atmospheric chemistry of gasoline-related …atmospheric chemistry of gasoline-related emissions:...

ATMOSPHERIC CHEMISTRY OF GASOLINE-RELATED EMISSIONS:

FORMATION OF POLLUTANTS OF POTENTIAL CONCERN

January 2006

Reproductive and Cancer Hazard Assessment Branch

Office of Environmental Health Hazard Assessment California Environmental Protection Agency

AUTHORS, CONTRIBUTORS AND REVIEWERS

Primary Authors

Roger Atkinson, Ph.D. Air Pollution Research Center University of California, Riverside Under OEHHA Agreement Nos. 01-851, ATES-020211, and OEHHA 04-153

Sara Hoover, M.S. Research Scientist III Reproductive and Cancer Hazard Assessment Branch Office of Environmental Health Hazard Assessment

OEHHA Contributors

Elinor Fanning, Ph.D. Associate Toxicologist Reproductive and Cancer Hazard Assessment Branch

Daniel Sultana, B.S. Research Scientist I Reproductive and Cancer Hazard Assessment Branch

Janet Arey, Ph.D. Air Pollution Research Center University of California, Riverside Under OEHHA Agreement Nos. 01-850 and ATES-020210

Kim Preston, M.S. University of California, Los Angeles Under Interagency Agreement No. 00E0026 between OEHHA and UCLA

Martha S. Sandy, Ph.D., Chief Cancer Toxicology and Epidemiology Section Reproductive and Cancer Hazard Assessment Branch

Reviewers

OEHHA:

Robert Blaisdell, Ph.D., Chief Exposure Modeling Section Air Toxicology and Epidemiology Branch

Lauren Zeise, Ph.D., Chief Reproductive and Cancer Hazard Assessment Branch

George V. Alexeeff, Ph.D., D.A.B.T. Deputy Director for Scientific Affairs

California Air Resources Board:

Dongmin Luo, Ph.D., P.E. Air Resources Engineer Atmospheric Processes Research Section Research and Economics Studies Branch Research Division

Larry Hunsaker, P.E. Air Pollution Engineer Emission Inventory Analysis Section Emission Inventory Branch Planning and Technical Support Division

Eileen McCauley, Ph.D. Air Resources Supervisor (Manager) Atmospheric Processes Research Section Research and Economics Studies Branch Research Division

Atmospheric Chemistry Overview i January 2006

TABLE OF CONTENTS

List of Tables ..................................................................................................................... iii

List of Appendices ............................................................................................................. iii

Abstract ................................................................................................................................1

1. Introduction..............................................................................................................1

2. The Lifetime and Fate of Volatile Organic Compounds .........................................2

3. Removal Mechanisms For Gasoline Emissions in the Troposphere .......................5

3.1 The Hydroxyl Radical (OH): Key Reactive Species of the Troposphere...5

3.2 The Nitrate Radical (NO3): Nighttime Reactive Species.............................6

3.3 Ozone (O3) ...................................................................................................6

3.4 Photolysis.....................................................................................................8

3.5 Physical Removal – Wet and Dry Deposition .............................................8

3.6 Cl Atom........................................................................................................8

4. Tropospheric Reactions of VOCs Associated with Gasoline Usage .......................9

4.1 Alkanes (including Cycloalkanes) .............................................................12

4.2 Alkenes ......................................................................................................13

4.3 Aromatic Hydrocarbons.............................................................................16

4.4 Oxygen-Containing Organics (including Carbonyls) ................................17

4.4.1 Primary Emissions .........................................................................17

4.4.2 Secondary Transformation Products..............................................18

4.5 Peroxyacyl Nitrates....................................................................................21

5. Gaseous Criteria Air Pollutants: CO, NO2, Ozone, SO2........................................22

6. Gas/Particle Partitioning of Organic Compounds..................................................23

7. Conclusions............................................................................................................24

References..........................................................................................................................43

Atmospheric Chemistry Overview ii January 2006

LIST OF TABLES

Table 1. Lifetimes of selected VOCs due to gas-phase reactions with OH radicals, NO3 radicals and O3. ............................................................................................................................... 4

Table 2. Basis for selection of gasoline-related VOCs ....................................................... 10

Table 3. First-generation carbonyl yields from the tropospheric reactions of gasoline

constituents with O3, NO3 radicals and OH radicals..................................................................... 20

Table 4. Observed atmospheric transformation products of selected gasoline-related VOCs, sorted by parent............................................................................................................................. 25

Table 5. Observed atmospheric transformation products of selected gasoline-related VOCs,

sorted by product........................................................................................................................... 31

Table 6. Predicted or tentatively identified atmospheric transformation products for

selected gasoline-related VOCs .................................................................................................... 35

Table 7. Atmospheric transformation products of selected PAHS ..................................... 41

LIST OF APPENDICES

Appendix 1 Sources and Fate for Selected Gasoline-Related Pollutants ............................... A-1

Appendix 2 Estimation of OH Radical Reaction Rate Constants and Alkoxy Radical Reaction Rates ........................................................................................................... A-191

Appendix 3 Mass Emissions Ranking Methodology ......................................................... A-198

Atmospheric Chemistry Overview iii January 2006

ABSTRACT

Gasoline powered vehicles emit a variety of volatile organic pollutants, including alkanes, alkenes, aromatic hydrocarbons and oxygenates. The pollutants that are emitted at the tailpipe and that evaporate from the fuel system are not necessarily the same as those to which the population is exposed, due to the atmospheric transformation processes that these pollutants undergo. Transformation products need to be considered in addition to primary emissions as part of conducting a comprehensive assessment of potential human health hazards associated with gasoline usage. The primary objective of this research effort was to identify secondary products associated with gasoline combustion and evaporative emissions that may pose a toxicological concern or may be present at relatively high levels in the atmosphere. A secondary objective was to assess the atmospheric lifetimes of gasoline-related pollutants, as constituents with longer lifetimes would be of greater exposure concern. This document includes a general discussion of the lifetime and fate of volatile organic compounds (VOCs) and describes removal mechanisms for gasoline-related pollutants in the atmosphere. An overview of the reactions of classes of VOCs associated with gasoline usage is provided. Some brief notes on inorganic compounds of interest and gas/particle partitioning are also included. Gasoline-related pollutants were selected based on toxicological and/or exposure concerns for a detailed review of their atmospheric chemistry, which is provided in Appendix 1. Appendix 2 describes the procedures used to estimate hydroxyl radical reaction rates and the resulting distribution of alkyl radicals for alkanes that have not been studied experimentally, and also describes the procedures used to estimate the reaction rates of alkoxy radicals under atmospheric conditions. The mass emissions ranking method that was used to prioritize the potential for exposure to gasoline-related pollutants was based on 1998 data for California Phase 2 Reformulated Gasoline and is outlined in Appendix 3.

1. INTRODUCTION

Nearly all volatile organic compounds (VOCs) emitted into the atmosphere from gasoline-fueled vehicles undergo atmospheric transformation. An understanding of the atmospheric reactions of evaporative and combustion emissions of gasoline-fueled vehicles and the identity of the resultant products is an important element of characterizing the human health effects of complex mixtures associated with gasoline.

For the assessment of exposure to gasoline-derived air pollutants, only the transformation and transport of these pollutants in the lowest region of the atmosphere, the troposphere, need be considered. The troposphere is the region of the atmosphere that stretches from the ground to the tropopause, which is located approximately 10-18 km above the earth’s surface. The make-up of the troposphere consists of 78% N2, 21% O2, 1% Ar, and 0.036% CO2, with varying amounts of water vapor and trace gases. Above the troposphere is the stratosphere, which contains the ozone layer at 25-30 km above the Earth’s surface. The stratospheric ozone layer, together with molecular oxygen in the upper stratosphere, absorbs short wavelength solar radiation such that only wavelengths longer than 290 nm reach the troposphere. Reactions in the troposphere that

Atmospheric Chemistry Overview 1 January 2006

would otherwise occur with short wavelength solar ultraviolet (UV) radiation are thus prevented because of the UV-absorbing properties of the stratosphere.

Gasoline emissions are expected to travel in the direction of the wind at wind speeds typical of the local meteorology. During transport, gasoline VOC emissions undergo oxidation through a number of tropospheric reactions, all of which are ultimately initiated by sunlight. The most important reactions of VOCs are those with the hydroxyl radical (OH), the nitrate radical (NO3)1, and ozone (O3). Atmospheric lifetimes of gasoline VOC emissions in the troposphere generally range from a few hours or less (for example, for formaldehyde, 1,3-butadiene and 1,3,5-trimethylbenzene) to ten days or greater (for example, for methane, ethane, acetylene, propane and benzene).

The typical transformation products of gasoline emissions include carbonyl-containing compounds (aldehydes and ketones such as formaldehyde, acetaldehyde and acetone; dicarbonyls; and hydroxycarbonyls), organic nitrates (including alkyl nitrates and hydroxynitrates), hydroperoxides, and peroxynitrates (including peroxyacyl nitrates, [RC(O)OONO2] and peroxyalkyl nitrates [ROONO2]), phenolic compounds (for example, phenol, cresols and nitrophenols), O3, NO2, and secondary organic aerosol (SOA). This document focuses on the atmospheric chemistry of VOCs, including a short discussion of gas/particle partitioning processes. A detailed review of particle phase compounds and SOA is beyond the scope of the current report. The atmospheric chemistry of selected gasoline-related pollutants is described in more detail in Appendix 1. Appendix 2 describes the procedures used to estimate hydroxyl radical reaction rates and the resulting distribution of alkyl radicals for alkanes that have not been studied experimentally, and also describes the procedures used to estimate the reaction rates of alkoxy radicals under atmospheric conditions. The mass emissions ranking method used to prioritize the potential for exposure to gasoline-related pollutants was based on 1998 data for California Phase 2 Reformulated Gasoline (CaRFG2) and is discussed in Appendix 3.

