitute a minor

removal process (Bolzacchini et al., 1999) and the major atmospheric removal process is

via reaction with OH. For unsaturated compounds, atmospheric oxidation may involve

three reactions, i.e., with OH, with O3 and with NO3. Rate constants for the reactions of

unsaturated compounds with OH, O3 and NO3 are listed in Tables 3-3, 3-4, and 3-5,

respectively.

3.2.2 Estimated rate constants

The data in Tables 3-2 to 3-5 indicate that kinetic information is available for all

but eight of the compounds listed in Table 3-1. Rate constants for the eight compounds

(4 saturated and 4 unsaturated) for which no data are available have been estimated as are

described below.

For saturated compounds, no kinetic data are available for the reactions of OH

with hexanal, camphor, cineole, and p-cymene. For these compounds, the rate constants

given in Table 3-2 have been estimated from structure-reactivity considerations as

follows:


Table 3-2. OH reaction rate constants for p-cymene and saturated aliphatic

compounds.

Compound Reaction rate constant(a,b)

ethane 0.254(c)

formic acid 0.45

acetic acid 0.8

acetone 0.219

2-butanone 1.15

methanol 0.944

ethanol 3.27

formaldehyde 9.37

acetaldehyde 15.8

hexanal 33(d)

camphor 14(d)

cineole 20(d)

bornyl acetate 13.9(e)

p-cymene 11(d,f)

(a) at or near 298 K and 1 atm of air. Units: 10-12 cm3 molecule-1 s-1.

(b) from the review of Atkinson (1994) unless otherwise indicated.

(c) Atkinson, 1997a.

(d) estimated, see text.

(e) Coeur et. al., 1998.

(f) p-cymene also reacts with NO3, k = 10.0 ± 0.15 x 10-16 cm3 molecule-1 s-1 (Bolzacchini et al., 1999).


Table 3-3. OH reaction rate constants for unsaturated aliphatic compounds.

Compound Reaction rate Compound Reaction rate

constant(a, b) Constant(a, b)

ethylene 8.52 myrcene 215

propene 26.3 ocimene 252

1-butene 31.4 α-cedrene 67

cis-2-butene 56.4 α-copaene 90

trans-2-butene 64.0 longifolene 47

isoprene 101 β-caryophyllene 197

α-pinene 53.7 α-humulene 293

β-pinene 78.9 2-methyl-3-buten-2-ol 65 ± 6(d)

69 ± 10(e)

∆3-carene 88 cis-3-hexen-1-ol 108 ± 22(f)

sabinene 117 linalool 159 ± 40(f)

α-thujene 71(c) trans-2-hexenyl acetate 29(c)

camphene 53 cis-3-hexenyl acetate 78.4 ± 16.4(f)

d-limonene 171 trans-2-hexenal 44.1 ± 9.4(f)

terpinolene 225 cis-3-hexenal 50(c)

α-terpinene 363 piperitone 50(c)

γ-terpinene 177 6-methyl-5-hepten-2-one

157 ± 39(g)

β-phellandrene 168

(a) at or near 298 K and 1 atm of air. Units: 10-12 cm3 molecule-1 s-1.

(b) from the review of Atkinson (1997a) unless otherwise indicated.

(c) estimated, see text.

(d) Rudich et al., 1995.

(e) Ferronato et al., 1998.

(f) Atkinson et al., 1995a.

(g) Smith et al., 1996.


Table 3-4. Ozone reaction rate constants for unsaturated aliphatic compounds.


constant(a,b) constant(a, b)

ethylene 1.59 myrcene 470

propene 10.1 ocimene 540

1-butene 9.64 α-cedrene 28

cis-2-butene 125 α-copaene 160

trans-2-butene 190 longifolene < 0.5

isoprene 12.8 β-caryophyllene 11,600

α-pinene 86.6 α-humulene 11,700

β-pinene 15 2-methyl-3-buten-2-ol 10.0 ± 0.3(e)

12.2 ± 1.3(c)

cis-3-hexen-1-ol 64 ±17(f)

∆3-carene 37 105 ± 7(g)

sabinene 86 linalool 430 ± 160(f)

> 269 ± 68(h)

α-thujene 62(d) 310(i)

camphene 0.90 trans-2-hexenyl acetate 21.8 ± 2.8(j)

d-limonene 200 cis-3-hexenyl acetate 54.14(f)

59.0 ± 8.7(h)

terpinolene 1,880trans-2-hexenal 2.0 ± 1.0(f)

α-terpinene 21,100 1.28 ± 0.28(j)

γ-terpinene 140 cis-3-hexenal 120(d)

β-phellandrene 47 piperitone 60(d)

6-methyl-5-hepten-2-one 390 ± 150(k)

394 ± 40(j)

(a) at or near 298 K and 1 atm of air. Units: 10-18 cm3 molecule-1 s-1.(b) from the review of Atkinson (1997a) unless otherwise indicated.(c) Grosjean et al., 1993c.(d) estimated, see text.(e) Grosjean and Grosjean 1994.(f) Atkinson et al., 1995a.(g) Grosjean et al., 1993a.(h) Grosjean and Grosjean, 1998.(i) Calogirou et al., 1995.(j) Grosjean et al., 1996b.(k) Smith et al., 1996.


Table 3-5. NO3 reaction rate constants for unsaturated aliphatic compounds.


constant(a,b) constant(a, b)

ethylene 2.05 (-16) β-phellandrene 8.0 (-12)

propene 9.49 (-15) myrcene 1.1 (-11)

1-butene 1.35 (-14) ocimene 2.2 (-11)

cis-2-butene 3.50 (-13) α-cedrene 8.2 (-12)

trans-2-butene 3.90 (-13) α-copaene 1.6 (-11)

isoprene 6.78 (-13) longifolene 3.5 (-11)

α-pinene 6.16 (-12) β-caryophyllene 1.9 (-11)5.9 ± 0.8 (-12)(c)

α-humulene 3.5 (-11)β-pinene 2.51 (-12)

2.1 ± 0.4 (-12)(c) 2-methyl-3-buten-2-ol 1.2 (-14)(e)

2.1 (-14)(f)

∆3-carene 9.1 (-12)cis-3-hexen-1-ol 2.7 ± 0.8 (-13)(g)

sabinene 1.0 (-11)linalool 1.12 ± 0.40 (-11)(h)

α-thujene 7.6 (-12)(d)

trans-2-hexenyl acetate 1 (-13)(d)

camphene 6.6 (-13)6.2 ± 2.1 (-13)(c) cis-3-hexenyl acetate 2.46 ± 0.75 (-13)(g)

d-limonene 1.22 (-11) trans-2-hexenal 1.21 ± 0.44 (-14)(g)

terpinolene 9.7 (-11) cis-3-hexenal 4 (-13)(d)

α-terpinene 1.4 (-10) piperitone 4 (-12)(d)

γ-terpinene 2.9 (-11) 6-methyl-5-hepten-2-one 7.5 ± 3.0 (-12)(h)

394 ± 40(j)

(a) at or near 298 K and 1 atm of air. Units: cm3 molecule-1 s-1; read 2.0 (-16) as 2.0 x 10-16.

(b) from the review of Atkinson (1997a) unless otherwise indicated.

(c) Martinez et al., 1998.

(d) estimated, see text.

(e) Rudich et al., 1996.

(f) Hallquist et al., 1996.

(g) Atkinson et al., 1995a.

(h) Smith et al., 1996.


• hexanal: extrapolation of data for the homologous series of aldehydes

propanal, butanal, and pentanal (Atkinson, 1990).

• camphor and cineole: examination of data for structurally similar bicyclic

compounds including the hydrocarbons bicyclo [2.2.1] heptane, bicyclo

[2.2.2] octane, and 2,6,6-trimethylbicyclo [3.1.1] heptane (Atkinson, 1997a),

the ketones 6,6-dimethylbicyclo [3.1.1] heptan-2-one, and 3,3-

dimethylbicyclo [2.2.1]-heptan-2-one (Atkinson, 1994), and the bicyclic ester

bornyl acetate (Coeur et al., 1998), together with considerations of the

electronic effects of the keto and ether functional groups on reactivity towards

OH.

• p-cymene (1-methyl-4-isopropyl benzene): data for p-ethyltoluene multiplied

by the ratio of the rate constants of isopropylbenzene and ethyl benzene (the

rate constants for these three aromatic compounds are obtained from the

review of Atkinson, 1990).

For unsaturated compounds, which are the most important compounds with

respect to O3 and aerosol formation, no kinetic data are available for the reactions of OH,

O3 and NO3 with α-thujene, cis-3-hexenal, and piperitone and for the reactions of OH

and NO3 with trans-2-hexenyl acetate. For the terpene α-thujene, we have assumed that

the OH, O3 and NO3 reaction rate constants are intermediate between those of the

structurally similar terpenes α-pinene and ∆3-carene. For the carbonyls cis-3-hexenal and

piperitone and for the ester trans-2-hexenyl acetate, we have estimated reaction rate

constants from data for close structural homologues and structure-reactivity

considerations as follows:

• cis-3-hexenal (OH, O3, and NO3 reactions): data for the non-conjugated

unsaturated aldehyde cis-4-heptenal (Grosjean and Grosjean, 1999) and for


the alkenes cis-3-hexene, cis-2-pentene, and cis-2-butene (as reviewed by

Atkinson, 1997a).

• piperitone (OH, O3 and NO3 reactions): data for other α, β-unsaturated

ketones including 4-hexen-3-one (Grosjean and Grosjean, 1999) and for cyclic

unsaturated ketones including 4-acetyl-1-methyl cyclohexene (Atkinson and

Aschmann, 1993).

• trans-2-hexenyl acetate (OH and NO3 reactions): data for unsaturated esters

(Grosjean and Grosjean, 1998) including cis-3-hexenyl acetate (Atkinson et

al., 1995a) and comparison of the O3 reaction rate constants of the two non-

conjugated unsaturated esters cis-3-hexenyl acetate (Atkinson et al., 1995a,

Grosjean and Grosjean, 1998) and trans-2-hexenyl acetate (Grosjean et al.,

1996b).

3.2.3 Reactivity considerations and atmospheric lifetimes

The reaction rate constants given in Tables 3-2 to 3-5 are summarized in Table 3-

6. All compounds listed in Table 3-1 are removed from the atmosphere via their reaction

with OH:

- d [VOC]/dt = kOH [OH] [VOC] (3-1)

where [OH] is the concentration of OH, [VOC] is the concentration of the volatile

organic compound, and kOH is the OH-VOC reaction rate constant. The data in Table 3-6

indicate that OH reaction rate constants increase by a factor of ca. 150 for saturated

compounds (from acetone to hexanal) and by a factor of ca. 40 for unsaturated

compounds (from ethylene to α-terpinene). Unsaturated compounds are also removed

from the atmosphere via their reactions with O3 and with NO3:

- d [VOC]/dt = [VOC] (kOH [OH] + kO3 [O3] + kNO3 [NO3] ) (3-2)


Table 3-6. Range of reaction rate constants and atmospheric half-lives.

Range of reaction rate constants(a)

1012 x kOH 1018 x kO3kNO3

(b)

Saturated compounds 0.22 - 33(c) — —

Unsaturated compounds

alkenes 8.5 - 64(d) 1.6 - 190(d) 2 (-16) - 4 (-13)(d)

isoprene 101 12.8 6.8 (-13)

terpenes 53 - 363(e) 0.9 - 21,100(e) 6.2 (-13) - 1.4 (-10)(e)

sesquiterpenes 47 - 293(f) < 0.5 - 11,700(f) 8.2 (-12) - 3.5 (-11)(g)

oxygenates 29 - 160(h) 2 - 430(i) 1.2 (-14) - 1.1 (-11)(i)

(a) units: cm3 molecule-1 s-1, from Tables 3-2 to 3-5.

(b) read 2 (-16) as k = 2 x 10-16.

(c) from acetone to hexanal.

(d) from ethylene to trans-2-butene.

(e) from camphene to α-terpinene.

(f) from longifolene to α-humulene.

(g) from α-cedrene to α-humulene.

(h) from trans-2-hexenyl acetate to linalool.

(i) from trans-2-hexenal to linalool.


Table 3-6. Range of reaction rate constants and atmospheric half-lives (continued).

Compound Atmospheric half-lives, hours (unless otherwise indicated)

OH = 1.0 x 106

molecule cm-3 O3 = 30 ppb

acetone 36 days —

hexanal 5.8 —

ethylene 22.6 6.6 days

trans-2-butene 3.0 1.3

isoprene 1.9 20

camphene 3.6 11.8 days

α-terpinene 0.53 0.01

longifolene 4.1 ≥ 21 days

α-humulene 0.65 0.02

trans-2-hexenyl acetate 6.6 11.6

trans-2-hexenal 4.3 5.3 days

linalool 1.2 0.6


where [OH], [O3], and [NO3] are the concentrations of OH, O3, and NO3 and kOH, kO3,

and kNO3 are the VOC-OH, VOC-O3 and VOC-NO3 reaction rate constants. The data in

Table 3-6 indicate that O3 and NO3 reaction rate constants for unsaturated compounds

span ca. 5 orders of magnitude (from longifolene to α-terpinene for O3 reactions, and

from ethylene to α-terpinene for NO3 reactions), vs. only a factor of ca. 40 for OH

reaction rate constants. The relative importance of OH, O3, and NO3 with respect to

removal of a given unsaturated compound may vary substantially with actual

concentrations of OH, O3, and NO3. As an example of atmospheric lifetimes, we

calculate the half-lives of several compounds for a clean atmosphere daytime scenario,

i.e., we set [OH] = 1.0 x 106 molecule cm-3, [O3] = 7 x 1011 molecule cm-3 (30 ppb) and

[NO3] = 0. Atmospheric half-lives due to removal by reactions with OH and O3 are given

by:

t1/2, OH = ln2/(kOH [OH] ) (3-3a)

and:

t1/2, O3 = ln2/(kO3 [O3] ) (3-3b)

The atmospheric half-lives thus calculated are listed in Table 3-6. For the specific

concentrations of OH, O3, and NO3 used in our example, half-lives range from ca. 6

hours (hexanal) to 36 days (acetone) for saturated compounds; half-lives of unsaturated

compounds range from ca. 30 minutes (α-terpinene) to ca. 1 day (ethylene) for the

reaction with OH and from ca. 7 minutes (α-terpinene) to ca. 12 days (camphene) for the

reaction with O3. Several compounds have very short half-lives, e.g., the terpene α-

terpinene, the sesquiterpene α-humulene and the unsaturated alcohol linalool (also see

kinetic data for other reactive compounds in Tables 3-2 to 3-5). For these compounds,


emission fluxes from biogenic sources must be sufficiently large to offset rapid chemical

removal. The half-lives given in Table 3-6 also indicate that reaction with O3 may often

be the dominant removal process for the more reactive compounds. Removal of

unsaturated compounds by reaction with O3 results in the formation of carboxylic acids

and SOAs, see Sections 3.3.5 and 3.5.4, respectively.

3.3 First-Generation Reaction Products

Reaction products that have been identified in the laboratory under conditions

relevant to the atmosphere are listed in Table 3-7 for the reaction of OH with p-cymene

and with saturated compounds, and, for unsaturated compounds, in Table 3-8 for the

reaction with OH, in Table 3-9 for the reaction with O3 and in Table 3-10 for the reaction

with NO3. Formation yields of the major products are included in Tables 3-7 to 3-10

when available.

The reaction of O3 with alkenes and other unsaturated compounds leads to

products that include OH (e.g., Atkinson, 1997a, Donahue et al., 1998). Formation yields

of OH in the reaction of O3 with unsaturated compounds have been measured and are

listed in Table 3-11. Since OH reacts rapidly with unsaturated compounds, product

studies of the reaction of O3 with unsaturated compounds must be carried out in the

presence of a scavenger for OH. The formation yields given in Table 3-9 have been

measured in studies carried out with sufficient cyclohexane, methylcyclohexane or 2-

propanol added to scavenge most (≥ 90%) of the OH radical.

Examination of the data in Tables 3-7 to 3-11 indicates that, overall, less

information is available for reaction products than for reaction rate constants. The

amount of information on reaction products varies widely among compounds: detailed

information is available for isoprene and α-pinene, but there are no data for several

compounds. A summary of literature data is given in this section for each category of

compounds and each relevant reaction, i.e., the reaction of OH with saturated compounds

and the reactions of OH, O3, and NO3 with unsaturated compounds. Also included in this

section is a discussion of data for SOAs with focus on aerosol molecular composition.


Table 3-7. Products of the reaction of OH with saturated compounds.

Compound Reaction products

ethane(a) acetaldehyde(a)

methanol(a) formaldehyde(a)

ethanol(a) acetaldehyde(a,b)

formaldehyde(a) CO

acetaldehyde (a) formaldehyde(a), PAN(a)

hexanal pentanal, CH3(CH2)4C(O)OONO2(c)

acetone(a) formaldehyde(a), PAN(a)

2-butanone(a) acetaldehyde(a), PAN(a)

formic acid CO2

acetic acid no data

camphor no data

bornyl acetate no data

cineole no data

p-cymene no data

(a) the atmospheric oxidation of this compound has been studied in detail and the corresponding reaction

mechanism is included in computer kinetic models (e.g., Carter 1990, Atkinson 1994, 1997a, and

references therein).

(b) possibly also hydroxyacetaldehyde, see text.

(c) also lower MW products including C1-C4 aldehydes and C2-C5 peroxyacylnitrates (Grosjean et

al.,1996a), see text.


Table 3-8. Products of the reaction of OH with unsaturated compounds.

Compound Product Formation yield Reference

ethylene formaldehyde 1.56 Niki et al., 1981hydroxyacetaldehyde 0.22 Niki et al., 1981

propene formaldehyde 0.86 Niki et al., 1978acetaldehyde 0.98 Niki et al., 1978

1-butene propanal 0.94 ± 0.12 Atkinson et al., 1995b

cis-2-butene acetaldehyde 1.85, 1.58 Tuazon et al., 1998hydroxynitrate(a) 0.06 Tuazon et al., 1998

trans-2-butene no data no data

isoprene formaldehyde 0.60 ± 0.10 Carter and Atkinson, 1996methacrolein 0.23 Carter and Atkinson, 1996methylvinyl ketone 0.32 Carter and Atkinson, 19963-methyl furan 0.045 Carter and Atkinson, 1996alkyl nitrates 0.044(b) Chen et al., 1998C4 and C5 unsaturated +(c) Kwok et al., 1995hydroxycarbonyls


Table 3-8. Products of the reaction of OH with unsaturated compounds (continued).


α-pinene pinonaldehyde 0.56 ± 0.04 Hatakeyama et al., 19890.28 ± 0.05 Hakola et al., 1994

+ Aschmann et al., 1998+(d) Grosjean et al., 1992

0.31 ± 0.15 Vinckier et al., 1998

0.87 ± 0.20 Nozière et al., 1999a (with NOx)

0.37 ± 0.07 Nozière et al., 1999a (no NOx)

acetone 0.11 ± 0.03 Aschmann et al., 1998+(d) Grosjean et al., 1992

0.18 ± 0.02 Vinckier et al., 1998

0.09 ± 0.06 Nozière et al., 1999a (with NOx)

0.07 ± 0.02 Nozière et al., 1999a (no NOx)formaldehyde 0.23 ± 0.09 Nozière et al., 1999a (with NOx)

0.08 ± 0.01 Nozière et al., 1999a (no NOx)+(d) Grosjean et al., 1992

organic nitrates 0.17 ± 0.08 Nozière et al., 1999a

dihydroxycarbonyls, MW = 184 + Aschmann et al., 1998hydroxy nitrates, MW = 215 + Aschmann et al., 1998dihydroxy nitrates, MW = 231 + Aschmann et al., 1998

β-pinene formaldehyde +(d) Grosjean et al., 19920.54 ± 0.05 Hatakeyama et al., 1989

nopinone 0.79 ± 0.08 Hatakeyama et al., 19890.27 ± 0.04 Hakola et al., 1994

+ Aschmann et al., 1998+(d) Grosjean et al., 1992




acetone +(d) Grosjean et al., 19920.08 ± 0.02 Aschmann et al., 1998

dihydroxycarbonyls, MW = 184 + Aschmann et al., 1998hydroxynitrates, MW = 215 + Aschmann et al., 1998dihydroxynitrates, MW = 231 + Aschmann et al., 1998

∆3-carene caronaldehyde 0.34 ± 0.08 Hakola et al., 1994

acetone 0.15 ± 0.03 Reissell et al., 1999

sabinene sabinaketone 0.17 ± 0.03 Hakola et al., 1994acetone 0.19 ± 0.03 Reissell et al., 1999

α-thujene no data

camphene camphelinone not detected, <0.02 Hakola et al., 1994acetone 0.39 ± 0.05 Reissell et al., 1999

d-limonene 4-acetyl-1-methyl-cyclohexene 0.20 ± 0.03 Hakola et al., 1994 + (d) Grosjean et al., 1992

endolim 0.29 ± 0.06 Hakola et al., 1994acetone not detected, < 0.03 Reissell et al., 1999

terpinolene 4-methyl-3-cyclohexen-1-one 0.26 ± 0.06 Hakola et al., 1994acetone 0.32 Reissell et al., 19993-propenyl-6-oxoheptanal 0.08 ± 0.02 Hakola et al., 1994




α-terpinene acetone ca. 0.10 Reissell et al., 1999

γ-terpinene acetone 0.10 ± 0.03 Reissell et al., 1999

β-phellandrene 4-isopropyl-2-cyclohexen-1-one 0.29 ± 0.07 Hakola et al., 1994

myrcene acetone 0.36 Reissell et al., 1999

ocimene no products were identified Atkinson, 1997aacetone 0.18 Reissell et al., 1999

α-cedrene no data

α-copaene no data

longifolene no data

β-caryophyllene formaldehyde + Grosjean et al., 1993c

unsaturated C14 ketone, MW = 206 +(e) Grosjean et al., 1993cglyoxal + Grosjean et al., 1993cmethylglyoxal + Grosjean et al., 1993c3 unidentified carbonyls Grosjean et al., 1993c

α-humulene no data




2-methyl-3-buten-2-ol formaldehyde 0.35 ± 0.04 Ferronato et al., 19980.09 ± 0.03 Fantechi et al., 19980.29 ± 0.03 Alvarado et al., 1999

2-hydroxy-2-methyl propanal 0.19 ± 0.07 Alvarado et al., 1999hydroxyacetaldehyde 0.50 ± 0.25 Ferronato et al., 1998

0.28 ± 0.03 Fantechi et al., 19980.61 ± 0.09 Alvarado et al., 1999

acetone 0.52 ± 0.05 Ferronato et al., 19980.141 ± 0.002 Fantechi et al., 1998

0.58 ± 0.04 Alvarado et al., 1999formic acid + Fantechi et al., 1998organic nitrates 0.05 ± 0.02 Alvarado et al., 1999CO and CO2 + Fantechi et al., 1998

cis-3-hexen-1-ol propanal 0.75 ± 0.07 Aschmann et al., 19973-hydroxypropanal 0.48 Aschmann et al., 1997hydroxynitrate, MW = 179 + Aschmann et al., 1997dihydroxycarbonyl, MW = 132 + Aschmann et al., 1997

linalool formaldehyde +(c,f) Calogirou et al., 1995acetaldehyde +(f) Calogirou et al., 1995acetone +(f) Calogirou et al., 1995

0.505 ± 0.047 Shu et al., 19976-methyl-5-hepten-2-one 0.068 ± 0.006 Shu et al., 19974-hydroxy-4-methyl-5-hexen-1-al(g) 0.46 ± 0.11 Shu et al., 1997

+(f) Calogirou et al., 1995




trans-2-hexenyl acetate no data

cis-3-hexenyl acetate no data

trans-2-hexenal no data

cis-3-hexenal no data

piperitone no data

6-methyl-5-hepten-2-one 2-oxopentanal(h) 0.59 ± 0.13 Smith et al., 1996acetone 0.71 ± 0.05 Smith et al., 1996

(a) CH3CH(OH)CH(ONO2)CH3.

(b) sum of 7 compounds.

(c) + : product identified, yield not reported.

(d) in terpene-NOx-sunlight experiments, products identified before the NO/NO2 crossover (mostly OH chemistry, little or no O3 present).

(e) tentative identification.

(f) in linalool-NOx-sunlight experiments.

(g) CH2=CHC(CH3)(OH)CH2CH2CHO.

(h) CH3C(O)CH2CH2CHO.


Table 3-9. Products of the reaction of ozone with unsaturated compounds.

Compound Product Formation yield(a) Reference

ethylene formaldehyde 1.06 ± 0.07 Grosjean et al., 1996c0.99 ± 0.06 Grosjean and Grosjean, 1996

propene formaldehyde 0.780 ± 0.015 Grosjean et al., 1996c0.645 ± 0.048 Tuazon et al., 1997

acetaldehyde 0.520 ± 0.026 Grosjean et al., 1996c0.446 ± 0.092 Tuazon et al., 1997

methanol 0.055 ± 0.007 Tuazon et al., 1997ketene 0.036 ± 0.008 Tuazon et al., 1997CO 0.276 ± 0.031 Tuazon et al., 1997CO2 0.258 ± 0.018 Tuazon et al., 1997CH4 0.096 ± 0.010 Tuazon et al., 1997glyoxal 0.030 ± 0.005 Tuazon et al., 1997

1-butene formaldehyde 0.630 ± 0.031 Grosjean et al., 1996cpropanal 0.350 ± 0.018 Grosjean et al., 1996cformic acid 0.16 ± 0.04 Grosjean et al., 1994a

cis-2-butene formaldehyde 0.161 ± 0.030 Tuazon et al., 1997acetaldehyde 1.19 ± 0.14 Tuazon et al., 1997

1.08 ± 0.08 Tuazon et al., 1997methanol 0.098 ± 0.018 Tuazon et al., 1997ketene 0.74 ± 0.019 Tuazon et al., 1997CO 0.244 ± 0.034 Tuazon et al., 1997CO2 0.36 ± 0.10 Tuazon et al., 1997CH4 0.190 ± 0.025 Tuazon et al., 1997glyoxal 0.065 ± 0.012 Tuazon et al., 1997


Table 3-9. Products of the reaction of ozone with unsaturated compounds (continued).


trans-2-butene formaldehyde 0.168 ± 0.015 Tuazon et al., 1997acetaldehyde 1.14 ± 0.14 Tuazon et al., 1997

1.09 ± 0.09 Tuazon et al., 1997

trans-2-butene (continued) methanol 0.069 ± 0.008 Tuazon et al., 1997ketene 0.045 ± 0.009 Tuazon et al., 1997CO 0.217 ± 0.026 Tuazon et al., 1997CO2 0.229 ± 0.025 Tuazon et al., 1997CH4 0.113 ± 0.018 Tuazon et al., 1997glyoxal 0.099 ± 0.014 Tuazon et al., 1997

isoprene formaldehyde 0.90 ± 0.04 Grosjean et al., 1993fmethacrolein 0.44 Grosjean et al., 1993f

0.39 ± 0.03 Aschmann and Atkinson, 1994methylvinyl ketone 0.17 Grosjean et al., 1993f

0.16 ± 0.01 Aschmann and Atkinson, 1994methacrylic acid +(b) Chien et al., 1998acrylic acid + Chien et al., 1998other carboxylic acids(c) + Chien et al., 1998

α-pinene pinonaldehyde 0.19 ± 0.04 Hakola et al., 1994

0.143 ± 0.024 Alvarado et al., 1998apinene oxide 0.021 ± 0.017 Alvarado et al., 1998aacetone 0.08 ± 0.02 Reissell et al., 1999




β-pinene formaldehyde 0.42 Grosjean et al., 1993c

acetone 0.07 ± 0.05 Reissell et al., 1999nopinone 0.22 Grosjean et al., 1993c

0.23 ± 0.05 Hakola et al., 1994+ Griesbaum et al., 1998(h)

∆3-carene caronaldehyde not detected, < 0.08 Hakola et al., 1994


sabinene sabinaketone 0.50 ± 0.09 Hakola et al., 1994+ Griesbaum et al., 1998(h)


α-thujene no data

camphene camphelinone 0.36 ± 0.06 Hakola et al., 1994

d-limonene formaldehyde 0.10 Grosjean et al., 1993c4-acetyl-1-methyl-cyclohexene not detected, < 0.04 Hakola et al., 1994

+(b) Grosjean et al., 1993cendolim not detected, < 0.04 Hakola et al., 1994acetone not detected, < 0.02 Reissell et al., 1999

terpinolene 4-methyl-3-cyclohexen-1-one 0.29 ± 0.06 Hakola et al., 1994acetone 0.50 ± 0.05 Reissell et al., 1999




α-terpinene acetone 0.03 ± 0.01 Reissell et al., 1999

γ-terpinene acetone 0.11 ± 0.02 Reissell et al., 1999

β-phellandrene 4-isopropyl-2-cyclohexen-1-one 0.29 ± 0.06 Hakola et al., 1994

myrcene acetone 0.33 ± 0.07 Reissell et al., 1999

ocimene acetone 0.21 ± 0.04 Reissell et al., 1999

α-cedrene no data

α-copaene no data

longifolene no data

β-caryophyllene formaldehyde 0.08 Grosjean et al., 1993c

+ Calogirou et al., 1997unsaturated C14 ketone, MW = 206 + Grosjean et al., 1993cC15 unsaturated ketoaldehyde(d) + Calogirou et al., 1997C14 saturated tricarbonyl(d) + Calogirou et al., 1997

α-humulene no data




2-methyl-3-buten-2-ol formaldehyde 0.36 ± 0.09 Grosjean and Grosjean, 19950.48, 0.57(e) Fantechi et al., 19980.29 ± 0.03 Alvarado et al., 1999

2-hydroxy-2-methyl propanal 0.30 ± 0.02(f) Grosjean and Grosjean, 19950.30 ± 0.06 (GC), Alvarado et al., 1999

0.47 (FT-IR)tentative Fantechi et al., 1998

acetone 0.23 ± 0.06 Grosjean and Grosjean, 19950.15-0.48 (GC), Alvarado et al., 1999

0.12 ± 0.02 (FT-IR)0.125, 0.182(e) Fantechi et al., 1998

formic acid 0.01-0.03 Alvarado et al., 1999CO 0.30, 0.50(e) Fantechi et al., 1998

0.11 ± 0.02 Alvarado et al., 1999CO2 0.44, 0.30(e) Fantechi et al., 1998

0.09 ± 0.02 Alvarado et al., 1999

cis-3-hexen-1-ol propanal 0.49 ± 0.07 Aschmann et al., 19970.59 ± 0.12 Grosjean et al., 1993a

3-hydroxypropanal 0.33(g) Aschmann et al., 1997methylglyoxal 0.17 ± 0.05 Grosjean et al., 1993aacetaldehyde 0.13 ± 0.02 Grosjean et al., 1993aformaldehyde 0.03 ± 0.01 Grosjean et al., 1993ahydroxyacetaldehyde and / orglyoxal

0.02 ± 0.01 Grosjean et al., 1993a




linalool 4-hydroxy-4-methyl-5-hexen-1-al(d) 0.85 ± 0.14 Shu et al., 1997+ Grosjean and Grosjean, 1997

acetone 0.21 ± 0.02 Shu et al., 19970.28 ± 0.01 Grosjean and Grosjean, 1997

formaldehyde 0.014 ± 0.012 Grosjean and Grosjean, 19970.36 ± 0.06 Shu et al., 1997

5-ethenyldihydro-5-methyl-2(3H)furanone(d)

0.126 ± 0.025 Shu et al., 1997

methylglyoxal 0.11 ± 0.01 Grosjean et al., 1997

trans-2-hexenyl acetate butanal 0.47 ± 0.02 Grosjean et al., 1996b2-oxoethyl acetate(d) 0.58 ± 0.14 Grosjean et al., 1996bglyoxal 0.21 ± 0.01 Grosjean et al., 1996bpropanal 0.10 ± 0.01 Grosjean et al., 1996b2-oxobutanal(d) 0.09 ± 0.01 Grosjean et al., 1996bacetaldehyde 0.04 ± 0.01 Grosjean et al., 1996b

cis-3-hexenyl acetate propanal 0.76 ± 0.04 Grosjean and Grosjean, 19993-oxopropyl acetate(d) + Grosjean and Grosjean, 19992-oxoethyl acetate(d) 0.06 ± 0.02 Grosjean and Grosjean, 1999acetaldehyde 0.05 ± 0.01 Grosjean and Grosjean, 1999methylglyoxal 0.05 ± 0.01 Grosjean and Grosjean, 1999acetone 0.03 ± 0.01 Grosjean and Grosjean, 1999




trans-2-hexenal butanal 0.53 ± 0.06 Grosjean et al., 1996bglyoxal 0.56 ± 0.04 Grosjean et al., 1996b2-oxobutanal 0.07 ± 0.01 Grosjean et al., 1996bpropanal 0.07 ± 0.01 Grosjean et al., 1996bacetaldehyde 0.11 ± 0.02 Grosjean et al., 1996b

cis-3-hexenal no data

6-methyl-5-hepten-2-one 2-oxopentanal(d) 0.82 ± 0.21 Smith et al., 1996acetone 0.30 ± 0.05 Smith et al., 1996

0.28 ± 0.02 Grosjean et al., 1996bmethylglyoxal 0.32 ± 0.03 Grosjean et al., 1996bformaldehyde 0.04 ± 0.03 Grosjean et al., 1996b

piperitone no data

(a) measured in the presence of a scavenger for OH.

(b) +: product identified, yield not reported.

(c) formic, acetic, pyruvic and four unidentified acids.

(d) structures are shown in Appendix A.