References used in the preparation of the general overview of atmospheric chemistry include Seinfeld and Pandis (1998), Finlayson-Pitts and Pitts (2000), Atkinson (2000), Calvert et al. (2000, 2002), Jacob (2000), and the NARSTO Ozone Assessment – Critical Reviews (2000); additional references are noted for certain specific items.

2. THE LIFETIME AND FATE OF VOLATILE ORGANIC COMPOUNDS

VOCs in the troposphere are removed or transformed by reactive species (principally OH radicals, NO3 radicals and O3), photolysis, and the physical removal processes of wet and dry

1 Following the convention in atmospheric chemistry literature, the hydroxyl and nitrate radicals are represented throughout this document by OH and NO3 respectively, omitting the dot (•) which signifies the presence of an unpaired electron.


deposition). The lifetime of a VOC, τ, is the time required for the concentration of the VOC to decrease to 1/e (37%) of its original concentration. The lifetime of a particular gasoline constituent is an important factor in determining exposure potential, since longer residence in the atmosphere results in a greater likelihood that people will be exposed. For reaction of a VOC with OH radicals, NO3 radicals and O3, the lifetime is given by τ = 1/(kX[X]), where kX is the rate constant for reaction of species X with the VOC, and [X] is the ambient atmospheric concentration of species X (X = OH, NO3 or O3). For photolysis, τ = 1/kphotolysis, where kphotolysis depends on the absorption cross-section and photolysis quantum yield of the VOC and on the intensity of solar radiation (all of these being a function of wavelength).

The fate of a chemical in the atmosphere is determined by its transformation reactions, removal pathway(s), and geographical transport. An atmospheric constituent may undergo phase changes or physical removal mechanisms such as dry and wet deposition. Factors that influence the lifetime and fate of atmospheric constituents in the troposphere include meteorology (e.g., humidity, wind, cloud cover, rain, and temperature), topography, solar radiation, key reactive intermediate concentrations, and the effects of other anthropogenic and biogenic emissions into the troposphere on the chemistry that occurs. Temperature inversion layers in the atmosphere can effectively form a “lid” over an air basin, trapping high concentrations of pollutants beneath the top of the inversion layer and limiting their dispersion into the troposphere above. Rain can result in wet deposition of gasoline constituents. Wind can determine how far and how rapidly gasoline constituents are transported regionally. Mountain ranges can effectively trap air pollution in a particular region or urban location, such as occurs in the mountain-ringed Los Angeles basin. Pollutants released near ocean regions can be transported by on-shore and offshore winds induced by the temperature differences between the land and the water.

Solar radiation is a key component that drives the photochemical reactions of atmospheric constituents. Variations in available solar radiation, which fluctuates with latitude, weather, season and time of day, affect the concentrations of OH radicals, NO3 radicals and O3, and hence the lifetimes of anthropogenic emissions, in the troposphere. For example, lifetimes of gasoline-related pollutants in polar latitudes will be much longer during the dark months of wintertime than the lifetimes during the sunlit months of the polar summer. All three reactive intermediates (i.e., OH, NO3, and O3) undergo large diurnal variations in urban locations. NO3 radical concentrations are known to be particularly variable and difficult to predict.

Table 1 gives calculated tropospheric lifetimes of selected gasoline constituents for assumed ambient concentrations of OH radicals, NO3 radicals and O3. The overall lifetime of a chemical due to the various removal and/or transformation reactions is given by:

1/τoverall = 1/τOH + 1/τNO3 + 1/τO3 + 1/τphotolysis + 1/τwet deposition + 1/τdry deposition

The overall lifetime depends on the concentrations of the reactive species OH, NO3 and O3, light intensity, and, via their influence on wet and dry deposition rates and lifetimes, precipitation, atmospheric turbulence and nature of the ground surface. These parameters will vary with time of day, season and latitude. The dominant loss process(es) can also change with time of day, season and latitude. Therefore, to obtain an accurate estimate of the overall lifetime of a chemical, it is necessary to know the concentrations of OH, NO3 and O3, the light intensity (as a


function of wavelength), and the time-dependence of these parameters and potentially the time-dependence of the precipitation rate and the chemical’s deposition velocity.

Table 1. Lifetimes of selected VOCs due to gas-phase reactions with OH radicals, NO3 radicals and O3. VOC Lifetime due to reaction witha

OH radicals NO3 radicals O3 Acetaldehydeb 8.8 hr 17 d >4.5 yr

Acrolein 6.9 hr 4.2 d 57 d

Benzene 9.4 d >4 yr >4.5 yr

1,3-Butadiene 2.1 hr 5.6 hr 2.6 d

2,3-Dimethylbutane 2.0 d 105 d >4500 yr

Ethane 47 d >12 yr >4500 yr

Ethanol 3.6 d 23 d - c

Ethene 1.4 d 225 d 10 d

Formaldehyded 1.2 d 80 d >4.5 yr

2-Methylpropane 5.5 d 1.2 yr >4500 yr

Naphthalene 5.7 hr 1.4 yre >80 d

n-Octane 1.3 d 240 d >4500 yr

Phenol 5.1 hr 9 min ~50 df

Propane 10 d 7 yr >4500 yr

Propene 5.3 hr 4.9 d 1.6 d

Styrene 2.4 hr 22 min 1.0 d

Toluene 1.9 d 1.9 yr >4.5 yr

1,2,4-Trimethylbenzene 4.3 hr 26 d >4.5 yr

2,2,4-Trimethylpentane 3.5 d 1.4 yr >4500 yr

m-Xylene 5.9 hr 200 d >4.5 yr a. For assumed concentrations of: OH, 12-hr daytime average of 2.0 x 106 molecule cm-3 (Krol et al., 1998; Prinn

et al., 2001); NO3, 12-hr nighttime average of 5 x 108 molecule cm-3 (Atkinson, 1991); O3, a 24-hr average of 7 x 1011 molecule cm-3 (30 ppbv) (Logan, 1985).

b. Photolysis also occurs, with a lifetime due to photolysis of ~6 days. c. No data available; lifetime expected to be >4.5 yr. d. Photolysis also important, with a lifetime due to photolysis of ∼4 hr for overhead sun. e. For an assumed NO2 concentration of 2.5 x 1011 molecule cm-3 (10 ppbv); see Appendix 1 for a discussion of

the NO3 radical-initiated chemistry of naphthalene. f. No data available for the reaction of phenol with O3; assumed to be similar to the reaction of cresols with O3.


3. REMOVAL MECHANISMS FOR GASOLINE EMISSIONS IN THE TROPOSPHERE

OH radicals, NO3 radicals, and O3 react widely and rapidly with chemicals in the atmosphere, playing a principal role in the breakdown, transformation, and removal of anthropogenic emissions. Photolysis by natural sunlight at wavelengths ≥290 nm also plays an important role for certain classes of organic compounds (mainly carbonyls and organic nitrates); sunlight is also critical to the formation of OH radicals, NO3 radicals, and O3. Dry deposition is expected to play only a minor role in removal of most gasoline-derived VOCs from the atmosphere, while wet deposition may be important for some highly water soluble compounds (possibly including certain alcohols, hydroxycarbonyls, aldehydes, dicarbonyls and hydroperoxides) (Bidleman, 1988; Wesely and Hicks, 2000). Reactions with Cl atoms are possible, but are believed to be of minor importance in the atmospheric transformation and loss of VOCs.

3.1 The Hydroxyl Radical (OH): Key Reactive Species of the Troposphere

Capable of reacting with all organic compounds except chlorofluorocarbons (CFCs) and certain halons, the OH radical plays a major role in the degradation and transformation of VOCs in the atmosphere. Furthermore, the reactions of the OH radical with many gasoline evaporative and combustion emissions occur rapidly and, as a result, the dominant removal (and transformation) mechanism of many gasoline emissions is by reaction with the OH radical. Thus, atmospheric lifetimes of these VOCs are often dictated by reaction with the OH radical (see, for example, Table 1).

The major source of OH radicals in the troposphere is from the photolysis of ozone by sunlight, followed by reaction of the resultant electronically-excited oxygen atom, O (1D), with water vapor.

O3 + hν → O2 + O(1D)

O(1D) + H2O → 2 OH

Other tropospheric reactions which produce the OH radical include the photolysis of nitrous acid (HONO), photolysis of formaldehyde in the presence of NO, and dark reactions of alkenes with O3.

The average global concentration of the OH radical (annually, seasonally and diurnally averaged) is known with some confidence from computer model calculations involving the emissions and measured ambient tropospheric concentrations of methyl chloroform (CH3CCl3) and the rate constant for reaction of methyl chloroform with the OH radical (Prinn et al., 1995, 2001; Krol et al., 1998). However, the concentration of OH radicals depends on the time of day, season and latitude, and also on the intensity of solar radiation (and hence on cloud cover), local ozone and water vapor concentrations, and concentrations of other trace species including VOCs and NOx (Ehhalt, 1999). For clear sky conditions, OH radical concentrations reach a peak at midday (solar noon) and decrease to zero or near-zero levels at night. Traditionally, for mid-


latitude regions such as the continental U.S., OH radical concentrations are often approximated by a 24 hour average of 1.0 x 106 molecules cm-3 or by a 12 hour daytime average of 2.0 x 106 molecules cm-3 (the global tropospheric average). Measurements at two mid-latitude northern hemisphere sites during the months of August and September found peak daytime OH radical concentrations in the range 2 x 106 to 10 x 106 molecules cm-3, noting however that OH radical concentrations have been reported to vary by a factor of 9 ± 2 from summertime to wintertime at 43 oN (Goldstein et al., 1995). The southern hemisphere appears to have higher concentrations of OH radicals than does the northern hemisphere (Prinn et al., 2001).