(e) first and second values are for cyclohexane and methylcyclohexane, respectively, as scavengers for OH.

(f) tentative identification and estimated formation yield.

(g) measured without OH scavenger.

(h) Griesbaum et al., (1998) also report the secondary ozonide and two lactones, see structures in Appendix A.


Table 3-10. Products of the reaction of NO3 with unsaturated compounds.

Compound Product Formation yield

propene(a) formaldehyde 0.08, 0.10 ± 0.05

acetaldehyde 0.12, 0.10 ± 0.05, 0.60

methyl oxirane 0.28

CH3C(O)CH2ONO2 0.12

1-butene(a) formaldehyde 0.11

propanal 0.12, 0.65

CH3CH2C(O)CH2ONO2 0.17

ethyl oxirane 0.18

trans-2-butene(e) acetaldehyde 0.70, 0.34 ± 0.12, 1.0

CH3C(O)CH(ONO2)CH3 0.55, 0.41 ± 0.13, 0.38

CH3CH(ONO2)CH(ONO2)CH3 0.04

CH3CH(OH)CH(ONO2)CH3 0.15 ± 0.05

2,3-dimethyloxirane < 0.01, 0.12

isoprene(e) formaldehyde 0.11

methacrolein 0.035 ± 0.014

methylvinylketone 0.035 ± 0.004

O2NOCH2C(CH3) = CHCHO + (Major)(b)

O2NOCH2CH=C(CH3)CHO +

O2NOCH2C(O)C(CH3) = CH2 +

other C5 unsaturated compounds +

α-pinene pinonaldehyde 0.62(c)

0.62 ± 0.04(d)

ca. 0.71(e)

total nitrate 0.19(e)

β-pinene nopinone ca. 0.02(e)

total carbonyls 0.14(e)

total nitrates 0.61(e)


Table 3-10. Products of the reaction of NO3 with unsaturated compounds (continued).

Compound Product Formation yield

∆3-carene caronaldehyde ca. 0.03(e)

total carbonyls 0.29(e)

total nitrates 0.48(e)

limonene endolim 0.69(e,f)

total nitrates 0.48(e,f)

2-methyl-3-buten-2-ol acetone +(g)

nitrate and carbonyl nitrates +(g)

linalool acetone 0.21 ± 0.02(h)

4-hydroxy-4-methyl-5-hexen-1-al 0.19 ± 0.05(h)

(a) from studies reviewed by Atkinson (1997a)

(b) +: product identified, yield not reported

(c) Berndt and Böge, 1997

(d) Wängberg et al., 1997

(e) Hallquist et al., 1999

(f) only one experiment

(g) Fantechi et al., 1998

(h) Shu et al., 1997


Table 3-11. OH formation yields in the reaction of ozone with unsaturated aliphatic

compounds.

Compound OH formation yield(a) Compound OH formation yield(a)

ethylene 0.12 myrcene 1.15

propene 0.33 ocimene(b) 0.63

1-butene 0.41 α-cedrene 0.67

cis-2-butene 0.41 α-copaene 0.38, 0.32

trans-2-butene 0.64 longifolene no data

isoprene 0.27 β-caryophyllene 0.06

α-pinene 0.85, 0.76(c) α-humulene 0.22

β-pinene 0.35 2-methyl-3-buten-2-ol 0.19(d)

∆3-carene 1.06 cis-3-hexen-1-ol 0.26(e)

sabinene 0.26, 0.33(c) linalool 0.72(e)

α-thujene no data trans-2-hexenyl acetate no data

camphene ≤ 0.18 cis-3-hexenyl acetate 0.16(e)

d-limonene 0.86 trans-2-hexenal 0.62(e)

terpinolene 1.03 cis-3-hexenal no data

α-terpinene no data piperitone no data

γ-terpinene no data 6-methyl-5-hepten-2-one

0.75(f)

β-phellandrene 0.14

(a) from Atkinson (1997a) unless otherwise indicated.

(b) cis + trans.

(c) using 2-butanol as OH scavenger.

(d) Alvarado et al., 1999.

(e) Atkinson et al., 1995a.

(f) Smith et al., 1996.


3.3.1 Products of the reaction of OH with p-cymene and with saturated

compounds

Information on products of the reaction of OH with saturated compounds is

summarized in Table 3-7. The OH-initiated oxidation of ethane, methanol, ethanol,

formaldehyde, acetaldehyde, acetone and 2-butanone has been studied in detail, and the

corresponding reaction mechanisms are included in computer kinetic models (e.g., Carter

1990, 1995, Atkinson, 1994, 1997a, and references therein). The oxidation of hexanal

leads to pentanal and to the peroxyacyl nitrate CH3(CH2)4C(O)OONO2. The mechanism

of the aldehyde-OH reaction (including hexanal-OH) and the formation and reactions of

the corresponding peroxyacyl nitrates is outlined in Section 3.5.2. There is no information

on the OH reaction products of camphor, bornyl acetate, cineole, and p-cymene. Formic

acid and acetic acid, for which little product information is available, react slowly with OH

(see Table 3-2) and as a result they are removed from the atmosphere more efficiently by

physical processes (deposition, hydrometeor scavenging) than by chemical oxidation.

3.3.2 Products of the reaction of OH with unsaturated compounds

Information on products of the reaction of OH with unsaturated compounds is

summarized in Table 3-8. Most studies of the OH reaction have been carried out in the

presence of NOx, i.e., under conditions that result in the formation of alkoxy radicals from

peroxy radicals (RH + OH à H2O + R, R + O2 à RO2, RO2 + NO à NO2 + RO, see

Section 3.5.1). Much information is available for isoprene, whose oxidation has been

described in a detailed computer kinetic mechanism (Carter and Atkinson, 1996), from

which a condensed mechanism suitable for use in air quality models has been constructed

(Carter, 1996). Reasonably detailed information is available for the simple alkenes

(ethylene, propene, 1-butene, cis-2-butene), for the terpenes α-pinene and β-pinene and

for the unsaturated alcohols 2-methyl-3-buten-2-ol, cis-3-hexen-1-ol, and linalool. Less

information, generally limited to one or more of the first-generation carbonyl product(s)

and to acetone, is available for the terpenes ∆3-carene, sabinene, d-limonene, terpinolene,


and β-phellandrene. The information available for the terpenes camphene, α-terpinene, γ-

terpinene, myrcene, and ocimene is limited to acetone formation yields, and that available

for the sesquiterpene β-caryophyllene is limited to qualitative observation of several

carbonyls. There is no information on the OH reaction products of the terpene α-thujene,

the sesquiterpenes α-cedrene, α-copaene, longifolene, and α-humulene, and the

unsaturated oxygenates trans-2-hexenal, cis-3-hexenal, trans-2-hexenyl acetate, cis-3-

hexenyl acetate, and piperitone.

3.3.3 Products of the reaction of O3 with unsaturated compounds

Information on products of the reaction of O3 with unsaturated compounds is

summarized in Table 3-9. Overall, more information is available for O3 reaction products

than for OH reaction products. For several of the compounds listed in Table 3-9, reaction

products have been identified and their formation yields measured by two or more

research groups using different sampling and analytical methods, and for these compounds

the reported product formation yields are generally in reasonable agreement. Formation

yields of OH have been measured for all compounds and are listed in Table 3-11.

Reasonably detailed product information is available for all alkenes, isoprene, the

terpenes α-pinene, β-pinene, and ∆3-carene, the alcohols 2-methyl-3-buten-2-ol, cis-3-

hexen-1-ol, and linalool, the two unsaturated esters cis-3-hexenyl acetate and trans-2-

hexenyl acetate, and the ketone 6-methyl-5-hepten-2-one. Less information (generally

limited to the primary carbonyls and, for the terpenes, to the primary carbonyls and

acetone) is available for trans-2-hexenal, β-caryophyllene, and the terpenes sabinene,

camphene, d-limonene, terpinolene, and β-phellandrene. Acetone formation yields is the

only information available for α-terpinene, γ-terpinene, myrcene, and ocimene. No

product information is available for the terpene α-thujene, the sesquiterpenes α-cedrene,

α-copaene, longifolene, and α-humulene, and the unsaturated oxygenates cis-3-hexenal

and piperitone.

The reaction of O3 with unsaturated compounds results in the formation of

carboxylic acids, see Section 5. Examination of the data in Table 3-9 indicates that little


information exists regarding the nature and formation yields of low molecular weight

carboxylic acids, e.g., there is no data for 3-hydroxypropionic acid (CH2OHCH2COOH)

which is expected to form from cis-3-hexen-1-ol. Information on higher molecular weight

carboxylic acids, which are components of the aerosol formed in the reaction of O3 with

terpenes (see Section 3.3.5), is available for α-terpene and, with less detail, for ∆3-carene,

d-limonene, and terpinolene.

3.3.4 Products of the reaction of NO3 with unsaturated compounds

Information on products of the reaction of NO3 with unsaturated compounds is

summarized in Table 3-10. Overall, there is much less information on products of the NO3

reaction than on products of the OH and O3 reactions. Compounds for which some

product information is available include isoprene, the alkenes propene, 1-butene and trans-

2-butene, the terpenes α-pinene, β-pinene, ∆3-carene, and limonene and the alcohols 2-

methyl-3-buten-2-ol (information limited to qualitative identification of acetone and

“nitrates”) and linalool. For the few compounds studied by more than one group, reported

products and their formation yields vary substantially. One exception is the formation

yield of pinonaldehyde from α-pinene, for which data from three studies are in good

agreement (Berndt and Böge 1997, Wängberg et al., 1997, Hallquist et al., 1999). More

product studies for the reaction of NO3 with biogenic compounds are obviously needed.

3.3.5 Formation of SOA

Numerous studies of aerosol formation from terpenes have been made since Went

proposed a role for biogenic organic compounds in the formation of tropospheric aerosol

and reported the formation of O3 and particles in α-pinene-NOx mixtures exposed to light

(Went, 1960). The early literature on SOA formation, including several studies of SOA

formation from α-pinene, has been reviewed by Grosjean (1977) and Grosjean and

Seinfeld (1989). Studies of SOA formation from biogenic organic compounds have

gained momentum in recent years. These studies can be divided into three categories:


• aerosol dynamics studies, which focus on measuring aerosol size distribution

and optical properties (e.g., light absorption and light scattering), and on

understanding physical processes including nucleation and growth by

condensation.

• aerosol yield studies, whose objective is to measure the aerosol formation yield

as a function of initial precursor concentrations (e.g., α-pinene and NOx, or α-

pinene and O3), and this generally for the purpose of describing aerosol-

precursor relationships as components of computer air quality models.

• aerosol molecular composition studies, which focus on characterizing the

chemical composition of SOA and on elucidating the chemical reactions that

lead to SOA formation.

A summary of studies of SOA formation from biogenic organic compounds is

given in Table 3-12. Table 3-12 indicates, for each compound studied, whether the study

cited focused on aerosol yields or on aerosol molecular composition. Also indicated in

Table 3-12 is the type of reaction studied for each organic compound, i.e., reaction with

O3, reaction with OH (in the presence or absence of NOx), reaction with NO3, or aerosol

formation in BVOC-NOx mixture exposed to light. For the reaction with O3, studies have

been carried out with and without a scavenger for OH, and this is also indicated in Table

3-12.

Several observations can be made from examination of the data in Table 3-12.

• SOA formation has been documented for 14 biogenic compounds (of which

one, the unsaturated alcohol terpinene-4-ol, is not listed in Table 3-1): 10

terpenes, 2 sesquiterpenes (β-caryophyllene and α-humulene), and 2

unsaturated alcohols (linalool and terpinene-4-ol). The terpene α-pinene has

received by far the most attention.


Table 3-12. Studies of aerosol formation from biogenic organic compounds.

Focus of Study

Reference CompoundStudied

Reactionwith

Aerosolformation

yield

Aerosolmolecular

composition

Schwartz, 1974 α-pinene NOx 3 3

Schuetzle and Rasmussen, 1978 limonene NOx, O3 3

terpinolene O3 3

Yokouchi and Ambe, 1985 α-pinene O3(a) 3

β-pinene O3(a) 3

limonene O3(a) 3

Hatakeyama et al., 1989 α-pinene O3(a) 3 3

Hatakeyama et al., 1991 α-pinene OH 3 3

β-pinene OH 3 3

Pandis et al., 1991 β-pinene NOx 3

Zhang et al., 1992 α-pinene NOx 3

β-pinene NOx 3

Grosjean et al., 1994b α-pinene NOx 3

β-pinene NOx 3

d-limonene O3(b) 3

β-caryophyllene NOx, O3(b) 3

Hoffmann et al., 1997 α-pinene NOx, O3(a) 3

β-pinene NOx, O3(a) 3

∆3-carene NOx, O3(a) 3

d-limonene NOx 3

ocimene NOx, O3(a) 3


Table 3-12. Studies of aerosol formation from biogenic organic compounds (continued).

Focus of Study

Reference CompoundStudied

Reactionwith

Aerosolformation

yield

Aerosolmolecular

composition

Hoffmann et al., 1997 (cont.) linalool NOx, O3(a) 3

terpinene-4-ol NOx, O3(a) 3

β-caryophyllene NOx 3

Karasawa et al., 1998 α-pinene NOx 3

Christofferson et al., 1998 α-pinene O3(a,c) 3

Hoffmann et al., 1998 α-pinene O3(a) 3

Yu et al., 1998 α-pinene O3(d) 3(e)

∆3-carene O3(d) 3(e)

Virkkula et al., 1999(f) α-pinene NOx, O3(a) 3

β-pinene NOx, O3(a) 3

limonene NOx, O3(a) 3

Jang and Kamens, 1999 α-pinene O3(a) 3

Kamens et al., 1999 α-pinene O3(a) 3(g)

Nozière et al., 1999a α-pinene OH (with NO) 3

OH (no NOx) 3

Hallquist et al., 1999 α-pinene NO3 3

β-pinene NO3 3

∆3-carene NO3 3

limonene NO3 3


Table 3-12. Studies of aerosol formation from biogenic organic compounds (continued).

Focus of Study

Reference Compoundstudied

Reactionwith

Aerosolformation

yield

Aerosolmolecular

composition

Griffin et al., 1999(g) α-pinene NOx, O3(d) 3

β-pinene NOx, O3(d), NO3 3

∆3-carene NOx, O3(d), NO3 3

sabinene NOx, O3(d), NO3 3

limonene NOx 3

α-terpinene NOx 3

γ-terpinene NOx 3

terpinolene NOx 3

myrcene NOx 3

ocimene NOx 3

linalool NOx 3

terpinen-4-ol NOx 3

β-caryophyllene NOx 3

α-humulene NOx 3

(a) ozone reaction studied without scavenger for OH.

(b) with cyclohexane added to scavenger OH.

(c) with and without cyclohexane added to scavenge OH.

(d) with 2-butanol added to scavenger OH.

(e) no differentiation of gas phase and aerosol products (all products are from samples collected in impingers).

(f) focus on aerosol hygroscopicity.

(g) focus on modeling of aerosol formation.


• The studies listed in Table 3-12 all involve higher molecular weight (MW)

unsaturated organics, i.e., C10 and C15 compounds. Considerations regarding

the volatility of reaction products suggest that the lower MW compounds

listed in Table 3-1 should form little or no aerosol under atmospheric

conditions. These compounds include the C2-C6 saturated compounds (from

ethane to hexanal), the C2-C6 unsaturated compounds (the alkenes, the two

hexenals, the two hexenyl acetates, and the two alcohols 2-methyl-3-buten-2-ol

and cis-3-hexen-1-ol), and probably the C8 ketone 6-methyl-5-hepten-2-one.

Isoprene, a major component of biogenic emissions, does not form aerosols

under atmospheric conditions (e.g., Pandis et al., 1991, and references therein).

• Compounds not studied with respect to SOA formation include the high MW

saturated aliphatic compounds bornyl acetate, cineole, and camphor, the

aromatic compound p-cymene, the unsaturated ketones 6-methyl-5-hepten-2-

one, the terpene α-thujene, and the sesquiterpenes α-cedrene, α-copaene, and

longifolene. Examination of SOA data for structural homologues (Grosjean

and Seinfeld, 1989, Griffin et al., 1999, also see other references listed in Table

3-12) suggests that these compounds (with the possible exception of 6-methyl-

5-hepten-2-one, see above) are likely to form organic aerosols, with aerosol

formation yields probably ranging from low for the saturated compounds (on

account of their low reactivity with OH, see Table 3-2) to high for the

sesquiterpenes (on account of their high reactivity with OH, O3 and NO3 and of

their oxidation to high molecular weight (MW) condensable products).

• For the compounds that have been shown to form SOA (i.e., the compounds

listed in Table 3-12), few studies of the aerosol molecular composition have

been carried out. Molecular composition information is available for only five

compounds, the terpenes α-pinene (O3 and NOx reactions), β-pinene (very

limited data for the O3 reaction), d-limonene (O3 and NOx reactions),

terpinolene (O3 reaction), and ∆3-carene (O3 reaction). No molecular

composition data are available for SOA formation via reaction with NO3, a


pathway for SOA formation that has received limited attention until recently

(Hallquist et al., 1999, Griffin et al., 1999).

Compounds that have been identified in α-pinene aerosol are listed in Table 3-13

(Schwartz, 1974; Yokouchi and Ambe, 1985; Hatakeyama et al., 1989, 1991; Hoffmann

et al., 1998; Christofferson et al., 1998; Yu et al., 1998; Karasawa et al., 1998; Jang and

Kamens, 1999). Although more compounds have been identified in the most recent

studies (Jang and Kamens, 1999), reflecting advances in analytical chemistry over the last

ca. 25 years, there is good agreement among studies as to what compounds are the major

components of α-pinene aerosol. The compounds listed in Table 3-13 are all difunctional

compounds, as expected from the structure of α-pinene, which is a cyclic monoalkene,

and from literature data for simpler structural homologues of α-pinene, i.e., cyclohexene

and other cyclic olefins (Schwartz, 1974; Grosjean and Friedlander, 1979; Hatakeyama et

al., 1989). Thus, all aerosol products of α-pinene are 1,3-disubstituted-2,2-

dimethylcyclobutanes that result from reactions at the C=C bond, followed by ring

opening:

R1 R2

In the same way, the high MW compounds identified in the oxidation of ∆3-carene

(Yu et al., 1998), which, like α-pinene, contains only one C=C bond, are 2,3-

disubstituted-1,1-dimethylcyclopropanes:

R1

R2

For limonene and terpinolene, which contain two C=C bonds, the major

components of the aerosol include ring-opening difunctional products of the more


Table 3-13. Molecular composition of α-pinene aerosol(a).

R1 R2

Product R1 R2 Molecular Weight

cis-pinic acid -CH2COOH -COOH 186

cis-pinonic acid -CH2COOH -C(O)CH3 184

trans-pinonic acid -CH2COOH -C(O)CH3 184

cis-norpinonic acid -COOH -C(O)CH3 170

trans-norpinonic acid -COOH -C(O)CH3 170

P1(b) -CH2COOH -C(O)CHO 198

P2(b) -CH2COOH -C(O)CH2OH 200

P3(b) -COOH CH2CHO 170

2,2-dimethylcyclobutane- 1,3-dicarboxylic acid

-COOH -COOH 172

pinonaldehyde -CH2CHO -C(O)CH3 168

nor-pinonaldehyde -CHO -C(O)CH3 154

2,2-dimethylcyclobutane-1-carboxylic acid-3-carboxaldehyde

-COOH -CHO 156

2,2-dimethylcyclobutane- 1,3-dicarboxaldehyde

-CHO -CHO 140

(a) from Schwartz, 1974; Yokouchi and Ambe, 1985; Hatakeyama et al., 1989, 1991; Karasawa et al.,

1998; Christofferson et al., 1998; Hoffmann et al., 1998; Yu et al., 1998; and Jang and Kamens,

1999.

(b) following the notation of Jang and Kamens, 1999.


reactive internal C=C bond as well as products resulting from oxidation at both C=C

bonds (Schuetzle and Rasmussen, 1978):

OR1

,O

R1O

OR1

,O

R1

O

Of the difunctional products listed in Table 3-13 and identified in laboratory

studies, not all are expected to be major components of terpene aerosols under

atmospheric conditions. This is because the particle phase/gas phase (P/G) partitioning of

condensable compounds increases with increasing aerosol concentration (Pankow, 1994)

and that, since initial terpene concentrations are typically much higher in laboratory studies

than in the atmosphere, the P/G ratio for a given condensable product is likely to be much

higher in laboratory studies than in ambient air. For example, of the 13 compounds

identified as components of α-pinene aerosol in the laboratory (Table 3-13), it is likely that

only those with the lowest vapor pressures (Grosjean, 1977), i.e., the dicarboxylic acids

cis-pinonic acid and 2,2-dimethylcyclobutane-1,3-dicarboxylic acid and the ketoacid

pinonic acid, may be components of α-pinene aerosol in the atmosphere. Similarly, the

least volatile products identified as aerosol components in laboratory studies of d-limonene

(Schuetzle and Rasmussen, 1978), terpinolene (Schuetzle and Rasmussen, 1978), and ∆3-

carene (Yu et al., 1998), i.e., the dicarboxylic acids, are most likely to be components of

the aerosol from oxidation of these terpenes in the atmosphere.

Even the products with the lowest vapor pressure, i.e., the dicarboxylic acids, may

not have vapor pressures that are low enough to initiate aerosol formation in clean air, i.e.,

via nucleation. The formation of carboxylic acid dimers has been reported for d-limonene,

terpinolene (Schuetzle and Rasmussen, 1978), and α-pinene (Hoffmann et al., 1998).

Hoffmann et al. (1998) have argued that dimer formation (the dimers of pinic acid and/or

pinonic acid in the case of α-pinene aerosol) may be necessary to initiate nucleation.


Oxidation products of α-pinene (and possibly β-pinene) have been identified in

ambient aerosols. Dicarboxylic acids and other difunctional oxygenates, including pinic

acid, pinonic acid, and norpinonic acid, have been identified in early studies of the

molecular composition of ambient aerosols (Schuetzle et al., 1975; Cronn et al., 1977;

Schuetzle, 1980). Yokouchi and Ambe (1985) have identified aerosol products of α-

pinene, along with pinonaldehyde, in samples collected in a pine forest and a cedar forest

in Japan. Pinonaldehyde was also measured in ambient air in Japan by Satsumabayashi et

al. (1990). More recently, several studies have focused on aerosol formation in forests

(Mäkelä et al., 1997) and its relation to chemical oxidation of α-pinene and β-pinene

(Leaitch et al., 1999; Kavouras et al., 1998, 1999a, 1999b; Yu et al., 1999). Kavouras et

al. (1999b) have identified pinic acid, cis- and trans-pinonic acid, and cis- and trans-

norpinonic acid in ambient aerosol samples collected over a eucalyptus forest.

Pinonaldehyde (from oxidation of α-pinene) and nopinone (from oxidation of β-pinene)

were identified in the corresponding gas-phase samples. These measurements, together

with measurements of other parameters including condensation nuclei concentration, have

provided supportive experimental evidence for a direct relation between terpene oxidation

and particle formation in the clean troposphere.

3.4 Atmospheric Reactions of First-Generation Products

Reaction products of the compounds listed in Table 3-1 may in turn be oxidized

under atmospheric conditions, and the oxidation of these “first-generation” products must

be considered. The reactivity of these products with OH, O3 and NO3 has important

implications. Products that react slowly may accumulate in the troposphere, for example,

acetone which forms from several terpenes, see Tables 3-8 to 3-10. Acetone has received

attention for its role in global tropospheric chemistry (e.g., Singh et al., 1994, Reissell et

al., 1999, and references therein). Products that react rapidly, e.g., pinonaldehyde from α-

pinene and other saturated and unsaturated high MW carbonyls from several terpenes, are

also important since their reactions lead to “second-generation” products that may


contribute to O3 and/or aerosol formation. Information on the reactions of the first-

generation products is also necessary to construct computer kinetic mechanisms that

describe the atmospheric oxidation of biogenic organic compounds.

3.4.1 Products discussed in this section

Not all first-generation products that are listed in Tables 3-7 to 3-10 will be

discussed in this section. From the products of saturated compounds (Table 3-7), we

exclude formaldehyde, acetaldehyde, and PAN, whose atmospheric reactions are well

documented. This leaves the oxidation products of hexanal, i.e., pentanal and the

peroxyacyl nitrate n-C5H11C(O)OONO2. From the products of unsaturated compounds

(Tables 3-8 to 3-10), we exclude those organic compounds whose oxidation mechanisms

are reasonably well understood, i.e., formaldehyde, acetaldehyde, hydroxyacetaldehyde,

propanal, acetone, glyoxal, methylglyoxal, methanol, formic acid, methacrolein,

methylvinyl ketone, 3-methylfuran, and PAN. We also exclude the oxirane (epoxide)

products of the NO3-alkene reactions since the formation yields of these products are

small under atmospheric conditions (e.g., Atkinson, 1997a), pinene oxide from (O3 + α-

pinene) on account of its small formation yield (ca. 2%, Alvarado et al., 1998a) and, until

more specific information becomes available regarding molecular composition, products

such as “C5 unsaturated hydroxycarbonyls” from isoprene and “dihydroxycarbonyls, MW

= 184” from α-pinene or β-pinene. This leaves the following 26 compounds: acrylic acid,

methacrylic acid, and 4-nitroxy-3-methyl-2-butenal (O2NOCH2C(CH3)=CHCHO) from

isoprene, pinonaldehyde from α-pinene, nopinone from β-pinene, caronaldehyde from ∆3-

carene, sabinaketone from sabinene, camphelinone from camphene, 4-acetyl-1-methyl

cyclohexene and endolim from d-limonene, 4-methyl-3-cyclohexen-1-one and 3-propenyl-

6-oxoheptanal from terpinolene, 4-isopropyl-2-cyclohexen-1-one from β-phellandrene, 2-

hydroxy-2-methyl propanal from 2-methyl-3-buten-2-ol, 3-hydroxypropanal from cis-3-


hexen-1-ol, 3-oxopropyl acetate from cis-3-hexenyl acetate, 2-oxoethyl acetate from cis-

3-hexenyl acetate and from trans-2-hexenyl acetate, 2-oxopentanal from 6-methyl-5-

hepten-2-one, 2-oxobutanal and butanal from trans-2-hexenal and from trans-2-hexenyl

acetate, and 4-hydroxy-4-methyl-5-hexen-1-al and 5-ethenyldihydro-5-methyl-2(3H)

furanone from linalool (another product of linalool, 6-methyl-5-hepten-2-one, is listed in

Table 3-1 and has been discussed earlier in this report). Also included in this section is the

unsaturated peroxyacyl nitrate MPAN, CH2=C(CH3)C(O)OONO2, which forms in the

oxidation of methacrolein and of methacrolein's precursor isoprene.

The 26 first-generation products discussed in this section include 14 saturated

compounds and 12 unsaturated compounds. These compounds are listed in Table 3-14

according to chemical functionality. All but five of the compounds listed in Table 3-14 are

carbonyls, and the majority of these carbonyls also bear one or more other functional

group(s). Kinetic data, atmospheric lifetimes, and reaction products of the first-generation

compounds listed in Table 3-14 are discussed in the following sections.

3.4.2 Kinetic data for first-generation products

Rate constants for the reactions of first-generation products with OH, O3 and NO3

are listed in Table 3-15. Examination of the data in Table 3-15 indicates that kinetic data

are available for less than half of the compounds of interest. For saturated compounds,

atmospheric removal involves reactions with OH and with NO3, and, for saturated

aldehydes, photolysis. Photolysis data are available for only two compounds,

pinonaldehyde and caronaldehyde, for which Hallquist et al. (1997) calculated photolysis

lifetimes of 3.3 and 5.8 hours, respectively. For unsaturated compounds, atmospheric

removal involves reactions with OH, O3 and NO3. Although limited to a few unsaturated

compounds, the kinetic data in Table 3-15 clearly show the effect of the substituents on

the reactivity of the C=C bond. For example, O3 reaction rate constants span ca. 4 orders

of magnitude, from acrylic acid (on account of the strong electron-withdrawing effect of


Table 3-14. First-generation products listed according to chemical functionality.

Functionality Compound(a) Reaction product of

saturated ketone nopinone β-pinene

camphelinone camphene

sabinaketone sabinene

saturated aldehyde butanal trans-2-hexenal

trans-2-hexenyl acetate

pentanal hexanal

unsaturated ketone 4-methyl-3-cyclohexen-1-one terpinolene

4-acetyl-1-methyl cyclohexene d-limonene

4-isopropyl-2-cyclohexen-1-one β-phellandrene

6-methyl-5-hepten-2-one linalool

saturated dicarbonyl(ketoaldehyde)

2-oxobutanal trans-2-hexenaltrans-2-hexenyl acetate

2-oxopentanal 6-methyl-5-hepten-2-one

pinonaldehyde α-pinene

caronaldehyde ∆3-carene

unsaturated dicarbonyl endolim d-limonene

3-propenyl-6-oxoheptanal terpinolene

saturated hydroxycarbonyl 2-hydroxy-2-methyl-propanal 2-methyl-3-buten-2-ol

3-hydroxypropanal cis-3-hexen-1-ol

unsaturated hydroxycarbonyl 4-hydroxy-4-methyl-5-hexen-1-al linalool

oxoester (ester-aldehyde) 3-oxopropyl acetate cis-3-hexenyl acetate

2-oxoethyl acetate cis-3-hexenyl acetate

unsaturated acid acrylic acid isoprene

methacrylic acid isoprene


Table 3-14. First generation products listed according to chemical functionality

(continued).

Functionality Compound(a) Reaction product of

peroxyacyl nitrate n-C5H11C(O)OONO2 hexanal

CH2=C(CH3)C(O)OONO2 (MPAN) methacrolein

other unsaturated compounds 5-ethenyl-dihydro-5-methyl-2(3H)furanone

linalool

4-nitroxy-3-methyl-2-butenal isoprene

(a) see structures in Appendix A.


Table 3-15. Kinetic data for the reactions of OH, O3 and NO3 with first-generation

products.

Reaction rate constant(b)

Compound(a) 1012 x kOH 1018 x kO3 kNO3(c)

Saturated compounds

nopinone 14.3 ± 3.7(d) < 0.005(e) < 2(-15)(e)

17 ± 2(e)

camphelinone 5.15 ± 1.44(d)

sabinaketone

butanal 23.5(f)

pentanal 28.5(f)

2-oxobutanal

2-oxopentanal

pinonaldehyde 91 ± 18(g) 0.089 ± 0.014(g) 5.4 ± 1.8(-14)(g)

87.2 ± 11.4(h) 2.35 ± 0.37(-14)(h)

40 ± 10(i)

caronaldehyde 121 ± 36(h) 2.71 ± 0.15(-14)(h)

2-hydroxy-2-methyl propanal

3-hydroxypropanal

2-oxoethyl acetate

3-oxopropyl acetate

n-C5H11C(O)OONO2

Unsaturated compounds

4-methyl-3-cyclohexen-1-one

4-acetyl-1-methylcyclohexene 129 ± 33(d) 150 ± 53(d) 1.05 ± 0.38(-11)(d)

4-isopropyl-2-cyclohexen-1-one

6-methyl-5-hepten-2-one 157 ± 39 394 ± 40 7.5 ±3.0(-12)

endolim 110 ± 30(e) 8.3 ± 2.2(e) 2.5 ± 0.8(-13)(e)

3-propenyl-6-oxoheptanal


Table 3-15. Kinetic data for the reactions of OH, O3 and NO3 with first-generation

products (continued).

Reaction rate constant

Compound(a) 1012 x kOH 1018 x kO3 kNO3(c)

4-hydroxy-4-methyl-5-hexen-1-al(j)

74 ± 9(e) 3.8 ± 0.8(e) 2.0 ± 0.9(-14)(e)

acrylic acid 0.65 ± 0.13(k)

methyacrylic acid 4.1 ± 0.4(k)

CH2=C(CH3)C(O)OONO2

(MPAN)3.6 ± 0.4(l) 8.2 ± 2.0(m)

5-ethenyl-dihydro-5-methyl-2(3H) furanone

4-nitroxy-3-methyl-2-butenal

(a) see Table 3-14 for biogenic hydrocarbon whose oxidation leads to the compounds listed.

(b) at room temperature and 1 atm of air. Units: cm3 molecule-1 s-1. Data for 6-methyl-5-hepten-2-one are

from Tables 3-3, 3-4, and 3-5.

(c) read 1.0 (-11) as 1.0 x 10-11.

(d) Atkinson and Aschmann, 1993.

(e) Calogirou et al., 1999.

(f) Atkinson, 1997a.

(g) Glasius et al., 1997.

(h) Hallquist et al., 1997.

(i) Nozière et al., 1999b.

(j) studied as its cyclized form 5-methyl-5-vinyltetrahydrofuran-2-ol(e).

(k) Neeb et al., 1998.

(l) Grosjean et al., 1993e.

(m) Grosjean et al., 1993b.


the —COOH group) to the trisubstituted alkene 6-methyl-5-hepten-2-one (on account of

the combined inductive effects of the alkyl substituents).

Although kinetic data for first generation products are limited, reaction rate

constants that have not been measured could, with reasonable accuracy, be estimated from

structure-reactivity considerations since kinetic data are available for structural

homologues. More critical with respect to understanding the atmospheric chemistry of

first-generation products is the severe lack of information on their atmospheric oxidation

products, see Section 3.4.4.