3.2 The Nitrate Radical (NO3): Nighttime Reactive Species

While the OH radical is the key reactive species during daylight hours, the NO3 radical is a key reactive species at night for certain classes of VOCs. Because of rapid photolysis of the NO3 radical during daylight hours (with a lifetime of ∼5 seconds for overhead sun), the NO3 radical is present at measurable concentrations only during early evening hours and nighttime.

The NO3 radical originates from sources of NO produced by soil nitrification, fires, lightning, and (most importantly in urban areas) from combustion sources. NO from these biogenic and anthropogenic sources is then transformed to NO3 radicals in the troposphere by reaction with ozone.

NO + O3 → NO2 + O2

NO2 + O3 → NO3 + O2

Nighttime concentrations of NO3 radicals are highly variable, with measured nighttime concentrations at around ground level ranging from non-detectable levels (

OH + CH4 → H2O + •CH3

•CH3 + O2 → CH3OO•

CH3OO• + NO → CH3O• + NO2

CH3O• + O2 → HCHO + HO2 (HCHO = formaldehyde)

HO2 + NO → OH + NO2

These reactions lead to a net overall transformation of

OH + CH4 + 2NO + 2O2 ⇒ HCHO + H2O + 2NO2 + OH,

converting 2 molecules of NO to NO2 and regenerating the OH radical. Because NO2 photolyzes rapidly to form a “ground” electronic-state oxygen atom, O(3P), which then reacts rapidly with molecular oxygen to form O3,

NO2 + hν → NO + O(3P)

O(3P) + O2 + M → O3 + M (M = air)

the net process (in the presence of NO) can be written as:

OH + CH4 ⇒ HCHO + H2O + 2O3 + OH

Different VOCs have different abilities to form O3 from their reactions in the troposphere, and this is related to (a) how fast they react in the atmosphere to form organic peroxy (ROO•) radicals (termed “Kinetic Reactivity”), and (b) how much ozone each organic peroxy radical makes in its subsequent reactions (termed “Mechanistic Reactivity”) (Carter, 1994). “Ozone forming potential” is a number assigned to a particular VOC in an effort to represent an individual VOC’s ability to produce ozone. There are a number of different ozone reactivity scales and methods for quantifying ozone formation impacts, with one of the most often used being the Maximum Incremental Reactivity (MIR) scale (Carter, 1994). VOCs from gasoline usage with high ozone forming potentials include ethene, propene, xylenes, 1,3-butadiene, and formaldehyde (Carter, 1994).

In polluted urban atmospheres, O3 concentrations remain relatively low in the morning, and morning rush-hour NO emissions keep ozone levels low because NO reacts rapidly with O3.


NO + O3 → NO2 + O2

Around noon, when NO has been largely converted to NO2, O3 concentrations rise, sometimes dramatically. The time of peak ozone concentration can vary by location in an air-shed, and polluted air masses can transport already formed O3. O3 is not formed after sunset because NO2 photolysis no longer occurs (see above). Nighttime concentrations of O3 can fall to low levels, depending on NO emissions and the concentrations of NO2 and of VOCs containing unsaturated C=C bonds.

3.4 Photolysis

As noted above, sunlight plays a critical role in the formation of OH and NO3 radicals and O3, and direct photolysis of certain classes of VOCs also occurs. Because the stratosphere absorbs solar ultraviolet radiation of wavelengths

or more in the marine boundary layer at dawn (Spicer et al., 1998). Reactions with Cl atoms proceed by H-atom abstraction from the various C-H bonds in alkanes (Atkinson, 1997), by initial addition to the carbon atoms of the C=C bond(s) in alkenes (Atkinson, 1997) and by H-atom abstraction from the C-H bonds of the alkyl substituent groups in the alkylbenzenes (Wallington et al., 1988). The room temperature rate constants for the reactions of Cl atoms with alkanes (other than methane), alkenes and aromatic hydrocarbons (other than benzene) are in the range 5 x 10-11 cm3 molecule-1 s-1 to 5 x 10-10 cm3 molecule-1 s-1. Because many organic compounds have rate constants for reaction with Cl atoms that are 100 times higher than those for reaction with OH radicals (Atkinson 1997; Finlayson-Pitts and Pitts, 2000; IUPAC, 2004), the ambient data on Cl atom concentrations indicate that reactions of VOCs with Cl atoms could be significant in the marine boundary layer, and perhaps also in coastal areas.

4. TROPOSPHERIC REACTIONS OF VOCS ASSOCIATED WITH GASOLINE USAGE

The different classes of VOCs react in the troposphere in different ways, with the VOCs in a given class (alkanes, alkenes, aromatic hydrocarbons, etc.) reacting by analogous pathways. In the following discussion, we consider the tropospheric reactions of the major classes of VOCs; alkanes, alkenes, aromatic hydrocarbons (including polycyclic aromatic hydrocarbons [PAH]), oxygen-containing organics (including carbonyls), as well as peroxyacyl nitrates (an important product class from gasoline VOC reactions). Detailed discussions of the atmospheric fate of specific gasoline-derived VOCs are provided in Appendix 1.

The 43 VOCs and classes of VOCs were selected on the basis of their estimated mass emissions and/or their importance to health outcomes, as summarized in Table 2. The mass emissions ranking method used to prioritize the potential for exposure to gasoline-related VOCs was based on 1998 data for CaRFG2 and is outlined in Appendix 3. The top 25 ranked VOCs account for 77% of the estimated mass emissions by weight for nonmethane organic gas phase compounds associated with gasoline-related sources based on 1998 data. When all 37 of the individual VOCs in Table 2 with available mass emissions ranks are considered, the percentage accounted for rises to 80%.


Table 2. Basis for selection of gasoline-related VOCs Gasoline-related VOC

Basis for selection Carcinogena Respiratory toxicantb

Recognized Suspected Mass

emissions rankc

Other/comments

Acetaldehyde 9 9 45 Acetylene 12 Acrolein 9 86 Benzaldehyde 9 58 Benzene 9 9 13 1,3-Butadiene 9 9 37 n-Butane 5 2,3-Dimethylbutane 21 2,3-Dimethylpentane 19 2,4-Dimethylpentane 34 Isomer of 2,3

dimethylpentane Ethane 23 Ethanol 88 Nominated by

ARBd

Ethylbenzene 9 22 Ethylene 4 Formaldehyde 9 9 18 Formic acid 9 --e Product of

acetylene Furan 9 9 --e Product of 1,3

butadiene n-Hexane 9 16 Isobutene 9 11 Isopentane 1 Methylcyclopentane 10 3-Methylhexane 25 2-Methylpentane 7 3-Methylpentane 14 2-Methylpropane 17 Methyl t-butyl ether (MTBE)

9 2 Phased out in California in 2003

Naphthalenef 9 9 110 Nitro-PAHsg 9 9 --h

PAHsg 9 9 9 --i

n-Pentane 6 Peroxyacetyl nitrate (PAN)

9 - j

Phenol 9 --e Product of benzene Propylene (propene) 9 9 Styrene 9 9 85


Table 2. (continued) Gasoline-related Basis for selection VOC Carcinogena Respiratory toxicantb

Recognized Suspected Mass

emissions rankc

Other/comments

Toluene 9 3 1,2,3Trimethylbenzene

9 70

1,2,4Trimethylbenzene

9 24

1,3,5Trimethylbenzene

9 43

2,2,4Trimethylpentane

15

2,3,4Trimethylpentane

32 Isomer of 2,2,4trimethylpentane

m-Xylene 9 8 o-Xylene 9 20 p-Xylene 9 162 a. Carcinogens were identified from the California Proposition 65 list, California’s Toxic Air Contaminant (TAC) program

and/or the International Agency for Research on Cancer. b. Recognized respiratory toxicants were identified based on the supporting documentation for Chronic Reference Exposure

Levels developed by OEHHA under the Air Toxics Hot Spots Program. Suspected respiratory toxicants were identified from a preliminary screening of secondary sources and/or the scientific literature.

c. Based on an analysis by OEHHA of 1998 organic gas phase emissions data for CaRFG2 from the California Air Resources Board (ARB) for gasoline-related VOCs (see Appendix 3). Methane was removed from the list prior to ranking. Ranks in top 25 are shown in bold.

d. Based on increasing use of ethanol as an oxygenate due to phase out of MTBE. e. Formic acid, furan and phenol are identified here based on formation as secondary products; earlier studies have indicated

that these chemicals may be directly emitted in vehicle exhaust (see Appendix 1). Because these chemicals are not identified in ARB’s speciated organic profiles associated with CaRFG2 (see Appendix 3), mass emissions ranks were not available.

f. Discussed separately and also as part of the class of PAHs. g. PAHs = polycyclic aromatic hydrocarbons. For PAHs and nitro-PAHs, data for both gas phase and particle phase

compounds were reviewed. h. Some nitro-PAHs may be both directly emitted and formed as secondary products; others are likely formed as secondary

products only. Data are not available to develop mass emissions ranks for nitro-PAHs. i. Ranking will vary for different gas phase PAHs. j. Peroxyacetyl nitrate is a secondary product formed in the atmosphere. Mass emissions ranks are available only for directly

emitted chemicals identified in ARB’s speciated organic profiles associated with gasoline-related sources (see Appendix 3).