3.4.3 Atmospheric lifetimes of first-generation products

Reactivity considerations and atmospheric lifetimes of biogenic organic

compounds have been discussed in Section 3.2.3. The same considerations apply to the

first-generation products for which, similar to data for their precursors, O3 and NO3

reaction rate constants span a much larger range than the corresponding OH reaction rate

constants (ca. 4 orders of magnitude for O3 reactions, ca. 3 orders of magnitude for NO3

reactions, and a factor of only ca. 40 for OH reactions). As an example, we have

calculated the atmospheric half-lives of first-generation products for a daytime scenario.

To facilitate comparison, we use the same OH and O3 concentrations as those used to

calculate the half-lives of biogenic compounds (see Table 3-6), i.e., [OH] = 1.0 x 106

molecule cm-3 and [O3] = 30 ppb. The half-lives thus calculated are listed in Table 3-16.

The aldehydes are removed rapidly from the atmosphere by reaction with OH. The

unsaturated carbonyls are also removed by reaction with O3, e.g., the half-life of 6-methyl-

5-hepten-2-one is ca. 40 minutes when [O3] = 30 ppb. The less reactive compounds, i.e.,

MPAN, methacrylic acid and acrylic acid have half-lives of ca. 1.5, 2.5, and 16 days,

respectively, and for acrylic acid removal by chemical reactions is likely to be much less

important than removal by physical processes such as deposition and hydrometeor

scavenging.


Table 3-16. Atmospheric lifetimes of first-generation products.

Atmospheric half-lives, hours (unless otherwise indicated)

Compound(a)

OH = 1.0 x 106 molecule cm-3 O3 = 30 ppb

Saturated

nopinone 13.4

camphelinone 37.3

butanal 8.2

pentanal 6.7

pinonaldehyde 2.1 – 4.8(a)

caronaldehyde 1.6

Unsaturated

4-acetyl-1-methylcyclohexene 1.5 1.7

6-methyl-5-hepten-2-one 1.2 0.65

endolim 1.7 31

4-hydroxy-4-methyl-5-hexen-1-ol(b) 2.6 67

acrylic acid (c) 16 days

methacrylic acid (c) 62

MPAN 53 31

(a) from range of rate constants in Table 3-15.

(b) from kinetic data for the cyclized form 5-methyl-5-vinyltetrahydrofuran-2-ol.

(c) no data.


3.4.4 Second-generation oxidation products

The oxidation products of first-generation products, i.e., “second-generation”

products, have received little attention, and the limited information available (as of mid-

1999) is summarized in Table 3-17. Grosjean et al. (1992) carried out sunlight irradiations

of mixtures of NOx-nopinone and NOx-4-acetyl-1-methylcyclohexene (4-AMCH) in air

and identified formaldehyde, acetone, and PAN from nopinone and formaldehyde, glyoxal,

and PAN from 4-AMCH. Calogirou et al. (1999) also studied nopinone and identified the

three high MW products listed in Table 3-17. In the same study, Calogirou et al. (1999)

reported one high MW unsaturated carbonyl product from endolim, three products from

pinonaldehyde (norpinonaldehyde, acetone and tentatively, methylglyoxal), and two

products (4-oxo-pentanal and hydroxyacetaldehyde) from 5-methyl-5-vinyl-

tetrahydrofuran-2-ol (the cyclized form of 4-hydroxy-4-methyl-5-hexen-1-al). Nozière

and Barnes (1998) and Nozière et al. (1999a) have studied the reaction of pinonaldehyde

with OH in the presence of NO2 (leading to the peroxyacyl nitrate in high yield, 0.82 ±

0.16) and in the presence of NO (leading to acetone, formaldehyde, and other unidentified

carbonyls). Nozière et al. (1999a) also studied the photolysis of pinonaldehyde and

identified formaldehyde and acetaldehyde as reaction products. Grosjean et al. (1993b,

1993e) have identified several products of the MPAN-OH and MPAN-O3 reactions.

To summarize, product studies of the oxidation of first-generation products have

been carried out for only six compounds, MPAN and five carbonyls. For these six

compounds, limited information is available regarding second-generation products. No

information is available on second-generation products of most of the biogenic compounds

listed in Table 3-1. As a result, it is difficult at the present time to describe the

atmospheric oxidation of biogenic compounds and, with the exception of isoprene and α-

pinene, to construct the corresponding computer kinetic mechanisms.


Table 3-17. Second-generation oxidation products.

Compound and reaction Product Formation yield Reference

pinonaldehyde + OH(b) 3-acetyl-2,2-dimethylcyclobutaneacetyl peroxynitrate (αP-PAN)(a)

0.82 ± 0.16 Nozière and Barnes, 1998

PAN ca. 0.08 Nozière and Barnes, 1998

pinonaldehyde + OH(c) norpinonaldehyde(a) + Calogirou et al., 1999

not detected Nozière et al., 1999a

acetone + Calogirou et al., 1999

0.15 ± 0.08 Nozière et al., 1999a

methyl glyoxal tentative Calogirou et al., 1999

formaldehyde 1.52 ± 0.56 Nozière et al., 1999a

other carbonyls 1.07 ± 0.50 Nozière et al., 1999a

pinonaldehyde photolysis(d) formaldehyde 0.12 ± 0.03 Nozière et al., 1999a

acetone 0.07 ± 0.02 Nozière et al., 1999a

endolim + OH(c) 2-isopropenyl-5-oxohexanal(a) + Calogirou et al., 1999

nopinone + OH(c) formaldehyde + Grosjean et al., 1992

acetone + Grosjean et al., 1992

PAN + Grosjean et al., 1992

2-hydroxy nopinone(a) + Calogirou et al., 1999

2,5-dihydroxynopinone(a) + Calogirou et al., 1999

2-oxo-nopinone(a) + Calogirou et al., 1999


Table 3-17. Second-generation oxidation products (continued).

Compound and reaction Product Formation yield Reference

4-acetyl-1-methylcyclohexene + OH(e) formaldehyde + Grosjean et al., 1992

glyoxal + Grosjean et al., 1992

PAN + Grosjean et al., 1992

4-hydroxy-4-methyl-5-hexen-1-al(f) + OH 4-oxopentanal(g) + Calogirou et al., 1999

hydroxyacetaldehyde + Calogirou et al., 1999

MPAN + OH(e) hydroxyacetone 0.59 ± 0.12 Grosjean et al., 1993

formaldehyde + Grosjean et al., 1993

MPAN + ozone(d) formaldehyde 0.6 ± 0.1 Grosjean et al., 1993

(a) see structure in Appendix A.

(b) in the presence of NO2

(c) in the presence of NO

(d) no NOx present

(e) in organic-NOx experiments

(f) studied as its cyclized form 5-methyl-5-vinyltetrahydrofuran-2-ol

(g) CH3C(O)CH2CH2CHO


3.5 Reaction Mechanisms

The atmospheric oxidation of BVOC, and of their first-generation and second-

generation products, involves pathways initiated by reactions with OH, O3, and NO3

(photolysis is also important for aldehydes). The overall features of these reactions are

reasonably well-known for OH and O3 reactions, and, to a more limited extent, for NO3

reactions. Our current knowledge of atmospheric oxidation mechanisms has been

described in several reviews (see for example Atkinson, 1997a, for reactions of alkanes

and alkenes with OH, O3 and NO3; and Atkinson and Arey, 1998, for several of the

biogenic compounds discussed in this report) and it is not our intent to repeat this

information here. Thus, rather than describing “generic” reaction mechanisms, we focus in

this section on specific compounds in order to illustrate atmospheric oxidation

mechanisms for biogenic compounds and to identify knowledge gaps. This section

includes examples for five types of reactions, i.e., the reaction of OH with saturated

compounds, the reaction of OH with aldehydes and the formation of peroxyacyl nitrates,

and the reactions of OH, O3 and NO3 with unsaturated compounds. The examples given

below include biogenic compounds as well as first-generation products.

3.5.1 The reaction of OH with saturated compounds

The OH radical reacts with saturated compounds by H-atom abstraction, with the

preferential (but not exclusive) reaction center being the weakest C-H bond. The alkyl

radical thus formed reacts with O2, and the corresponding peroxy radical reacts with NO

via two pathways, one yielding NO2 + an alkoxy radical (major) and the other yielding an

organic nitrate:

RH + OH H2O + R (1)

R + O2 RO2 (2)

RO2 + NO RO + NO2 (3a)

RO2 + NO RONO2 (3b)


Alkoxy radicals RO may decompose (C-C bond scission), react with oxygen (if H-

atom abstraction from the oxygen-bearing carbon is possible), or isomerize (if a 1,5-

intramolecular hydrogen shift is possible, e.g., Atkinson, 1997b). The relative importance

of these three pathways depends in part on the chemical structure of the biogenic

compound and dictates the nature and relative abundance of the reaction products.

For the saturated compounds listed in Table 3-1 and for which product studies

have been carried out, the relative importance of RO decomposition, RO reaction with O2,

and RO isomerization is reasonably well understood. The possible exception is ethanol,

whose reaction with OH leads to acetaldehyde as the major product:

CH3CH2OH + OH H2O + CH3

•C HOH (4a)

CH3

•C HOH + O2 HO2 + CH3CHO (4b)

The measured acetaldehyde yield is 0.85 ± 0.15 (Carter et al., 1979). The reported

uncertainty on this yield implies that 15 ± 15% of the OH-ethanol reaction may involve

another pathway, i.e., H-atom abstraction from the CH3 group:

CH3CH2OH + OH H2O + •C H2CH2OH (5)

If Reaction 5 takes place, it is followed by:

•C H2CH2OH + O2 O2CH2CH2OH (6a)

O2CH2CH2OH + NO NO2 + •

O CH2CH2OH (6b)

and the alkoxy radical OCH2CH2OH may decompose to yield formaldehyde or react with

O2 to yield hydroxyacetaldehyde (this radical cannot isomerize):

•

O CH2CH2OH HCHO + CH2OH (7a)

(followed by CH2OH + O2 HO2 + HCHO)


•

O CH2CH2OH + O2 HO2 + CH2OHCHO (7b)

The branching ratio k7a/k7b is not known. This branching ratio is unimportant if

Pathway 4a accounts for nearly all of the overall ethanol-OH reaction, but becomes

important if Reaction 5 accounts for 15-30% of the overall reaction. Therefore, the key

parameter needed to describe the overall ethanol-OH reaction is the measured yield of

acetaldehyde.

The mechanism of the reaction of OH with saturated compounds is illustrated in

Figure 3-1 for nopinone, the saturated ketone which forms as a major product of the

oxidation of β-pinene by OH, O3 and NO3. The initial step may involve five reaction

sites, i.e., H-atom abstraction from the two —CH groups (major) and from the three —

CH2 groups (minor). The five alkyl radicals thus formed react with O2, and the

corresponding peroxy radicals RO2 react with NO to form alkoxy radicals RO (major)

and alkyl nitrates RONO2 (minor). The five alkoxy radicals are shown in Figure 3-1 (the

five alkyl nitrates are omitted for clarity). Further reactions are outlined for only one

alkoxy radical. This alkoxy radical cannot react with oxygen (there is no abstractable H

atom at the C-O carbon). It decomposes via C-C bond scission (2 pathways), leading to

two alkyl radicals. The alkyl radicals react according to R + O2 RO2 and RO2 + NO

RO (we omit again the two alkyl nitrates RONO2 for clarity). The two alkoxy

radicals may decompose, yielding formaldehyde, acetone, and two alkyl radicals, which, in

turn, yield two alkoxy radicals whose subsequent reactions are not shown in Figure 3-1.

Figure 3-1 illustrates an important aspect of the atmospheric chemistry of

biogenic compounds: even for a simple compound such as nopinone, numerous oxidation

pathways are possible, and experimental studies of reaction products are required to

establish which pathways are important. In turn, knowledge of the important pathways

and product yields is necessary input to computer mechanisms that describe the

atmospheric chemistry of biogenic compounds.


Figure 3-1. Reaction mechanism of OH with nopinone (u.d. = unimolecular

decomposition).

O

O•O

O

•O

O

•O

O

•O

O

OO

O

•

OO2, NO O2, NO

OO

O Ou.d. u.d.

OO

HCHO +

O O

O2, NO O2, NO

+ CH3C(O)CH3

OO

O O

other products other products

OH, O2, NO

u.d.

O

O•

•

•CH2

CH2

•O

•

•O

•O

•


3.5.2 The reaction of OH with aldehydes: peroxyacyl nitrates

An important group of reactions between OH and saturated compounds is that

involving the reaction of OH with saturated aldehydes. In this case the major, if not

exclusive, initial pathway involves H-atom abstraction from the carbonyl carbon:

RCHO + OH H2O + RCO (8)

followed by:

RCO + O2 RCO3 (9)

These reactions are followed by the competing reactions of RCO3 with NO and

with NO2. The reaction of RCO3 with NO2 leads to a peroxyacyl nitrate, and the reaction

of RCO3 with NO leads to CO2, NO2 and an alkyl radical:

RCO3 + NO2 RC(O)OONO2 (10a)

RCO3 + NO NO2 + CO2 + R (10b)

Reactions 8, 9 and 10a describe the formation of PAN from acetaldehyde, MPAN

from methacrolein (a product of isoprene), n-C5H11C(O)OONO2 from hexanal (Grosjean

et al., 1996a) and “αP-PAN” from pinonaldehyde (Nozière and Barnes, 1998). Reactions

8, 9, and 10a also describe the formation of peroxyacyl nitrates that have yet to be

characterized, e.g., those that possibly form from 3-hydroxypropanal (a product of cis-3-

hexen-1-ol) or from caronaldehyde (a product of ∆3-carene):

cis-3-hexen-1-ol HOCH2CH2CHO HOCH2CH2C(O)OONO2 (11)

∆3-carene CH3C(O)CH2 CH2CHO

CH3C(O)CH2 CH2C(O)OONO2 (12)


In the presence of NOx, and when NO2 > NO, the peroxyacyl nitrates PAN,

MPAN, n-C5H11C(O)OONO2 and "αP-PAN" are major products of the reaction of OH

with acetaldehyde, methacrolein, hexanal, and pinonaldehyde, respectively. In the same

way, peroxyacyl nitrates such as those shown in Reactions 11 and 12 above are expected

to form from the many carbonyls that are first-generation and second-generation products

of biogenic compounds.

The reaction of the RCO3 radicals with NO (Reaction 10b) and the thermal

decomposition of the peroxyacyl nitrates (the reverse of Equilibrium 10a followed by

Reaction 10b) lead to products that includes Cn-1 aldehydes, where n is the number of

carbon atoms in RCO3. Reaction 10b is followed by:

R + O2 RO2 (2)

RO2 + NO RO + NO2 (3a)

RO2 + NO RONO2 (3b)

RO + O2 HO2 + carbonyl (analogous to 7b)

RO decomposition products including carbonyls (analogous to 7a)

This sequence of reactions initiates a “cascade effect”, i.e., Cn carbonyls

Cn-1 carbonyls + other products, Cn-1 carbonyls Cn-2 carbonyls + other products,

and so on. When NO2 > NO, this sequence also includes the formation of the Cn-1, Cn-2,

etc. peroxyacyl nitrates. For example, hexanal leads to n-C5H11CO3, which leads to n-

C5H11C(O)OONO2 when NO2 > NO and to pentanal when NO > NO2. The peroxyacyl

nitrate n-C5H11C(O)OONO2 decomposes to pentanal, whose reaction with OH leads to n-

C4H9CO3, to n-C4H9C(O)OONO2 when NO2 > NO, and to butanal when NO > NO2, etc.

(Grosjean et al., 1994a, 1996a). For higher molecular weight carbonyls, and taking

pinonaldehyde as an example, the sequence of reactions outlined above leads to

pinonaldehyde from α-pinene, then to the Cn-1 carbonyl nor-pinonaldehyde (which was

tentatively identified by Calogirou et al., 1999, but was not detected by Nozière et al.,

1999a) and to the corresponding peroxyacyl nitrate:


CH3C(O)— —CHO CH3C(O)— —C(O)OONO2 (13)

The relevant sequence of reactions is shown in Figure 3-2. Peroxyacyl nitrates are

important reservoirs for the long-range transport of NOx. The high MW peroxyacyl

nitrates that form in the oxidation of terpenes should be studied in the laboratory and in

the atmosphere.

The simplest peroxyacyl nitrate, i.e., peroxyacetyl nitrate (PAN,

CH3C(O)OONO2), is a reaction product of the atmospheric oxidation of isoprene, several

terpenes, and other biogenic compounds in the presence of NOx (Grosjean et al., 1992,

Grosjean, 1995). Reissell et al. (1999) have measured the formation yields of acetone,

which is a product of the reactions of OH (see Table 3-8) and O3 (see Table 3-9) with

many terpenes. Acetone is also a product of the oxidation of other biogenic compounds,

e.g., 2-methyl-3-buten-2-ol (Fantechi et al., 1999). The oxidation of acetone leads to

PAN (e.g., Carter, 1995), and therefore the oxidation of biogenic compounds may play an

important role in global tropospheric PAN formation.