With regard to the following overview of the tropospheric chemistry of gasoline combustion and evaporative emissions, it is important to point out the limitations in our knowledge of this chemistry, and thus the limitations and appropriate use of the following presentation. The tropospheric chemistry summarized in the following text is based upon published literature in which the tropospheric chemistry of selected gasoline emission constituents is postulated based on experimental kinetic and product studies, by using empirical estimation methods developed from the experimental database, and by analogy with the known chemistry of VOCs in a homologous series (for example, the atmospheric chemistry of more complex alkanes is analogous in many details to that of methane). In many cases, experimental studies identify and


quantify only a fraction of the total number of products formed (because of analytical difficulties in analyzing for labile and multifunctional compounds), and losses of chemicals to the reaction vessel walls can occur for low volatility VOCs (including products). The chemical mechanisms leading to the observed products are postulated based on current (and evolving) understanding of the atmospheric chemistry of VOCs.

4.1 Alkanes (including Cycloalkanes)

In the troposphere, alkanes and cycloalkanes react with OH radicals and, to a much lesser extent, with NO3 radicals (Atkinson, 1997). Alkanes and cycloalkanes also react with Cl atoms, but the importance of this transformation process in the troposphere is not yet known. Photolysis and reaction with O3 are of no importance. The reactions of alkanes and cycloalkanes with OH radicals, NO3 radicals, and Cl atoms proceed by H-atom abstraction from the various C-H bonds to form alkyl radicals (R•), which then rapidly (and solely) react with O2 to form alkyl peroxy radicals (ROO•).

X + RH → HX + R• (X = OH, NO3, or Cl)

R• + O2 → ROO•

In the troposphere, alkyl peroxy radicals react with NO, NO2, HO2 radicals and organic peroxy radicals, and may react with NO3 radicals at night, with the dominant reaction depending on the ambient atmospheric concentrations of these species. The reaction with NO2 leads to the formation of alkyl peroxynitrates, ROONO2, which rapidly thermally decompose back to reactants; therefore reaction of alkyl peroxy radicals with NO2 is of no importance in the lower troposphere.

Reaction of alkyl peroxy radicals with NO is expected to dominate in the troposphere when NO concentrations are greater than 2 x 108 to 7 x 108 molecule cm-3 (greater than 10 to 30 parts-pertrillion by volume, pptv). The reaction of alkyl peroxy radicals with NO has two pathways, one leading to formation of the corresponding alkyl nitrate, RONO2, and the other leading to formation of NO2 and an alkoxy (RO•) radical.

ROO• + NO (+M) → RONO2 (+M)

ROO• + NO → RO• + NO2

Formation of the alkoxy radical plus NO2 dominates. However, for alkyl peroxy radicals with more than three carbon atoms, formation of alkyl nitrates (RONO2) does occur, and in general this reaction pathway increases in importance as the size of the alkyl peroxy radical increases, and with increasing pressure and decreasing temperature.

Reaction of alkyl peroxy radicals with HO2 radicals leads to formation (at least in part) of hydroperoxides,


ROO• + HO2 → ROOH + O2

which react with OH radicals and may also be removed by wet and dry deposition.

In the troposphere, alkoxy radicals can react with O2, decompose by C-C bond breakage, and isomerize through a six-membered transition state. For example, for the 2-pentoxy radical formed from n-pentane:

CH3CH(O)CH 2CH2CH3

CH3 + CH 3CH2CH2CHO

or

CH3CHO + CH 3CH2CH2

HO2 + CH 3C(O)CH 2CH2CH3

CH3CH

CH2

H

CH2 CH2

O

O2

isomerization decomposition

CH3CH(OH)CH 2CH2CH2

For cycloalkanes, decomposition of cycloalkoxy radicals (i.e., the alkoxy radical being located on one of the ring carbons) is expected to lead to breakage of the ring in most cases (Aschmann et al., 1997). There is also evidence that breakdown products of cycloalkanes tend to produce more SOA than straight-chain alkanes.

The typical products of the atmospheric photooxidations of alkanes and cycloalkanes are alkyl nitrates, carbonyls (aldehydes and ketones), hydroxyalkyl nitrates, and hydroxycarbonyls, with formation of carbonyl-nitrates, hydroxydicarbonyls and hydroxycarbonyl-nitrates being additional products formed from cycloalkanes (Aschmann et al., 1997).

4.2 Alkenes

Alkenes, including ethene, propene and 1,3-butadiene, react with OH radicals, NO3 radicals, and O3. These reactions proceed mainly (OH) or totally (NO3 and O3) by initial addition to the C=C double bond. The dominant atmospheric reaction pathway(s) is specific to each individual alkene.


The primary reaction of OH radicals with alkenes involves addition of the OH radical to one of the carbons of the double bond, forming 1,2-hydroxyalkyl radicals. For example, for propene

OH + CH3CH=CH2 → CH3C•HCH2OH and CH3CH(OH)C•H2

Analogous to the reactions described above for alkyl radicals formed from alkanes, 1,2hydroxyalkyl radicals react rapidly with O2 to form 1,2-hydroxyalkyl peroxy radicals. As for alkyl peroxy radicals formed from alkanes, 1,2-hydroxyalkyl peroxy radicals can then react with NO, NO2, HO2, organic peroxy radicals, and possibly NO3 (at night). As discussed above, the NO2 reaction is unimportant as the resulting peroxynitrates rapidly thermally decompose in the lower troposphere. Reaction of 1,2-hydroxyalkyl peroxy radicals with NO are similar to the corresponding reactions of alkyl peroxy radicals, and form a 1,2-hydroxyalkoxy radical plus NO2 or a 1,2-hydroxyalkyl nitrate (the latter in small yield [O’Brien et al., 1998] at room temperature and atmospheric pressure). Reaction of 1,2-hydroxyalkyl peroxy radicals with HO2 radicals appears to form hydroxyalkyl hydroperoxides.

As for alkoxy radicals formed from alkanes, the 1,2-hydroxyalkoxy radicals can react with O2, decompose, or isomerize (noting that the O2 reaction and isomerization may not be feasible for a given hydroxyalkoxy radical). The available data indicate that at room temperature the decomposition of 1,2-hydroxyalkoxy radicals dominates over reaction with O2 (except for the HOCH2CH2O• radical formed in the ethene reaction), and hence the products expected from the OH radical-initiated reactions of alkenes in the presence of NO are 1,2-hydroxyalkyl nitrates, carbonyls (aldehydes and ketones), dihydroxynitrates, and dihydroxycarbonyls.

The reaction of NO3 radicals with alkenes is similar to the corresponding OH radical reaction, and proceeds by initial addition of the NO3 radical to one of the carbons of the double bond, forming a 1,2-nitrooxyalkyl radical. As described above for the OH radical reaction, the nitrooxyalkyl radical reacts with O2 to form a 1,2-nitrooxyalkyl peroxy radical which can then react with NO, NO2, HO2, NO3 or other organic peroxy radicals. Expected products of the reactions of NO3 radicals with alkenes include carbonyl compounds (aldehydes and ketones), carbonyl nitrates, hydroxynitrates and dinitrates.

Ozone reacts with alkenes by adding to the double bond to form a chemically activated, or energy-rich, “primary ozonide”. The primary ozonides rapidly decompose to form carbonyls and energy-rich “Criegee” intermediates.


O O

*

OR1 R3

O3 + C=C R1 C C R3 R2 R4 R2 R4

R1C(O)R2 + [R3R4COO]* R3C(O)R4 + [R1R2COO]*

The Criegee intermediates then undergo decomposition and/or isomerization, or are collisionally stabilized to form a “thermalized” intermediate. If the Criegee intermediate has the correct configuration (i.e., is dialkyl substituted or is a syn- mono-substituted intermediate), then it can isomerize and decompose to form an OH radical (from the excited intermediates and possibly also from the thermalized intermediates).

(CH3)2COO → [CH3C(OOH)=CH2]* → OH + CH3C(O)C•H2

Other decomposition pathways include the elimination of CO2 (apparently from intermediates of structure RCHOO). In the atmosphere, thermalized intermediates (and presumably mainly the anti- mono-substituted intermediates) react with water vapor, to form α−hydroxyhydroperoxides which may be thermally stable or may decompose to a carboxylic acid plus water or to a carbonyl plus H2O2. For di-substituted intermediates, the only decomposition pathway is to form the carbonyl plus H2O2.

RCHOO + H2O → RCH(OH)OOH → RC(O)OH + H2O

↓

RCHO + H2O2

The products formed from the reaction of O3 with alkenes include carbonyls (the “primary” carbonyls formed as a co-product to the Criegee intermediates), OH radicals (the yield of which is often close to 100%), organic acids, hydroxyhydroperoxides, and the products formed from the atmospheric reactions of the carbonyl-substituted alkyl radicals formed as co-products with OH radicals (typically including dicarbonyls, carbonyl-hydroperoxides, hydroxycarbonyls and carbonyls).

In most cases, a complete accounting of the products from the atmospheric reactions of alkenes has not been obtained, especially for the larger alkenes where alkoxy radical isomerization reactions can occur leading to the formation of multifunctional products (often of low volatility).


4.3 Aromatic Hydrocarbons

Because of a number of studies carried out during the past two to three years, details of the atmospheric reactions of aromatic hydrocarbons are becoming evident. In the atmosphere, the only important reaction of benzene and the alkyl-substituted benzenes such as toluene, ethylbenzene, the xylenes and the trimethylbenzenes is with the OH radical. Reaction with the OH radical proceeds by two pathways; H-atom abstraction from an alkyl substituent group to form a benzyl or alkyl-substituted benzyl radical, and initial addition of the OH radical to the

CH3

CH3

H

OH

CH2

OH +

H2O +

carbon atoms of the aromatic ring to form a hydroxyalkylcyclohexadienyl radical (or OH-aromatic adduct).

The kinetics of this initial reaction are well understood, as is the relative importance of the two reaction pathways. For toluene, the xylenes and the trimethylbenzenes,

OH

+ HO2

OH

H HC(O)CH=CHCH=CHCHOH

OO

OH HC(O)CHOO H

O +

HC(O)CH=CHCHO

HO OH

H products, including H H

OO HC(O)CH=CHCH CHCHO

OH

H + O2

H

Formation of nitroaromatics (a product arising after reaction of the OH-aromatic adduct with NO2) is predicted to be of no importance under realistic atmospheric conditions (Atkinson and Aschmann, 1994), although these compounds are observed in laboratory studies of monocyclic aromatic hydrocarbons conducted at elevated NOx concentrations.

The atmospheric reactions of PAH are similar to those of the monocyclic aromatic hydrocarbons, with some differences. The dominant atmospheric reaction is with the OH radical, and the reaction proceeds by analogous pathways to the monocyclic aromatic hydrocarbons. However, the small amount of data available, and especially the observed presence in ambient air of nitro-PAH formed from gas-phase OH radical-initiated reactions, indicate that the OH-PAH adducts must react dominantly with NO2 under atmospheric conditions. For the PAH, the NO3 radical reaction occurs by reversible addition of the NO3 radical to the aromatic ring(s), with the back-decomposition of the NO3-PAH adduct competing with its reaction with NO2 (and potentially O2) (Atkinson et al., 1994). This complex reaction mechanism means that the rate of reaction of a PAH not containing an unsaturated substituent group with the NO3 radical depends on both the NO3 radical concentration and the NO2 concentration.

4.4 Oxygen-Containing Organics (including Carbonyls)

4.4.1 Primary Emissions

Gasoline consists of non-oxygenated hydrocarbons, to which oxygenated additives and other additives (such as detergents) may be added. Oxygenated additives in reformulated gasolines can be emitted both by evaporative losses as well as by emission of unburnt gasoline fuel in the


vehicle exhaust. Oxygenated compounds are also formed in the combustion process and found in primary gasoline vehicle emissions. MTBE previously, or ethanol currently, acetaldehyde, and formaldehyde are the oxygen-containing organics emitted in highest mass as primary emissions from gasoline evaporative and combustion processes (Grosjean et al., 2001; Kean et al., 2001). Other oxygen-containing organics present in gasoline-fueled vehicle exhaust include acrolein, 2-butanone (methyl ethyl ketone), acetone, propanal, other carbonyls, and methanol and other alcohols.

MTBE was formerly used as the oxygenate in reformulated gasoline in California fuel; ethanol is now the oxygenate of choice. As noted above, MTBE (previously) or ethanol (currently) is subject to both evaporative and combustion emissions. The fates of MTBE and ethanol in the troposphere and their tropospheric breakdown products are discussed in Appendix 1. The tropospheric fates of formaldehyde and acetaldehyde, found in primary emissions and also formed as secondary products, are discussed below and in Appendix 1.

Methanol is released in moderate mass yields from the primary emissions of gasoline combustion. The dominant tropospheric reaction for aliphatic alcohols including methanol and ethanol is reaction with OH radicals. Ethanol reacts with OH radicals to form acetaldehyde together with much smaller amounts of formaldehyde and glycolaldehyde.

4.4.2 Secondary Transformation Products

Combustion sources both emit carbonyls directly and emit VOCs which are photooxidized in the troposphere to form carbonyls. A wide variety of carbonyl products are derived from gasoline-related VOCs (Grosjean et al., 2001; Kean et al., 2001). Many carbonyl-containing compounds have not been quantitatively measured in ambient air. This is largely due to the difficulty in measuring hydroxycarbonyls, dihydroxycarbonyls, hydroxydicarbonyls, dihydroxydicarbonyls, and di-unsaturated dicarbonyls by analytical techniques. Many of these have been detected qualitatively and some semi-quantitatively (Destaillats et al., 2002), suggesting that significant concentrations of these compounds may be present in the ambient air. In laboratory studies of C5-C10 n-alkanes, hydroxycarbonyls have been estimated to account for ∼35-70% of the total transformation products (Arey et al., 2001; Aschmann et al., 2001).

Aldehydes are both primary air pollutants emitted from gasoline-fueled vehicles and secondary pollutants formed in situ in the atmosphere from the atmospheric transformation of combustion emissions. The atmospheric reactions of ≥C2 aldehydes lead (at least in part) to the formation of aldehydes containing one less carbon atom, and hence the atmospheric reactions of the higher aldehydes lead to a “cascade” effect in that a progression of smaller aldehydes are sequentially formed, finally leading to formation of formaldehyde.

The tropospheric reactions of many constituents of gasoline combustion and evaporative emissions, including MTBE, ethene, propene, 1,3-butadiene, n-pentane, 2-methylpentane, and isopentane produce aldehydes in significant yields (Table 3; see Appendix 1 for individual mechanisms and product yields). In an assessment of compound-specific aldehyde yields resulting from tropospheric reactions of VOCs, it is important to distinguish between first-generation aldehyde yields and total aldehyde yields formed from first- and later-generation reactions; in Table 3 for example, only the first-generation (or primary) carbonyl yields are given


and they exclude subsequent reactions of first-generation products which may form the same, or other, carbonyl compounds. For example, the OH radical-initiated reaction of propene forms acetaldehyde as a primary, or first-generation, product in large yield; this acetaldehyde then reacts in the atmosphere to form formaldehyde, which is then both a first- and second-generation product from propene. It should be noted that indoor releases from products containing formaldehyde, such as furniture, paneling and carpet resins may create significant indoor air formaldehyde exposures many times higher than ambient air exposures.

In the troposphere, aldehydes undergo primarily photolysis and reaction with OH radicals. The dominant transformation process depends on the specific aldehyde, with photolysis dominating for formaldehyde (lifetime of ∼4 hrs for photolysis and 1.3 days for reaction with the OH radical). In contrast, reaction with the OH radical dominates for acetaldehyde.

The OH radical-initiated reaction of a Cn straight-chain aldehyde leads, in part, to formation of the Cn-1 aldehyde and so on, leading ultimately to formation of acetaldehyde and then formaldehyde, which photolyzes to form CO as the sole carbon-containing molecule.


Table 3. First-generation carbonyl yields (molar, in %) from the tropospheric reactions of gasoline constituents with O3, NO3 radicals and OH radicals.a

Gasoline Constituent

Product Molar Yieldb from OH Radical Reaction

(%)

Molar Yield from NO3 Radical Reaction

(%)

Molar Yield from O3 Reaction

(%)

Ethene Formaldehyde 156% No data 100%

Propene Formaldehyde

Acetaldehyde

∼100%

∼100%

10%

10%

65-78%

45-52%

2-Methylpropene Formaldehyde

Acetone

92%

78-90%

24%

24%

95-101%

29-34%

2-Butenes Acetaldehyde 160-200% 34% 115%

1,3-Butadiene Formaldehyde

Acrolein

62%

58%

6%

5% 52%

MTBE Formaldehyde

t-Butyl formate

Methyl acetate

Acetone

48%

76%

18%

6%

No data No reaction

a. Data have been taken from Atkinson (1994, 1997), which should be consulted for the primary references. The data for 1,3-butadiene are from Tuazon et al. (1999) and Kramp and Paulson (2000), and the data cited for the NO3 radical reactions with the monoalkenes are from the study of Hjorth et al. (1990) in which NO was not added after the initial reaction (see Atkinson, 1994).

b. Note that because more than one molecule of a specific carbonyl can be formed from certain of the alkenes, molar yields greater than 100% can occur.


4.5 Peroxyacyl Nitrates

Peroxyacyl nitrates (PANs, RC(O)OONO2) are important secondary transformation products of gasoline emissions linked both to human health effects and to plant toxicity. Peroxyacyl nitrates have not been measured routinely as part of national, state, or local air quality surveillance programs. The available database concerning the formation and reactions of PANs mainly deals with the two simplest PANs, peroxyacetyl nitrate (CH3C(O)OONO2; PAN) and peroxypropionyl nitrate (CH3CH2C(O)OONO2; PPN). PAN is the most prevalent and most studied peroxyacyl nitrate in the troposphere (IUPAC, 2004). For a discussion of the atmospheric chemistry of PPN, which is generally found in significantly lower concentrations than PAN, refer to IUPAC (2004); for ambient and spatial measurements, refer to Grosjean et al. (1996).

Peroxyacyl nitrates are formed exclusively by tropospheric photochemistry; there are no direct emission sources of these compounds. For this reason, PAN is often used as an indicator of photochemical air pollution. Peroxyacyl nitrates are formed from the reaction of acyl radicals, RC•O, where R ≠ H.

RC•O + O2 → RC(O)OO•

RC(O)OO• + NO2 → RC(O)OONO2

Therefore, any reaction which creates an acyl radical can result in the formation of a peroxyacyl nitrate. There are multiple chemical reaction pathways by which VOCs present in the troposphere react to form acyl radicals. Aldehydes are among the most important sources of acyl radicals through their reaction with the OH radical.

OH + RCHO → H2O + RC•O (R ≠ H)

Other sources of acyl radicals include the decomposition of intermediate alkoxy radicals. For example, acetyl radicals are formed from the OH radical-initiated reaction of methyl ethyl ketone, and from the photolysis and OH radical-initiated reactions of methylglyoxal [CH3C(O)CHO] formed from alkyl benzenes (such as toluene, xylenes and trimethylbenzenes).

PANs thermally decompose to the acyl peroxy radical plus NO2, the reverse reaction to their formation reaction.

RC(O)OONO2 → RC(O)OO• + NO2

The lifetime of PANs due to thermal decomposition is ∼30 min at 298 K. In the presence of NO, the thermal decomposition of PAN leads to formation of the methyl radical.