3.5.3 The reaction of OH with unsaturated compounds

The OH radical reacts with unsaturated compounds mainly by addition at the C=C

bond. The addition of OH at the C=C bond yields two β-hydroxyalkyl radicals, which,

under atmospheric conditions, react with O2 to form β-hydroxyperoxy radicals:

RCH=CH2 + OH R•

C HCH2OH (14a)

RCH(OH)•

C H2 (14b)

R•

C HCH2OH + O2 RCH(O2)CH2OH (15)

RCH(OH)•

C H2 + O2 RCH(OH)CH2O2 (16)

Each peroxy radical reacts with NO to form NO2 + a β-hydroxyalkoxy radical

(major pathway) and a β-hydroxynitrate (minor pathway):


others

Figure 3-2. Reaction mechanism of OH with pinonaldehyde: the formation of Cn

peroxyacyl nitrate (A), Cn-1 aldehyde (B), and Cn-1 peroxyacyl nitrate (C).

CH3C(O)

OH

CH3C(O)

O2

CH2CO

CH3C(O)

NO

CH3C(O) CH2 + CO2 + NO2

CH2CO3 CH3C(O) CH2C(O)OONO2

O2, NO

CH3C(O) CH2O CH3C(O) + HCHO u.d.

O2

HO2 + CH3C(O) CHO

OH

CH3C(O) CO

O2

CH3C(O) CO3

CH2CHO

NO2

CH3C(O) C(O)OONO2

CH3C(O) + CO2 + NO2

NO

C

B

A

NO2

•

•

•

•

•

•


RCH(O2)CH2OH + NO NO2 + RCH(•

O )CH2OH (17a)

RCH(ONO2)CH2OH (17b)

RCH(OH)CH2O2 + NO NO2 + RCH(OH)CH2

•

O (18a)

RCH(OH)CH2ONO2 (18b)

The two alkoxy radicals formed in Reactions 17a and 18a may decompose,

isomerize, and react with O2. The relative importance of these pathways is a function of

the structure of the unsaturated compound. This is illustrated in Figure 3-3 for the

reaction of OH with cis-3-hexen-1-ol, CH3CH2CH=CHCH2CH2OH. In this case the two

β-hydroxyalkoxy radicals are CH3CH2CH(OH)CH(•

O )CH2CH2OH and

CH3CH2CH(•

O )CH(OH)CH2CH2OH, for which all three pathways of decomposition,

isomerization and reaction with O2 are possible. The products identified experimentally

are propanal (yield = 0.75) and 3-hydroxypropanal (yield = 0.48), along with smaller

amounts of 3,4-dihydroxyhexanal and hydroxynitrates (Aschmann et al., 1997). The

experimental results indicate that decomposition is the dominant pathway for both β-

hydroxyalkoxy radicals (leading to propanal and 3-hydroxypropanal), that the alkoxy

radical CH3CH2CH(•

O )CH(OH)CH2CH2OH also isomerizes (leading to 3,4-

dihydroxyhexanal) and that reaction with O2 is negligible for both alkoxy radicals. We

note that alkoxy radical isomerization following reaction of OH with unsaturated

compounds leads to polyfunctional oxygenates, e.g., the reaction of cis-3-hexen-1-ol with

OH leads to the dihydroxycarbonyl 3,4-dihydroxyhexanal. A number of first-generation

products of biogenic compounds are unsaturated dicarbonyls, hydroxycarbonyls, etc. (see

Tables 3-2 and 3-3) whose reactions with OH are expected to yield dihydroxydicarbonyls,

trihydroxycarbonyls, and other polar, water-soluble products.


Figure 3-3. Reaction mechanism of OH with cis-3-hexen-1-ol (isom. = isomerization,

u.d. = unimolecular decomposition).

CH3CH2 CH2CH2OH

C = C

H H

OH

CH3CH2CH HCH2CH2OH + CH3CH2 HCHCH2CH2OH•C

OHOH

O2, NO O2, NO

CH3CH2CH(OH)CHCH2CH2OH CH3CH2CHCH(OH)CH2CH2OH

•O

•O

Isom. + O2 u.d.

CH3CH2 HOH + HCCH2CH2OH

O2 HO2

CH3CH2CHO

HO2 + CH3CH2CH(OH)CCH2CH2OH

H2CH2CH(OH)CH(OH)CH2CH2OH

O2, NO

CH2CH2CH(OH)CH(OH)CH2CH2OH

isom.

HOCH2CH2CH(OH) (OH)CH2CH2OH

O2

HO2 + HOCH2CH2CH(OH)CCH2CH2OH

O

O

•C

•

•C

O

•C

O


3.5.4 The reaction of O3 with unsaturated compounds

O3 reacts with unsaturated compounds by addition at the C=C bond. The initial

adduct decomposes into two carbonyls and two carboxy oxide biradicals:

O3 + R1R2C=CR3R4 R1R2C CR3R4 (1,2,3-trioxolane) (19)

1,2,3-trioxolane α (R1C(O)R2 + R3

•

C (R4)O•

O ) + (1-α) (R3C(O)R4 + R1

•

C (R2)O•

O ) (20)

where the coefficient α in Reaction 20 varies with the number and nature of the

substituents R1, R2, R3, R4. The mechanism summarized by Reaction 20 has been shown

to apply to the reaction of O3 with unsaturated hydrocarbons (including the alkenes and

terpenes listed in Table 3-1) and to the reaction of O3 with unsaturated aliphatic

oxygenates including alcohols, ethers, esters and carbonyls (e.g., Grosjean and Grosjean,

1997). Thus, the reaction of O3 with the unsaturated oxygenates listed in Table 3-1 and

with first- and second-generation unsaturated oxidation products can be described in a

manner similar to that summarized in Reactions 19 and 20:

O3 + R1R2C=CR3X α (R1C(O)R2 + R3

•

C (X)O•

O ) + (1 - α) (R3C(O)X + R1

•

C (R2)O•

O ) (21)

where R1, R2 and R3 are alkyl substituents and X is the substituent that bear the oxygen-

containing functional group.

Shown in Figure 3-4 as an example is the mechanism for the reaction of O3 with

the unsaturated ester trans-2-hexenyl acetate. The two primary carbonyl products are n-

butanal and 2-oxoethyl acetate, for which the sum of the measured formation yields, 1.05

± 0.16 (Grosjean et al., 1996b), is close to the value of [α + (1 - α)] = 1.0 that is

consistent with Reaction 21. The other carbonyl products identified (see Table 3-9) are

O O

O


Figure 3-4. Reaction mechanism of O3 with trans-2-hexenyl acetate.

CH3C(O)OCH2 H

C = C

H CH2CH2CH3

O3

CH3C(O)OCH2

C C

H

O O

O

H

CH2CH2CH3

(1-α)(α)

CH3C(O)OCH2CHO

+ CH3CH2CH2 HO •C

•O

HCO + CH3CH2 HOH•C

CH3CH2CH2COOH

(CH3CH2CH=CH(OOH))*

(CH3CH2CHOHCHO)*

CH3CH2CHOHCHO

O2 HO2

CH3CH2CHO

CH3CH2CH2CHO

+ CH3C(O)OCH2 HO •C

HCHO + CH3C(O)O HOH•C

CH3C(O)OCH2COOH

(CH3C(O)OCH=CH(OOH))*

(CH3C(O)OCHOHCHO)*

CH3C(O)OCHOHCHO

O2 HO2

CH3C(O)OCHO

•O


formed in subsequent reactions of the biradicals R3

•

C (X)O•

O and R1

•

C (R2)O•

O as is

shown in Figure 3-4.

Carboxylic acids constitute an important category of products of the reaction of O3

with unsaturated biogenic compounds. Carboxylic acids form from monosubstituted

biradicals, e.g., for R2 and R3 = H in Reaction 21 above:

R1

•

C CHO•

O R1C(O)OH (22a)

X•

C HO•

O XC(O)OH (22b)

There is only limited information on carboxylic acid formation in the reaction of O3

with unsaturated biogenic compounds. Carboxylic acids have been identified (and their

formation yields measured in a few instances) for only a few compounds, e.g., formic acid

and acetic acid from simple alkenes (see Table 3-9), several saturated and unsaturated

acids from isoprene (pyruvic, acrylic, and methacrylic, see Section 3.4.2) and several

monocarboxylic and dicarboxylic acids from α-pinene (pinic, pinonic, see Section 3.4.5).

The formation of these acids is consistent with the mechanism summarized by Reactions

20-22. This mechanism also predicts, for example, the formation of propionic acid and 3-

hydroxypropionoic from cis-3-hexen-1-ol, and of butyric acid and CH3C(O)OCH2COOH

from trans-2-hexenyl acetate (see Figure 3-4):

cis-3-hexen-1-ol + O3 α CH3CH2

•

C HO•

O + (1 - α) CH2OHCH2

•

C HO•

O (23a)

CH3CH2

•

C HO•

O CH3CH2COOH (23b)

CH2OHCH2

•

C HO•

O CH2OHCH2COOH (23c)

trans-2-hexenyl acetate + O3 α CH3CH2CH2

•

C HO•

O + (1 - α) CH3C(O)OCH2

•

C HO•

O (24a)

CH3CH2CH2

•

C HO•

O CH3CH2CH2COOH (24b)

CH3C(O)OCH2

•

C HO•

O CH3C(O)OCH2COOH (24c)


In the same way, ketoacids are among the expected products of the reaction of O3

with terpenes that bear an internal C=C bond (e.g., pinonic acid from α-pinene), and the

two unsaturated acids (CH3)2C=CH(CH2)2C(OH)(CH3)COOH and

CH2=CHC(CH3)(OH)CH2CH2COOH are among the expected products of the reaction of

O3 with linalool. Monocarboxylic acids (generally present in the gas phase) and

dicarboxylic acids (typically present in aerosols) are ubiquitous in ambient air, and their

formation by reaction of O3 is predicted from mechanistic considerations. Thus, the lack

of data on carboxylic acids in laboratory studies is a major gap in our understanding of the

atmospheric oxidation of biogenic compounds, especially with regard to the abundance

and role of higher molecular weight acids in atmospheric aerosols.

3.5.5 The reaction of NO3 with unsaturated compounds

While the mechanisms of the reactions of OH and O3 with unsaturated compounds

are reasonably well understood, much less is known regarding the mechanism of the

reaction of these compounds with NO3. The initial steps are similar to those for the

reaction of OH, i.e., addition at the C=C bond followed by reaction of the alkyl radical

with O2:

RCH=CH2 + NO3 R•

C CHCH2ONO2 (25a)

RCH(ONO2)•

C H2 (25b)

R•

C HCH2ONO2 + O2 RCH(O2)CH2ONO2 (26a)

RCH(ONO2)•

C H2 + O2 RCH(ONO2)CH2O2 (26b)

For NO3 to be present, NO must be very low, and as a result the peroxy radicals

formed in Reactions 26a and 26b recombine, i.e., RO2 + RO2 2RO + O2 and RO2 +

RO2 --> aldehyde + alcohol + O2. The alkoxy radicals RO may decompose, isomerize,

or react with O2 as described earlier, thus leading to a number of carbonyls (RO

decomposition), nitroxycarbonyls (RO reaction with O2) and other products. The

mechanism of the reaction of NO3 with unsaturated compounds is illustrated in Figure 3-5,


Figure 3-5. Reaction mechanism of NO3 with 2-methyl-3-buten-2-ol (u.d. =

unimolecular decomposition).

CH3C - CH=CH2 + NO3

OH

CH3

CH3C - HCH2(ONO2)

OH

CH3

O2

CH3CCH(OO)CH2(ONO2)

OH

CH3

CH3CCH(ONO2) H2

OH

CH3

•C

O2

CH3CCH(ONO2)CH2O2

OH

CH3A B

A A+ O2 + 2 CH3C CHCH2(ONO2)

OH•O

CH3

C

OH + HCCH2ONO2

O

CH3

CH3

u.d. O2

O2

HO2 + CH3C(O)CH3

HO2 + CH3C CCH2(ONO2)

OH

CH3

O

NO2

CH3C CHCH2(ONO2)

OH

CH3

ONO2

B + O2 + 2 CH3CCH(ONO2)CH2

u.d., + O2, + NO2, similar to above.

B

OH

CH3

D

C

•O

•C

•C


using 2-methyl-3-buten-2-ol as an example. We emphasize again that information on

products and their yields is limited (see Table 3-10) and that as a result our understanding

of the NO3-unsaturated compound reaction mechanism under atmospheric conditions is

less than adequate at the present time.


4. SECONDARY ORGANIC AEROSOL GAS/PARTICLE PARTITION

4.1 Introduction

Secondary organic aerosols (SOA) are formed in the atmosphere via the oxidation

of anthropogenic and biogenic volatile organic compounds (VOC). Since SOA can

constitute a significant fraction of atmospheric particulate matter (PM) (e.g., Strader et al.,

1999; Turpin and Huntzicker, 1995), it is essential to simulate SOA formation in PM air

quality models. We present here an overview of the current status of SOA modules.

First, we summarize the three basic theories used to describe the partition of organic

compounds between the gas phase and the particulate phase. Next, we review SOA

modules currently used in three-dimensional (3-D) air quality models. Finally, we briefly

describe two SOA modules that are currently under development and will incorporate the

state-of-the-science in their formulations.

4.2 SOA Partition Theories

Saturation Theory. The basic assumption behind the saturation theory (Grosjean

and Seinfeld, 1989; Pandis et al., 1992) is that the capacity of air to hold a condensable

compound is limited. This capacity is determined by the saturation vapor pressure, which

is the density of a gas above a pure liquid of the condensable compound at the same

temperature. If the gas-phase concentration is above the saturation vapor pressure,

thermodynamics dictates that the excess must be transferred to another phase, in this case,

the particulate phase. A liquid secondary organic aerosol (SOA) phase is consistent with

the saturation theory as it is currently formulated, although the physical state of the

particulate phase is not specified in most modeling applications. The condensable

compound exists in the particulate phase if and only if the total concentration exceeds the

saturation vapor pressure. In this case, the gas-phase concentration is determined by the

saturation vapor pressure, and the particulate-phase concentration is the difference

between the total concentration and the saturation vapor pressure. If the gas-phase

concentration falls below the saturation vapor pressure, the condensable species must


evaporate from the particulate phase until the gas phase saturates or until there is no more

of the condensable species in the particulate phase.

The key shortcoming of the saturation theory is that it does not allow for the

partitioning that occurs when the gas-phase concentration is below saturation vapor

pressure. A variation of the saturation theory, in which the saturation vapor pressure is

set to zero, is used in many model applications. This assumption results in immediate

formation of a stoichiometric amount of SOA upon the reaction of precursors, and is

referred to as the fixed yield approach.

Absorption Theory. Pankow (1994a, b) proposed that the absorption of a

condensable compound i into the particulate phase is characterized by an equilibrium

constant Ki:

i

i

i G

MA

K 0= , (4-1)

where Ai is the mass concentration of i in the particulate phase, M0 is the mass

concentration of the absorbing organic phase, and Gi is the mass concentration in the gas

phase, all expressed in µg/m3 of air. The key feature of this method is that it calculates the

yield as a function of the organic material already present in the particulate phase. As a

result, an organic compound can enter the particulate phase even when the gas-phase

concentration is below saturation. This equation forms the basis for the work of several

recent studies including those of Odum et al. (1997), Strader et al. (1998), and Griffin et

al. (1999). The key differences between the approach used by Odum, Griffin and co-

workers, and that used by Strader et al. lie in the determination of Ki and in the

precursor/product species that are modeled.

Aqueous dissolution. Water-soluble organic compounds have been identified in

atmospheric PM (Saxena and Hildemann, 1996, and references therein). The partition of

hydrophilic compounds between the gas phase and aqueous particles is governed by

Henry’s law, which relates the partial pressure of the solute to its aqueous-phase

concentration. The dissolution of a strong acid or base is enhanced by the formation of

ions at favorable pH. Meng et al. (1995) and Jacobson (1997) modeled the dissolution


behavior of several soluble organic species within the same framework as the inorganic

aerosols: Meng et al. (1995) modeled formic acid and acetic acid, and Jacobson (1997)

modeled these acids plus formaldehyde, methylhydroperoxide, peroxyacetic acid, and

nitrocresol. A major hurdle to modeling the aqueous dissolution of organic compounds is

the difficulty of treating the activity of organic molecules and ions. The standard activity

coefficient methods of the inorganic modules require a wealth of experimental data to

describe the interactions between the ions. The data base for organic ions is quite limited

compared to the inorganic ions. In addition, these methods typically do not account for

the interactions between molecular and ionic components. One assumption made in both

Meng et al. (1995) and Jacobson (1997) is that the organic compounds do not affect the

water content of PM. This assumption may be reasonable given the low solubilities of the

compounds modeled in these studies. However, in general, the effects of organics on the

liquid water content of PM cannot be ignored (e.g., Saxena et al., 1995).

4.3 Review of Existing Secondary Organic Aerosol Modules

We present here an overview of the SOA modules currently used in four

applications-oriented air quality models: Models-3, DAQM2, SAQM-AERO, and UAM-

AERO.