CH3C(O)OONO2 → CH3C(O)OO• + NO2


CH3C(O)OO• + NO → CH3C(O)O• + NO2

↓

CO2 + •CH3

The methyl radical reacts further to form formaldehyde, as shown in Section 3.3 above.

In addition to reacting with NO, acyl peroxy radicals can also react with NO2 (to reform PANs, see above), HO2 radicals and NO3 radicals.

RC(O)OO• + HO2 → RC(O)OH + O3

RC(O)OO• + HO2 → RC(O)OOH + O2

and

RC(O)OO• + NO3 → RC(O)O• + NO2 + O2

↓

R• + CO2

The fraction of an acyl peroxy radical which forms a PAN therefore depends on the NO2/NO concentration ratio, with the fraction forming a PAN = kNO2[NO2]/(kNO[NO] + kNO2[NO2]), where kNO and kNO2 are the rate constants for the reactions of acyl peroxy radicals with NO and NO2, respectively.

Peroxyacyl nitrates may also be removed by dry deposition, especially at nighttime when NO concentrations are low (and hence the effective lifetimes of PANs become long). The diurnal and spatial variations of PAN and PPN measured in the Los Angeles air basin mirror expected photochemical formation with concentrations increasing as polluted air masses move eastward (similar to ozone concentrations [Grosjean et al., 1996]).

5. GASEOUS CRITERIA AIR POLLUTANTS: CO, NO2, OZONE, SO2

CO is both directly emitted from gasoline-fueled vehicle exhaust as well as being formed in the atmosphere from the atmospheric reactions of VOCs. Direct emissions of CO dominate over atmospheric formation (the emission standard for CO from light-duty vehicles is about an order


of magnitude higher than that for VOCs). The formation of O3 and of NO2 from directly-emitted NO have been described above and elsewhere (see, for example, Finlayson-Pitts and Pitts, 2000; Atkinson 2000). SO2 is emitted in tailpipe exhaust as a combustion product of the sulfur in gasoline. The introduction of California Phase 2 Reformulated Gasoline in 1996 significantly reduced sulfur levels in fuel. Sulfur dioxide emissions decreased concomitantly, such that gasoline-powered vehicles are not currently a major source of SO2 in California. SO2 is removed from the atmosphere by gas-phase reaction with OH radicals (to form sulfuric acid, with an SO2 lifetime of approximately 10 days), dry deposition to the Earth's surface with a lifetime of a few days, and incorporation into cloud, fog and rain water with oxidation to sulfate in these aqueous systems, followed by precipitation.

6. GAS/PARTICLE PARTITIONING OF ORGANIC COMPOUNDS

As organic pollutants are photooxidized in the troposphere, secondary products are produced, some of which have sufficiently low vapor pressures that the compound homogeneously nucleates or partitions onto/into already existing aerosol.

For gas/particle partitioning involving surface adsorption, the determining factors are the liquid-phase vapor pressure of the chemical, PL, and the surface area of particles in the atmosphere, θ (in units of surface area per volume of air), with the fraction of the chemical in the particle phase, Φ, being given by,

Φ = cθ/(cθ + PL)

where c is a constant. For gas/particle partitioning proceeding by absorption of the chemical into organic material present in the aerosol, it is assumed that the organic material in the aerosol can be treated as behaving like octanol (and hence the gas/particle partitioning coefficient is described by the inverse of the octanol-air partition coefficient KOA [Finizio et al., 1997]). For an absorption process, the gas/particle partitioning depends on PL and the amount, and activity, of aerosol-phase organic matter present in the atmosphere. The available database suggests that these two approaches are complementary (Finizio et al., 1997); as a rough approximation organic chemicals with vapor pressures PL >1 Pa (>10-2 Torr) at the ambient temperature are present essentially totally in the gas phase, those with vapor pressures PL

7. CONCLUSIONS

The reactions of gasoline-related VOCs in the atmosphere lead, in general, to formation of oxygenates and, to a lesser extent, organic nitrates. The oxygenated compounds are largely carbonyls (including aldehydes, ketones, and hydroxycarbonyls). Atmospheric formation of many of these carbonyl compounds may dominate over direct emissions, in particular during summertime when photochemical activity is highest. This has been demonstrated for formaldehyde and acetaldehyde in the Los Angeles basin during summertime, where the majority of these two aldehydes are formed from atmospheric reactions of other VOCs. The first generation products of VOCs go on to react further, leading to the formation of progressively more oxygenated compounds that may partition into the particle phase. SOA formation is more likely for first-generation products that are of higher molecular weight than their precursor VOC. Specific reaction products arising from the gasoline-related pollutants selected for analysis are discussed in Appendix 1 and summarized in Tables 4-6. Table 4 summarizes the observed products for selected gasoline-related VOCs, sorted according to the parent compound. Table 5 shows the observed product with the parent VOCs (included in this analysis) that give rise to that product. Table 6 summarizes the products that are predicted to form, or for which there is some evidence of formation, from atmospheric reactions of VOCs. Table 7 summarizes observed and predicted (or tentatively identified) atmospheric transformation products for selected PAHs.

VOCs emitted into the atmosphere react at varying rates, so their radius of impact will be local, regional, and in some cases global, depending on their atmospheric lifetimes. For example, 1,3-butadiene, with an estimated lifetime of a few hours, will degrade over a local scale, ethene, with an estimated lifetime of one to two days, will degrade over a regional scale, and benzene, with an estimated lifetime of approximately ten days, will degrade over a global scale. Lifetimes of other gasoline-related VOCs were discussed in the body of the document.

The rate constants for the initial reactions of VOCs with OH radicals, NO3 radicals, and O3 are well understood, and this is also the case for the reactions of a number of VOC reaction products. However, the reaction products and mechanisms are less well understood, and these are the areas in which most of the current research into the atmospheric chemistry of VOCs is being focused. Additionally, in contrast to the semi-quantitative understanding of the atmospheric chemistry of VOCs that has been developed, the toxicology of the reaction products is essentially unknown but likely significant. Monitoring for potentially important products and investigating the toxicology of those products are areas of critically needed research.


Table 4. Observed atmospheric transformation products of selected gasoline-related VOCs, sorted by parent

Parent Observed productsa

Acetaldehyde Carbon dioxide Carbon monoxideb

Formaldehyde Methaneb

Peroxyacetyl nitrate (PAN) Acetylene Carbon monoxide

Formic acid Glyoxal

Acrolein Acryloylperoxy nitrate Carbon dioxide Carbon monoxide Formaldehyde Glycolaldehyde Glyoxal Ketene

Benzaldehyde 2-Nitrophenol Peroxybenzoyl nitrate

Benzene Glyoxal Phenol

1,3-Butadiene Acrolein 3,4-Epoxy-1-butene Formaldehyde Furan 4-Nitrooxy-2-butenal 4-Nitrooxy-3-oxo-1-butene

n-Butane Acetaldehyde Butanal 2-Butanone 1-Butyl nitrate 2-Butyl nitrate

2,3-Dimethylbutane Acetone Ethane Acetaldehyde

Ethanol Ethyl hydroperoxide

Ethanol Acetaldehyde


Table 4. (continued)

Parent Observed productsa Ethene Carbon dioxide

Carbon monoxide Formaldehyde Formic acidc

Glycolaldehyde Hydroperoxymethanol 1-Hydroxy-2-ethyl nitrate

Ethylbenzene Acetophenone Benzaldehyde

Formaldehyde Carbon monoxide Hydrogen

Furan 2-Butenedial Formic acid 3H-Furan-2-one Glyoxal Maleic anhydride

n-Hexane 2-Hexyl nitrate 3-Hexyl nitrate 3-Hexanone 4-Hydroxyhexanal 5-Hydroxy-2-hexanone 6-Hydroxy-3-hexanone

Isobutene Acetone Carbon dioxide Carbon monoxide Formaldehyde Formic acid Hydroperoxymethanol Methanol

Isopentane Acetone Acetaldehyde 2-Methyl-2-butyl nitrate 3-Methyl-2-butyl nitrate 2-Propyl nitrate



Parent Observed productsa 2-Methylpentane Acetone

2-Methyl-2-pentyl nitrate 2-Methyl-3-pentyl nitrate 4-Methyl-2-pentyl nitrate Propanal

3-Methylpentane 3-Methyl-2-pentyl nitrate MTBE Acetone

t-Butyl formate Formaldehyde Methyl acetate

n-Pentane Acetaldehyde 4-Hydroxypentanal 5-Hydroxy-2-pentanone 2-Pentanone 3-Pentanone 2-Pentyl nitrate 3-Pentyl nitrate Propanal

Peroxyacetyl nitrate Carbon dioxide (PAN) Formaldehyde Phenol 2-Nitrophenol

1,2-Dihydroxybenzene 1,4-Benzoquinone

Propylene Acetaldehyde Acetic acid Carbon monoxide Carbon dioxide Formaldehyde 2-Hydroxy-1-propyl nitrate 1-Hydroxy-2-propyl nitrate Methane 2-Nitrooxypropanal 1-Nitrooxy-2-propanone



Parent Observed productsa Styrene Benzaldehyde

Carbon dioxide Carbon monoxide Formaldehyde Formic acid

Toluene Benzaldehyde Benzyl nitrate 2-Butenedial o-Cresol m-Cresol p-Cresol Glyoxal 2-Methyl-2-butenedial Methylglyoxal 4-Oxo-2-pentenal

1,2,3-Trimethylbenzene 2,3-Butanedione Glyoxal Methylglyoxal

1,2,4-Trimethylbenzene 2,3-Butanedione 2,4-Dimethylbenzaldehyde 2,5-Dimethylbenzaldehyde 3,4-Dimethylbenzaldehyde Glyoxal 3-Hexene-2,5-dione 2-Methyl-2-butenedial 3-Methyl-3-hexene-2,5-dione 2-Methyl-4-oxo-2-pentenal Methylglyoxal 2,3,5-Trimethylphenol 2,3,6-Trimethylphenol 2,4,5-Trimethylphenol



Parent Observed productsa 1,3,5-Trimethylbenzene 3,5-Dimethylbenzaldehyde

3,5,-Dimethyl-3(2H)-2-furanone 3,5,-Dimethyl-5(2H)-2-furanone Methylglyoxal Methylmaleic anhydride 3-Methyl-5-methylidene-5(2H)-2furanone 2-Methyl-4-oxo-2-pentenal 2,4,6-Trimethylphenol

2,2,4-Trimethylpentane Acetone C4-Alkyl hydroxy nitratesd

C7-Alkyl hydroxy nitratese

C8-Alkyl hydroxy nitratesf

C8-Alkyl nitratesg

5-Hydroxy-4,4-dimethyl-2pentanone 4-Hydroxy-4-methyl-2-pentanone 2-Methylbutanal 2,2,4-Trimethylpentanal

2,3,4-Trimethylpentane Acetaldehyde Acetone C8-Alkyl nitratesh

3-Methyl-2-butanone 3-Methyl-2-butyl nitrate 2-Propyl nitrate

m-Xylene 2,4-Dimethylphenol 2,6-Dimethylphenol Glyoxal m-Methylbenzyl nitrate 2-Methyl-2-butenedial Methylglyoxal 4-Oxo-2-methyl-2-pentenal 4-Oxo-2-pentenal m-Tolualdehyde



Parent Observed productsa o-Xylene 2-Butenediali

2,3-Butanedione 2,3-Dimethyl-2-butenedial 2,3-Dimethylphenol 3,4-Dimethylphenol Glyoxal o-Methylbenzyl nitrate Methylglyoxal 4-Oxo-3-methyl-2-pentenal 4-Oxo-2-pentenal o-Tolualdehyde

p-Xylene 2,5-Dimethylphenol Glyoxal 3-Hexene-2,5-dione p-Methylbenzyl nitrate 2-Methyl-2-butenedial Methylglyoxal p-Tolualdehyde

a. NO2 and O3 form during the photooxidation of VOCs in the presence of NO (see section 3.3 for more explanation), and thus would be expected atmospheric transformation products of all the VOCs listed in this Table.

b. From photolysis. c. From decomposition of hydroperoxymethanol. d. Specific isomers not elucidated. Predicted isomer: 2-hydroxy-2-methyl-1-propyl nitrate. e. Specific isomers not elucidated. Predicted isomers: 4-hydroxy-2,4-dimethyl-2-pentyl nitrate; 4-hydroxy-2,2

dimethyl-1-pentyl nitrate. f. Specific isomers not elucidated. Predicted isomers: 1-hydroxy-2,2,4-trimethyl-4-pentyl nitrate; 4-hydroxy

2,2,4-trimethyl-1-pentyl nitrate. g. Specific isomers not elucidated. Predicted isomers: 2,2,4-trimethyl-3-pentyl nitrate; 2,4,4-trimethyl-2-pentyl

nitrate; 2-nitrooxymethyl-2,4-dimethylpentane; 2-nitrooxymethyl-4,4-dimethylpentane. h. Specific isomers not elucidated. Predicted isomers: 2,3,4-trimethyl-3-pentyl nitrate; 2,3,4-trimethyl-1-pentyl

nitrate; 2,3,4-trimethyl-2-pentyl nitrate; 3-nitrooxymethyl-2,4-dimethylpentane. i. Expected co-product to 2,3-butanedione (o-xylene reaction); observed but not quantified.


Table 5. Observed atmospheric transformation products of selected gasoline-related VOCs, sorted by product

Product Parent(s)a

Acetaldehyde n-Butane; ethane; ethanol; isopentane; npentane; propylene; 2,3,4-trimethylpentane;

Acetic acid Propylene Acetone 2,3-Dimethylbutane, isobutene; isopentane; 2

methylpentane; MTBE; 2,2,4-trimethylpentane; 2,3,4-trimethylpentane

Acetophenone Ethylbenzene Acrolein 1,3-Butadiene Acryloylperoxy nitrate Acrolein C4-Alkyl hydroxy nitratesb 2,2,4-Trimethylpentane C7-Alkyl hydroxy nitratesc 2,2,4-Trimethylpentane C8-Alkyl hydroxy nitratesd 2,2,4-Trimethylpentane C8-Alkyl nitrates 2,2,4-Trimethylpentanee ; 2,3,4

trimethylpentanef

Benzaldehyde Ethylbenzene; styrene; toluene 1,4-Benzoquinone Phenol Benzyl nitrate Toluene Butanal n-Butane 2,3-Butanedione 1,2,3-Trimethylbenzene; 1,2,4

trimethylbenzene; o-xylene 2-Butanone n-Butane 2-Butenedial Furan; toluene; xylenes t-Butyl formate MTBE 1-Butyl nitrate n-Butane 2-Butyl nitrate n-Butane Carbon dioxide Acetaldehyde; acrolein; ethene; isobutene; PAN;

propylene; styrene Carbon monoxide Acetaldehyde; acetylene; acrolein; ethene;

formaldehyde; isobutene; propylene; styrene m-Cresol Toluene o-Cresol Toluene p-Cresol Toluene 1,2-Dihydroxybenzene Phenol 2,4-Dimethylbenzaldehyde 1,2,4-Trimethylbenzene 2,5-Dimethylbenzaldehyde 1,2,4-Trimethylbenzene 3,4-Dimethylbenzaldehyde 1,2,4-Trimethylbenzene 3,5-Dimethylbenzaldehyde 1,3,5-Trimethylbenzene 2,3-Dimethyl-2-butendial o-Xylene



Product Parent(s)a 3,5-Dimethyl-3(2H)-2furanone

1,3,5-Trimethylbenzene

3,5-Dimethyl-5(2H)-2furanone


2,3-Dimethylphenol o-Xylene 2,4-Dimethylphenol m-Xylene 2,5-Dimethylphenol p-Xylene 2,6-Dimethylphenol m-Xylene 3,4-Dimethylphenol o-Xylene 3,4-Epoxy-1-butene 1,3-Butadiene Ethanol Ethane Ethyl hydroperoxide Ethane Formaldehyde Acetaldehyde; acrolein; 1,3-butadiene; ethene;

isobutene; MTBE; PAN; propylene; styrene Formic acid Acetylene; ethene; furan; isobutene; styrene Furan 1,3-Butadiene 3H-Furan-2-one Furan Glycolaldehyde Acrolein; ethene Glyoxal Acetylene; acrolein; benzene; furan; toluene;

1,2,3-trimethylbenzene; 1,2,4-trimethylbenzene; xylenes

3-Hexanone n-Hexane 3-Hexene-2,5-dione 1,2,4-Trimethylbenzene; xylenes 2-Hexyl nitrate n-Hexane 3-Hexyl nitrate n-Hexane Hydrogen Formaldehyde Hydroperoxymethanol Ethene, isobutene 5-Hydroxy-4,4-dimethyl-2pentanone

2,2,4-Trimethylpentane

1-Hydroxy-2-ethyl nitrate Ethene 4-Hydroxyhexanal n-Hexane 5-Hydroxy-2-hexanone n-Hexane 6-Hydroxy-3-hexanone n-Hexane 4-Hydroxy-4-methyl-2pentanone

2,2,4-Trimethylpentane

4-Hydroxypentanal n-Pentane 5-Hydroxy-2-pentanone n-Pentane 2-Hydroxy-1-propyl nitrate Propylene 1-Hydroxy-2-propyl nitrate Propylene



Product Parent(s)a Ketene Acrolein Maleic anhydride Furan Methane Acetaldehyde; propylene Methanol Isobutene Methyl acetate MTBE m-Methylbenzyl nitrate m-Xylene o-Methylbenzyl nitrate o-Xylene p-Methylbenzyl nitrate p-Xylene 2-Methylbutanal 2,2,4-Trimethylpentane 3-Methyl-2-butanone 2,3,4-Trimethylpentane 2-Methyl-2-butenedial Toluene; 1,2,4-trimethylbenzene; xylenes 3-Methyl-2-butyl nitrate 2,3,4-Trimethylpentane Methylglyoxal Toluene; 1,2,3-trimethylbenzene; 1,2,4

trimethylbenzene; 1,3,5-trimethylbenzene xylenes

3-Methyl-3-hexene-2,5-dione 1,2,4-Trimethylbenzene Methylmaleic anhydride 1,3,5-Trimethylbenzene 3-Methyl-5-methylidene5(2H)-2-furanone


2-Methyl-4-oxo-2-pentenal 1,2,4-Trimethylbenzene; 1,3,5-trimethylbenzene 2-Methyl-2-pentyl nitrate 2-Methylpentane 2-Methyl-3-pentyl nitrate 2-Methylpentane 3-Methyl-2-pentyl nitrate 3-Methylpentane 4-Methyl-2-pentyl nitrate 2-Methylpentane Nitrogen dioxide All VOCs 2-Nitrophenol Benzaldehyde; phenol 4-Nitrooxy-2-butenal 1,3-Butadiene 4-Nitrooxy-3-oxo-1-butene 1,3-Butadiene 2-Nitrooxypropanal Propylene 1-Nitrooxy-2-propanone Propylene 4-Oxo-2-methyl-2-pentenal Xylenes 4-Oxo-3-methyl-2-pentenal Xylenes 4-Oxo-2-pentenal Toluene; xylenes Ozone All VOCs 2-Pentanone n-Pentane 3-Pentanone n-Pentane 2-Pentyl nitrate n-Pentane 3-Pentyl nitrate n-Pentane