Models-3. Two options are available to simulate SOA formation in Models-3

(EPA, 1999). The first option uses the fixed aerosol yield approach. Organic aerosol

formation is quantified by the amount (µg m-3) of aerosol produced per ppm of organic gas

reacted with hydroxy radicals (OH), ozone (O3) , or nitrate radicals (NO3). Aerosol

production is assumed to occur from reactions involving five different generic organic

groups: (1) long-chain alkanes, (2) alkyl-substituted benzenes such as toluene and xylenes,

(3) cresol and phenols, (4) long-chain olefins, and (5) monoterpenes. The formation rates

of aerosol mass in terms of the reaction rates of the precursors are obtained from Pandis et

al. (1992). These yield factors are given in Table 4-1.

The alternative option is based on an aerosol absorption model following Odum et

al. (1996). Odum et al. (1996) reformulated the absorption equation as follows:


Table 4-1. Aerosol yields in terms of amount of precursor reacted (Pandis et al.,

1992).

Parent Compound Aerosol Yield (µµg m-3 ppm-1)

C8 and Higher Alkanes 380

Anthropogenic Internal Alkenes 247

Monoterpenes 740

Toluene 424

Xylenes 342

Cresol 221


∑

+=

i iom

iomi

MK

KMY

0,

,0 1

α(4-2)

where Y is the measured aerosol yield (defined as the mass of SOA produced per mass of

precursor reacted), αi is the mass-based stoichiometric coefficient for the reaction

generating the condensable product, and Kom,i is the partition coefficient in m3/µg. This

equation was fitted to data obtained in smog chamber experiments to determine

empirically the yield and partition parameters of the model. Using a two-product

assumption, Odum et al. (1996) applied the theory successfully to model SOA formation

from xylenes, 1,2,4-trimethylbenzene, and α-pinene in a smog chamber. The fitted

parameters, shown in Table 4-2, are used in the Models-3 implementation. The

condensable product partitions onto an existing organic aerosol phase containing both

primary and secondary compounds. Note that the Models-3 implementation is based on

the rate of precursor reaction within each model grid cell at each time step. Since each

counter species is set back to zero after the partition function calculation, the gaseous

condensable species does not accumulate over time or space. Moreover, the secondary

compounds in the particulate phase cannot partition back to the gas phase. Therefore, the

Models-3 implementation consists in an irreversible process that is not consistent with the

thermodynamic equilibrium basis of the smog chamber data of Odum et al. (1997).

DAQM2. DAQM2 offers two options to simulate SOA formation. These two

SOA modules are formulated very similarly to those in Models-3. They are the fixed yield

approach (Table 4-1) and the irreversible absorption approach (Table 4-2).

SAQM-AERO. The SOA module is based on the fixed yield approach. Precursor

species include long-chain alkanes, olefins, aromatics, cresols and other phenols. The

fixed yields for SOA formation were derived from Pandis et al. (1992).

UAM-AERO. Strader et al. (1998) implemented the absorption theory in UAM-

AERO under CRC sponsorship. However, their approach differs from that of Odum and

coworkers. Strader et al. invoked Raoult’s law in the determination of the partition

coefficients. Raoult’s law describes the equilibrium partition of an ideal gas-ideal solution

system:


Table 4-2. Aerosol yield parameters used in Models-3 (Odum et al., 1996; Figures 1,

2, 4).

Parent Compound αα1 αα2 Kom,1 (m3 µµg-1) Kom,2 (m

3 µµg-1)

m-xylene 0.03 0.167 0.032 0.0019

1,2,4-trimethylbenzene 0.0324 0.166 0.053 0.002

α-pinene 0.038 0.326 0.171 0.004


satiii PxPy = (4-3)

where yiP is the partial pressure of compound i in the gas phase, xi is the mole fraction of i

in the organic liquid phase, and Pisat is the saturation vapor pressure of i at the system

temperature. Converting the units from pressure to mass concentration in the gas phase,

.,satgasii

gasi cxc = (4-4)

Instead of the equilibrium constant Ki, the partition of compound i is calculated via

the saturation concentration, cigas,sat, of i in the gas phase. Since the identities of the

condensing compounds are frequently unknown in the modeling of SOA, cigas,sat were

chosen based on smog chamber results and laboratory experiments on the temperature

dependence of the saturation concentrations. Strader et al.’s methodology provides the

framework for incorporating mechanistic information of the condensing species when such

information becomes available. At present, however, it relies on empirical data to

determine the partition parameters.

Strader et al. (1998) used a model with six condensable products (6 partition

parameters): one from alkane precursors (also for alkenes, benzaldehyde, phenol, cresol,

and nitrophenol), three from aromatic precursors, and two from monoterpenes. Each

precursor forms one or two condensable products, resulting in 14 yield parameters and 6

saturation concentration parameters used in the model (Table 4-3). According to the

authors, the stoichiometric parameters (of the condensable species) were determined from

available experimental data, although the data base was not identified.

Griffin/Odum Approach. The SOA module implemented by AER under EPRI

sponsorship calculates the equilibrium partition of condensable organic products based on

the results of the smog chamber experiments of Odum et al. (1997) and Griffin et al.

(1999). In Odum et al. (1997), most of the aromatic compounds were fitted by two sets

of parameters (each set contains 2 stoichiometric parameters and two partition parameters,

see Table 4-4), with the more highly substituted aromatics producing less SOA. Griffin et

al. (1999) analyzed the formation of SOA from biogenic compounds


Table 4-3. Aerosol yield parameters and saturation concentrations in UAM-AERO

(Strader et al., 1998, Table 2-4).

ParentCompound

CondensableSpecies

Aerosol Yield(µµg m-3

ppm-1)Saturation

Concentration (µµg m-3)

ALK1 SOA4 1.9 0.007

ALK2 SOA4 131 0.007

ARO1 SOA1 430 0.023

SOA2 836 0.572

ARO2 SOA1 268 0.023

SOA3 1178 0.776

OLE1 SOA4 9.2 0.007

OLE2 SOA4 19 0.007

OLE3 SOA5 749 0.008

SOA6 0.1 0.008

BALD SOA4 5 0.007

PHEN SOA4 192 0.007

CRES SOA4 221 0.007

NPHE SOA4 285 0.007

Table 4-4. Aerosol yield parameters for the oxidation of aromatic compounds (Odum

et al., 1997, Figure 1).


3 µµg-1)

High Yield Aromatics(1) 0.071 0.138 0.053 0.0019

Low Yield Aromatics(2) 0.038 0.167 0.042 0.0014

(1) e.g., Toluene, ethyl-benzene, ethyl-toluene, n-propyl-benzene

(2) e.g., Xylenes, trimethyl-benzenes, dimethylethyl-benzenes, tetramethyl-benzenes


using the same framework. Parameters for 34 products from 12 biogenic precursors were

determined experimentally (see Table 4-5). The AER SOA module is designed to treat all

38 species. Following Bowman et al. (1997), the partitioning compounds form part of the

absorbing phase, so that the partition constant is defined as:

( )i

k kiniti

i G

AMAK

∑+= (4-5)

where Minit is the initially-present organic absorbing mass (non-volatile). The model

receives external input of the total amount of available (gas + particulate) condensables

and solves simultaneous equations of the form of Equation (4-5) to determine the partition

of each condensable compound between the gas and particulate phases.

4.4 Description of Secondary Organic Aerosol Modules under Development

AER/Caltech/EPRI Model. AER, in collaboration with John Seinfeld and

coworkers at Caltech, is developing under EPRI sponsorship an organic aerosol module

that combines the absorption and aqueous dissolution theories by modeling two types of

condensable compounds: (A) hydrophilic compounds and (B) hydrophobic compounds

(Pun and Seigneur, 1999). Hydrophilic compounds are assumed to enter aqueous

particles that may also contain inorganic species. Hydrophobic compounds are absorbed

into liquid organic PM. Each type of compounds is represented by surrogate species that

are known to exist in the atmosphere.

Due to the lack of activity data and suitable activity coefficient models for the

interactions between the surrogate Type A molecules, organic ions, and inorganic ions,

molecular-level interactions between the organic compounds and inorganic compounds are

not currently represented in the AER model. Instead, the inorganic module and the Type

A organic module interact via the effects on pH and the liquid water content of the

aqueous particles. Within the Type A module, the Henry’s law and acid dissociation

relationships for the organic compounds are satisfied. The activity coefficients for the


Table 4-5. Aerosol yield parameters for the oxidation of biogenic organic compounds

(Griffin et al., 1999, Tables 3 and 4).


3 µµg-1)

OH Reactions

∆3-Carene 0.054 0.517 0.043 0.0042

β-Caryophyllene 1.00 -- 0.0416 --

α-Humulene 1.00 -- 0.0501 --

Limonene 0.239 0.363 0.055 0.0053

Linalool 0.073 0.053 0.049 0.0210

Ocimene 0.045 0.149 0.174 0.0041

α-Pinene 0.038 0.326 0.171 0.0040

β-Pinene 0.130 0.406 0.044 0.0049

Sabinene 0.067 0.399 0.258 0.0038

α- & γ-Terpinene 0.091 0.367 0.081 0.0046

Terpinen-4-ol 0.049 0.063 0.159 0.0045

Terpinolene 0.046 0.034 0.185 0.0024

O3 Reactions

∆3-Carene 0.128 0.068 0.337 0.0036

α-Pinene 0.125 0.102 0.088 0.0788

β-Pinene 0.026 0.485 0.195 0.0030

Sabinene 0.037 0.239 0.819 0.0001

NO3 Reactions

∆3-Carene 0.743 0.257 0.0088 0.0091

β-Pinene 1.000 -- 0.0163 --

Sabinene 1.000 -- 0.0115 --


molecules and water are determined using the group contribution method UNIFAC. Since

acidic compounds are currently chosen as surrogate Type A compounds, activity

coefficients must also be determined for the ions; currently the activity coefficients of the

molecules are also applied to the organic ions. The amount of water associated with the

organic compounds is estimated using UNIFAC, given the relative humidity of the system.

Type B organic compounds are modeled according to the absorption theory

described above, where the partition coefficient is determined based on the characteristics

of the condensable compound and the absorbing medium (Pankow, 1994a, b).

omisat

i

om

i

i

MWP

fTR

G

TSPAPankowK

)(10

760)(

6 γ

== (4-6)

where TSP is the total suspended particulate matter (µg/m3), R is the gas constant (m3

atm/mol/K), T is temperature (K), fom is the fraction of the TSP assumed to be absorbing,

Pisat is the saturation vapor pressure of the condensing compound (torr), γi is the liquid

phase activity coefficient in mole fraction scale, and MWom is the molecular weight of the

liquid phase. Both γi and MWom depend on an assumed composition of the organic phase,

which contains both condensable compounds and representative non-volatile primary

compounds identified in ambient PM samples. From the assumed composition, γi is

determined using the UNIFAC method.

The modeling framework has been developed using three surrogate compounds

(Table 4-6). It is now being expanded to include additional compounds and will be

coupled with a gas-phase chemical mechanism for SOA formation developed by Griffin

and coworkers at Caltech.

Caltech/AER/ARB Model. Under the sponsorship of ARB, Caltech (John Seinfeld,

PI) and AER are developing a SOA model based on the entire set of smog chamber

experiments conducted at Caltech. These experiments include those already reported in

the literature (e.g., Odum et al., 1997; Griffin et al., 1999) as well as more recent

experiments conducted at higher humidities. An attempt will be made to reconcile

fundamental mechanistic descriptions of SOA formation with experimental data.


Table 4-6. Surrogate compounds in AER module (Pun and Seigneur, 1999).

AerosolType

Surrogate Species MolecularFormula

Characteristics

A Malic Acid COOH-CHOH-CH2-COOH

Deliquescent at 79% relativehumidity; dissociates twice;highly soluble

A Glyoxalic Acid COH-COOH Miscible with water; dissociatesonce; very volatile

B Octadecanoic Acid C17H35COOH Not soluble in water


5. KNOWLEDGE GAPS AND RECOMMENDATIONS

FOR FUTURE WORK

5.1 BVOC Emissions

For isoprene, we have a relatively complete understanding of the emission

mechanism, a good basis for predicting leaf-level emissions as a function of current

temperature and light conditions, and considerable experience making isoprene emission

measurements over a variety of scales. Remaining gaps in our knowledge concerning

isoprene emissions from vegetation include:

• Lack of well defined emission factors for vegetation types other than oaks,

poplars, aspen, and spruce;

• Lack of a well established quantitative description of the effect of temperature

and light history upon isoprene emission factors;

• Lack of quantitative seasonal descriptions of isoprene emissions in a manner

suitable for inclusion in regional emission inventories.

For monoterpenes, the number of compounds emitted by vegetation has

complicated attempts to develop separate emission factors for individual monoterpene

compounds. We seem to have a reasonable basis for predicting monoterpene emissions as

a function of temperature, although uncertainties about the temperature coefficient remain.

Gaps in our understanding for monoterpene emissions include:

• Lack of well established emission factors for specific compounds for specific

vegetation species;

• Lack of quantitative information of the effects of wounding and stress upon

emission rates;

• Lack of experimental scale-up of emissions for dominant ecosystems to

confirm consistency between leaf-level emission factors and canopy or larger

scale ecosystem fluxes of monoterpenes;


• Lack of identification of light dependent monoterpene emitters in U.S.

ecosystems;

• Need for establishment of seasonal emission patterns for inclusion in regional

emission inventories;

• Lack of information concerning canopy escape efficiencies for specific

monoterpenes for the dominant U.S. terpene-emitting ecosystems.

For sesquiterpenes, we know very little about emissions. There have only been a

handful of measurements of sesquiterpene emissions, and no information is available

concerning emission rate dependence upon temperature or other environmental factors.

Because of the similarity in structure and formation mechanisms to monoterpenes, it is

assumed that sesquiterpene emissions will be similar in pattern to monoterpene emissions

with suitable modification to account for differences in molecular weights. However, this

assumption is unsubstantiated. Specific gaps in our understanding include:

• Lack of quantitative emission factors for sesquiterpene class from specific

vegetation types;

• Lack of identity of dominant sesquiterpene compounds for dominant

ecosystems;

• Lack of confirmation of the emission relationship to temperature or other

environmental factors;

• No experimental scale-up of emissions for dominant ecosystems;

• No information related to seasonal emission patterns.

Compared to sesquiterpenes, there is considerably more information related to

oxygenated BVOC, although the overall confidence in estimating emissions of oxygenated

BVOC is limited. Specific gaps in our understanding of oxygenated BVOC emissions

include the following:

• The emission factors for MBO for dominant vegetation species have not been

widely confirmed;


• The dependence of MBO emissions upon light and temperature has not been

clearly established, although it appears to mimic that of isoprene;

• The emission factors for specific compounds of the hexene family and other

compounds for specific vegetation species are not well established;

• There is no method available to account for emissions due to intermittent

wounding or harvesting events;

• There has been no experimental scale-up of emissions for dominant

ecosystems;

• Seasonal emission patterns have not been identified.

We have made considerable progress in our ability to make quantitative emission

measurements using methods applicable at all scales of interest. Further progress is

needed in the development and application of these methods to a wider range of

compounds and for a wider range of ecosystems. These include:

• Development and application of REA methods for monoterpenes,

sesquiterpenes, and oxygenated BVOC for dominant ecosystems throughout

the growing season;

• Confirmation of leaf and canopy-scale emissions through further use of aircraft

and balloon surveys to establish landscape-scale emission patterns;

• Continued development of fast analytical methods to allow greater use of eddy

covariance methods for emission measurements.

There have been only a small handful of efforts to reconcile emission inventories

with ambient BVOC measurements. Emission inversion methods based upon numerical

modeling techniques need further development and application for a wider range of

compounds and a wider range of locations. Similarly, more work is needed in the

development and application of isotopic methods to determine the relative importance of

BVOC for specific locations.

As the gaps described above are filled with results from ongoing and new studies,

the BEIS-type emission inventory models will need to be updated. BEIS3 provides a


good framework for incorporation of this new information. Specific tasks in the continued

improvement of BEIS3 include:

• Performance of ground truth tests of landcover data sets;

• Incorporation of better emission factors for specific compounds for regionally

dominant vegetation types;

• Incorporation of canopy escape factors for major compound classes;

• Development of a temperature history algorithm for isoprene (and other

BVOC classes);

• Establishment of a seasonal correction term for each major emission class;

• Continued refinement of the BVOC compound emission list.

5.2 Atmospheric Chemistry

In Section 3, we have attempted to organize and summarize information available

as of mid-1999 regarding the gas-phase atmospheric chemistry of BVOC, noting the lack

of detailed information regarding multiphase chemistry. Knowledge gaps have been

identified for the following topics: kinetic data, reaction products, aerosol formation,

kinetic data for first-generation products, second-generation products, and reaction

mechanisms.

5.2.1 Kinetic data

The information now available for the biogenic compounds listed in Table 3-1 is

reasonably complete. Relevant rate constants (OH reaction for all compounds, also O3

and NO3 reactions for unsaturated compounds) have been measured for most compounds.

Rate constants not yet measured are those for (1) the reaction of OH with cineole,

camphor, hexanal, p-cymene, α-thujene, piperitone, cis-3-hexenal, and trans-2-hexenyl

acetate, (2) the reactions of O3 and NO3 with α-thujene, piperitone, and cis-3-hexenal, and

(3) the reaction of NO3 with trans-2-hexenyl acetate. For these compounds and these


reactions, rate constants can be estimated with reasonable accuracy from structure-

reactivity considerations.

Kinetic data are also available for first-generation products, although for these

compounds the information available is not as complete as that for their precursors.

Kinetic data are available for many structural homologues of the first-generation products,

thus allowing reasonable estimates to be made of “missing” reaction rate constants using

structure-reactivity considerations.

5.2.2 First-generation products

Less information is available for reaction products than for reaction rate constants.

For the two compounds isoprene and MBO, reaction products and their formation yields

are well documented, and the information available for these compounds is sufficient to

construct detailed computer kinetic mechanisms (this has been done for isoprene, Carter

and Atkinson, 1997). The atmospheric oxidation of α-pinene is also well documented,

although one important uncertainty should be resolved, namely, that associated with the

yield of pinonaldehyde for the reaction of OH with α-pinene (literature yields range from

0.28 to 0.82, see Table 3-8).

For a number of the unsaturated compounds listed in Table 3-1, major products of

the OH and O3 reactions have been identified and their yields measured (including OH

yields in the O3 reaction), thus allowing to describe at least the overall features of the

corresponding oxidation mechanisms.

No product data are available for many compounds. Knowledge gaps regarding

products of the OH and O3 reactions are listed in Table 5-1. The compounds listed in

Table 5-1 include several terpenes, sesquiterpenes, and unsaturated oxygenates. Also

missing for most unsaturated compounds is information on the nature and formation yields

of carboxylic acids in the O3 reaction. Even less information is available regarding

products of the reaction of NO3 with unsaturated compounds. Data are available only for

isoprene, and, to a lesser extent, for three alkenes, four terpenes and two alcohols (Table

3-10). More product studies of the NO3-biogenic hydrocarbon reaction are obviously

needed.


Table 5-1. Summary of knowledge gaps for products of the OH and ozone reactions.

Product information not available for reaction with

CompoundOH ozone

camphor 3 3 (a)

bornyl acetate 3 3 (a)

cineole 3 3 (a)

β-cymene 3 3 (a)

α-thujene 3 3

camphene 3 (b)

α-terpinene 3 (b) 3 (b,c)

γ-terpinene 3 (b) 3 (b,c)

myrcene 3 (b) 3 (b,c)

ocimene 3 (b) 3 (b,c)

α-cedrene 3 3 (c)

α-copaene 3 3 (c)

longifolene 3 3 (c)

β-caryophyllene (d)

α-humulene 3 3 (c)

cis-3-hexenal 3 3

trans-2-hexenal 3

cis-3-hexenyl acetate 3

trans-3-hexenyl acetate 3

piperitone 3 3

(a) no data are available but reaction with ozone is a negligible atmospheric loss process for this

compound.

(b) acetone formation yields have been measured, see Tables 3-8 and 3-9.

(c) OH formation yields have been measured, see Table 3-11.

(d) very limited information, see Table 3-8.


5.2.3 Reaction of first-generation products

First-generation products can be divided into two groups, i.e., low MW

compounds whose atmospheric oxidation is reasonably well known (e.g., formaldehyde,

acetaldehyde, acetone) and higher MW products (carbonyls and oxygenates) for which

much less information exists. Of the higher MW first-generation products listed in Tables

3-7 to 3-10, we selected 26 compounds for review of available information on kinetic data

and atmospheric oxidation products. For these 26 higher MW compounds (14 saturated

and 12 unsaturated), of which 21 are carbonyls, some kinetic data are available (for about

half of the compounds) but little is known about oxidation products, i.e., second-

generation products. One first-generation product, pinonaldehyde, has been studied in

some detail. Some information is also available for four carbonyls and for MPAN, and

there is no information on second-generation products for the remaining 19 compounds.

5.2.4 Aerosol formation

The formation of SOA has been documented for linalool, two sesquiterpenes and

ten terpenes, of which α-pinene has received by far the most attention. More information

is available on aerosol formation yields and aerosol physics (e.g., size distribution) than on

aerosol molecular composition.

Regarding aerosol molecular composition, reasonably detailed information is

available for α-pinene aerosol, whose major components are 1,3-disubstituted-2,3-

dimethylcyclobutanes including dicarboxylic acids (e.g., cis-pinic acid) and carboxylic

acids that bear other oxygenated substituents (e.g., pinonic acid). Molecular composition

data are also available for β-pinene, d-limonene and ∆3-carene, although data for these

compounds are much more limited than for aerosol from α-pinene. For the several

compounds studied, there are no data on formation yields of individual products, and the

chemical mechanisms that lead to dicarboxylic acids (the major aerosol products) remain

uncertain. No information on aerosol molecular composition is available for the other

higher MW compounds listed in Table 3-1, including nine terpenes and the five


sesquiterpenes. Recent studies carried out in forests have provided strong evidence that

formation of new particles results from chemical oxidation of α-pinene. Studies of this

type should be extended to other terpenes and to sesquiterpenes, for which there is no

information at the present time.

5.2.5 Reaction mechanisms

The overall features of the mechanisms initiated by the reaction of biogenic

compounds with OH, O3 (and, with less certainty, NO3) are reasonably well understood.

The subsequent reaction pathways are well known from studies of other compounds, e.g.,

alkoxy radicals may decompose, isomerize, or react with O2. However, it is difficult to

describe the relative importance of these reaction pathways (of which many are possible,

see for example the OH-nopinone reaction mechanism discussed in Section 3.5) since (1)

only incomplete information is available on formation yields of first-generation products,

and (2) little or no information is available on formation yields of second-generation

products. Even in the case of α-pinene, a much-studied compound, we know that

reaction with OH leads to pinonaldehyde, but the formation yield is uncertain (from 0.28

to 0.82). We also know that pinonaldehyde reacts with OH as rapidly as α-pinene does,

but there are little or no data on high MW products of the OH + pinonaldehyde reaction.

Other aspects of the atmospheric oxidation of biogenic hydrocarbons that are difficult to

describe due to lack of experimental data include the nature and yields of carboxylic acids

for the O3 reaction and of high MW peroxyacetyl nitrates for the OH-aldehyde reaction in

the presence of NOx.

5.3 Gas/Particle Partition of Organic Aerosols

A major data gap in the current understanding of biogenic SOA is the lack of

information regarding SOA species, which are discussed in Section 3.3.5. This knowledge

gap results in the treatment of unidentified surrogate compounds in many current partition

modules. Instead of deriving thermodynamic parameters based on the structure and

properties of individual molecules, partition parameters are frequently determined


empirically. The application of empirical parameters are limited to the ranges of

temperature, relative humidity, and other atmospheric characteristics that have been tested

in environmental chambers, and uncertainties in SOA predictions exist because of

extrapolation of the empirical parameters to different environmental conditions.

Existing partition theories for organic compounds have been discussed in Section

4.2. The ambient partition of biogenic organic products likely involves multiple channels.

Some compounds may nucleate (see Section 3.3) in the clean atmosphere, but condense

onto existing particles that are available in polluted atmospheres. The absorption theory

dictates that the affinity of a condensable compound to particles depends on the

composition of the particles. Finally, many first- and second-generation products of

biogenic compounds contain polar groups such as carbonyls, alcohols, and acids. Their

partition may involve aqueous dissolution or absorption. It is likely that all three

processes play a role in the formation of organic particles from biogenic compounds in the

ambient atmosphere under different circumstances. However, the interplay of the

processes remains difficult to decipher given the current data base for evaluating aerosol

processes. Pun and Seigneur (1999) classified organic aerosols into a hydrophilic group

and a hydrophobic group to model the formation of particles by both aqueous dissolution

and organic-phase absorption. Though an improvement over existing modules, the model

does not account for the possibility of multiphase aerosols (e.g., an aqueous core

surrounded by an organic film), which have been suggested by several groups (e.g.,

Ellison et al., 1999; Xiang et al., 1998). A comprehensive theoretical framework remains

to be developed for modeling SOA partition.

In the modeling of aqueous particles that may contain organic compounds, the

interactions between the inorganic species (e.g., ammonium, sulfate, nitrate) and the

organic compounds need to be defined. Thermodynamic information on the interactions

between the inorganic and organic species is extremely limited, and theoretical

development is also needed to predict the activity of mixed solutions.


5.4 Recommendations for Future Work

The review presented in this report indicates that some data are available

concerning the emission and atmospheric oxidation of biogenic compounds, and the

partition of condensable products. However, the lack of data on emission composition

and reaction products constitutes a severe limitation with respect to the development of a

better understanding of the atmospheric chemistry of biogenic compounds, the

construction of reliable schemes that relate products observed in the atmosphere to their

biogenic precursors (e.g., using retrosynthetic analysis, Pun et al., 1999), and the

development of computer kinetic mechanisms that describe the contribution of biogenic

compounds to tropospheric O3 and to the formation of SOA.

There is a critical need to establish emission factors for specific terpenes,

sesquiterpenes, and oxygenated compounds from specific vegetation species. Some of

these BVOC remain unidentified from potential source vegetation species. In addition,

evaluation of current BEIS-type emission models and reconciliation of emissions with

ambient measurements need to be performed for compounds other than isoprene and for a

variety of landscapes.

Important topics for future research for atmospheric chemistry are:

• studies of the nature and formation yields of first-generation products for

several terpenes, sesquiterpenes and unsaturated oxygenates, including

carboxylic acid yields (O3 reaction) and aerosol molecular composition (O3,

OH, and NO3 reactions).

• for those compounds for which first-generation products have been identified,

studies of the nature and formation yields of second-generation products,

including high MW peroxyacyl nitrates from carbonyls, aerosol formation from

high MW compounds, and carbonyl and carboxylic acid yields from

unsaturated first-generation products.

• the role of multiphase, especially aqueous-phase, chemistry in the oxidation of

BVOC and products, and in the formation of condensable compounds in SOA.


The research topics listed above include more compounds and more reaction

systems than can be realistically studied given limited research funding resources.

Research priorities may be based on regulatory considerations (e.g., O3 formation

potential, ability to form aerosols/impact on PM2.5 concentrations), on reactivity

considerations (priority being given to the most reactive compounds), on health effects

considerations, i.e., the known or possible toxicity of reaction products, or on the

magnitude of their biogenic emissions. Most likely, the best approach for prioritizing

BVOC for emission and atmospheric chemistry research involves feedback between the

two fields. Efforts in atmospheric chemistry may focus on compounds that are abundantly

emitted. State, regional, nationwide and global inventories of biogenic emissions are now

available (e.g., Lamb et al., 1993, Guenther et al., 1999, and references therein) that can

be used to scope experimental research projects relevant to air quality issues on scales

ranging from one urban area to the global troposphere. On the other hand, priorities for

field measurements of emissions should be set on those frequently emitted compounds

whose atmospheric processes lead to significant formation of O3 and/or PM. The rapidly

growing literature on emissions and reactions of BVOC will require effective

communication between the fields and frequent updates of priorities for field and

experimental research.

Information on the condensable products of BVOC will reduce the reliance of

SOA partition modeling on smog chamber data and parametric approaches that use

unspecified surrogate model species. Further investigation of the composition and phase

properties of atmospheric particles is needed to determine the applicability of various

aerosol partition theories. The atmospheric interactions between inorganic and organic

compounds need to be incorporated in the modeling of hydrophilic organic compounds,

which may also undergo chemical reactions in the aqueous phase.


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Review of the Emissions, Atmospheric Chemistry, and Gas/Particle Partition of Biogenic Volatile Organic Compounds and Reaction Products A-1

Appendix A1. Chemical structures of biogenic compounds listed in Table 3-1.

Saturated bicyclic compounds:

camphor (2-bornanone, 1,7,7-trimethylbicyclo [2.2.1]heptan-2-one)

O

bornyl acetate (1,7,7-trimethylbicyclo [2.2.1] heptan-2-ol acetate)

C(O)OCH3

cineole (eucalyptol)

O

Terpenes:

p-cymene (4-methyl-isopropyl benzene)

α-pinene

β-pinene


∆3-carene

camphene (2,2-dimethyl-3-methylene bicyclo [2.2.1]heptane)

,

sabinene (1-isopropyl-4-methylene bicyclo [3.1.0]hexane)

α-thujene

d-limonene


terpinolene

α-terpinene

γ-terpinene

β-phellandrene

myrcene (3-methylene-7-methyl-1,6-octadiene)


ocimene (3,7-dimethyl-1,3,6-octatriene)

Sesquiterpenes:

α-cedrene

α-copaene

longifolene

β-caryophyllene


α-humulene

Unsaturated oxygenates

linalool OH

piperitone (4-isopropyl-1-methyl-1-cyclohexen-3-one)

O

6-methyl-5-hepten-2-one O


Appendix A2. Chemical structures of first-generation products (listed here as in

Table 3-14, which also includes their precursors).

nopinone (6,6-dimethylbicyclo [3.1.1] heptan-2-one)O

camphelinone (3,3-dimethylbicyclo [2.2.1]heptan-2-one)

,

O

O

sabinaketone (4-isopropylbicyclo [3.1.0] hexan-1-one) O

butanal CH3CH2CH2CHO

pentanal CH3(CH2)3CHO

4-methyl-3-cyclohexen-1-one

O


4-acetyl-1-methylcyclohexene

O

4-isopropyl-2-cyclohexen-1-one O

6-methyl-5-hepten-2-one O

2-oxobutanal CH3CH2C(O)CHO

2-oxopentanal CH3CH2CH2C(O)CHO

pinonaldehyde (cis-4-acetyl-(2,2-dimethylcyclobutyl)ethanal) O

CHO , HC CCH3

O O

caronaldehyde (2,2-dimethyl-3-(2-oxopropyl)-cyclopropyl ethanal)

, HC-CH2 CH2CCH3

O O

O

CHO•


endolim (3-isopropenyl-6-oxoheptanal)

O CHO

3-propenyl-6-oxoheptanal

O CHO

2-hydroxy-2-methylpropanal CH3

CH3 C CHO

OH

3-hydroxypropanal HOCH2CH2CHO

4-hydroxy-4-methyl-5-hexen-1-al (cyclized form: 5-methyl-5-vinyltetrahydrofuran-2-ol) CH2=CH C CH2 CH2 CHO C CHOH

OCH3

CH2=CH CH2-CH2

OH

CH3

2-oxoethyl acetate CH3C(O)OCH2CHO

3-oxopropyl acetate CH3C(O)OCH2CH2CHO

acrylic acid CH2=CHCOOH

methacrylic acid CH2=C(CH3)COOH

5-ethenyl-dihydro-5-methyl-2(3H) furanone (4-methyl-4-vinyl-γ-butyrolactone)

C C=O

CH2=CH

CH3

CH2-CH2

O

4-nitroxy-3-methyl-2-butenal O2NOCH2C(CH3)=CHCHO


Appendix A3. Structures of second-generation products (listed in Table 3-17

along with their precursors).

α-P-PAN(a)

O

CH3C CH2C(O)OONO2

norpinonaldehydeCH3C

O

CHO

methylglyoxal CH3C(O)CHO

2-isopropenyl-5-oxohexanal CH3CCH2CH2CHCHO

O C

CH3CH2

2-hydroxynopinone (b)O

OH

2,5-dihydroxynopinone (b)O

OH

HO

2-oxo-nopinone (b)O

O

4-oxopentanal CH3C(O)CH2CH2CHO

(a) not a nomenclature name, PAN homologue from α-pinene, see text, also see pinonaldehyde in

Appendix A-2.

(b) not nomenclature name, see nopinone in Appendix A-2.


Appendix A4. Structures of other compounds listed in Tables 3-8 to 3-10.

hydroxyacetaldehyde HOCH2CHO

methacrolein CH2=C(CH3)CHO

methylvinylketone CH2=CHC(O)CH3

3-methylfuran

CH C

CH CH

O

CH3

methyl oxirane

CH3 C H CH2

O

ethyl oxirane

CH3CH2 C H CH2

O

2,3-dimethyloxirane

CH3 C H CHCH3

O

pinene oxide O

terpinen-4-ol

OH

products of β-pinene(a):O - O

O

H

H O

OO

O


Products of sabinene(a): O O

O

H

H O

O

O

O

Products of β-caryophyllene(b):

3,3-dimethyl-γ-methylene-2-(3-oxobutyl)-cyclobutanebutanal

O

CHO

3,3-dimethyl-γ-oxo-2-(3-oxobutyl)-cyclobutanebutanal

O

CHO

O

(a) Griesbaum et al., 1998.

(b) Calogirou et al., 1997.

review of the emissions, atmospheric chemistry, and gas/particle

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