Product Parent(s)a Peroxyacetyl nitrate (PAN) Acetaldehyde Peroxybenzoyl nitrate Benzaldehyde Phenol Benzene Propanal 2-Methylpentane; n-pentane 2-Propyl nitrate 2,3,4-Trimethylpentane; isopentane m-Tolualdehyde m-Xylene o-Tolualdehyde o-Xylene p-Tolualdehyde p-Xylene 2,2,4-Trimethylpentanal 2,2,4-Trimethylpentane 2,3,5-Trimethylphenol 1,2,4-Trimethylbenzene 2,3,6-Trimethylphenol 1,2,4-Trimethylbenzene 2,4,5-Trimethylphenol 1,2,4-Trimethylbenzene 2,4,6-Trimethylphenol 1,3,5-Trimethylbenzene

a. Includes only gasoline-related compounds that were included in the current analysis.

b. Predicted isomer: 2-hydroxy-2-methyl-1-propyl nitrate.

c. Predicted isomers: 4-hydroxy-2,4-dimethyl-2-pentyl nitrate; 4-hydroxy-2,2-dimethy-1-pentyl nitrate

d. Predicted isomers: 4-nitrooxy-2,2,4-trimethyl-4-pentyl nitrate; 4-hydroxy-2,2,4-trimethyl-1-pentyl nitrate

e. Predicted isomers: 2,2,4-trimethyl-3-pentyl nitrate; 2,4,4-trimethyl-2-pentyl nitrate; 2-nitrooxymethyl-2,4dimethylpentane; 2-nitrooxymethyl-4,4-dimethylpentane

f. Predicted isomers: 2,3,4-trimethyl-3-pentyl nitrate; 1-nitrooxy-2,3,4-trimethyl-1-pentyl nitrate; 2,3,4trimethyl-2-pentyl nitrate; 3-nitrooxymethyl-2,4-dimethylpentane


Table 6. Predicted or tentatively identifieda atmospheric transformation products for selected gasoline-related VOCs

Parent Predicted or tentatively identified products

Benzaldehyde Benzene Carbon monoxide 4-Nitrophenol Nitrosobenzene

Benzene 2-Butenedial 2,3-Epoxy-4-hexenedialb

Hexa-2,4-dienedialc

1,3-Butadiene 3-Hydroperoxy-4-nitrooxy-1-butene 1-Hydroperoxy-4-nitrooxy-2-butene 4-Hydroxy-2-butenal 2-Hydroxy-4-nitrooxy-1-butene 4-Hydroxy-1-nitrooxy-2-butene 4-Hydroxy-2-nitrooxy-1-butene

n-Butane 2,3-Dihydrofuran 4-Hydroxybutanal 4-Hydroxy-1-butyl nitrate

2,3-Dimethylbutane Acetaldehyde 2,3-Dimethylbutanal 2,3-Dimethyl-1-butyl nitrate 2,3-Dimethyl-2-butyl nitrate Formaldehyde 4-Hydroxy-2,3,-dimethylbutanal 2-Propyl nitrate

2,3-Dimethylpentane Acetone Acetaldehyde 2-Butanone 2-Butyl nitrate 2,3-Dimethyl-2-pentyl nitrate 3,4-Dimethyl-2-pentyl nitrate 2,3-Dimethyl-3-pentyl nitrate Formaldehyde 4-Hydroxy-2,3-dimethylpentanal 4-Hydroxy-3,4-dimethylpentanal 4-Hydroxy-2-ethyl-3-methylbutanal 3-Methyl-2-butanone 2-Methyl-2-butyl nitrate 2-(1-Methylethyl)-4-hydroxybutanal 2-Propyl nitrate




2,4-Dimethylpentane Acetone 4,5-Dihydroxy-4-methyl-2-pentanone 2,4-Dimethyl-4-hydroxy-1-pentyl nitrate 2,4-Dimethyl-3-pentanone 2,4-Dimethyl-1-pentyl nitrate 2,4-Dimethyl-2-pentyl nitrate 2,4-Dimethyl-3-pentyl nitrate 2,4-Dimethyl-5-hydroxy-2-pentyl nitrate Formaldehyde 4-Hydroxy-2,4-dimethylpentanal 4-Methyl-4-hydroxy-2-pentyl nitrate 2-Methyl-1-propyl nitrate 2-Methylpropanal 2-Propyl nitrate

Ethanol Formaldehyde Glycolaldehyde 1-Hydroxy-2-ethyl nitrate

Ethylbenzene 2-Butenedial 2-Ethyl-2-butenedial 2-Ethyl-2,3-epoxy-4-hexenedial 2-Ethyl-4,5-epoxy-2-hexenedial 3-Ethyl-4,5-epoxy-2-hexenedial Ethylglyoxal 2-Ethyl-2,4-hexadienedial 3-Ethyl-2,4-hexadienedial 2-Ethylphenol 3-Ethylphenol 4-Ethylphenol Formaldehyde Glyoxal 1-Phenyl-1-ethyl nitrate 4-Oxo-2-hexenal 6-Oxo-2,4-octadienal

Formic acid Carbon dioxide n-Hexane 4,5-Dihydro-2,5-dimethylfuran

4,5-Dihydro-2-ethylfuran 4,5-Dihydro-5-ethylfuran 1-Hexyl nitrate




n-Hexane (continued) 4-Hydroxy-1-hexyl nitrate 5-Hydroxy-2-hexyl nitrate 1-Hydroxy-4-hexyl nitrate

Isobutene Hydrogen 2-Hydroperoxy-2-propanol 1-Hydroperoxypropanone 1-Hydroxypropanone Methylglyoxal Methylhydroperoxide 2-Methyl-2-hydroperoxy-1-propyl nitrate 2-Methyl-1-hydroperoxy-2-propyl nitrate 2-Methyl-2-hydroxy-1-propyl nitrate 2-Methyl-1-hydroxy-2-propyl nitrate 2-Nitrooxy-2-methylpropanal PAN

Isopentane 4-Hydroxy-2-methylbutanal 4-Hydroxy-3-methylbutanal 3-Methyl-2-butanone

Methylcyclopentane Carbon dioxide Formaldehyde 4-Hydroxybutanal 2-Hydroxy-3-methylpentanedial 2-Hydroxy-4-methylpentanedial 3-Hydroxymethylpentanedial 4-Hydroxy-2-methylbutanal 4-Hydroxy-3-methylbutanal 2-Hydroxy-5-oxohexanal 4-Hydroxy-5-oxohexanal 4-Hydroxypentanal 1-Methyl-1-cyclopentyl nitrate 2-Methyl-1-cyclopentyl nitrate 3-Methyl-1-cyclopentyl nitrate 5-Nitrooxyhexanal 6-Nitrooxy-2-hexanone 3-Nitrooxy-6-hydroxy-2-hexanone 5-Nitrooxy-3-methylpentanal




Methylcyclopentane 5-Nitrooxy-4-methylpentanal (continued) 5-Hydroxy-2-nitrooxyhexanal

5-Hydroxy-2-nitrooxy-3-methylpentanal 5-Hydroxy-2-nitrooxy-4-methylpentanal PAN

3-Methylhexane Acetaldehyde 2-Butanone 2-Butyl nitrate 4-Hydroxy-2-ethylpentanal 4-Hydroxy-3-methylhexanal 4-Hydroxy-4-methylhexanal 5-Hydroxy-3-methyl-2-hexanone 5-Hydroxy-4-methyl-2-hexanone 6-Hydroxy-4-methyl-3-hexanone 5-Hydroxy-2-pentanone 3-Methyl-2-hexyl nitrate 3-Methyl-3-hexyl nitrate 4-Methyl-2-hexyl nitrate 4-Methyl-3-hexyl nitrate 1-Pentyl nitrate 2-Pentyl nitrate Propanal

2-Methylpentane 4-Hydroxy-4-methylpentanal 5-Hydroxy-4-methyl-2-pentanone 2-Methyl-3-pentanone 1-Propyl nitrate 2-Propyl nitrate




3-Methylpentane Acetaldehyde 2-Butanone 2-Butyl nitrate 2-Ethyl-1-butyl nitrate 4-Hydroxy-3-methylpentanal 2-(2-Hydroxyethyl)butanal 3-(Hydroxymethyl)-1-pentyl nitrate 3-Methyl-5-hydroxy-2-pentanone 3-Methyl-1-hydroxy-4-pentyl nitrate 3-Methyl-2-pentanone 3-Methyl-1-pentyl nitrate 3-Methyl-3-pentyl nitrate 3-Methyl-4-hydroxy-1-pentyl nitrate 3-(Nitrooxymethyl)-1-pentanol

2-Methylpropane Acetone t-Butyl nitrate Formaldehyde 2-Methylpropanal 2-Methyl-1-propyl nitrate 2-Propyl nitrate

n-Pentane 4-Hydroxy-1-pentyl nitrated

5-Hydroxy-2-pentyl nitratee

1-Pentyl nitratef

1-Propyl nitratef

Styrene Hydrogen Hydroperoxymethanol 1-Phenyl-1-hydroperoxy-2-ethyl nitrate 1-Phenyl-2-hydroperoxy-1-ethyl nitrate 1-Phenyl-2-hydroxy-1-ethyl nitrate 1-Phenyl-1-hydroxy-2-ethyl nitrate 2-Phenyl-2-nitrooxyethanal 1-Phenyl-2-nitrooxyethanone




Toluene Di-unsaturated 1,6-dicarbonylsg

Unsaturated epoxy-1,6-dicarbonylsh

1,2,3-Trimethylbenzene 2,3-Dimethylbenzaldehyde 2,6-Dimethylbenzaldehyde 2,6-Dimethylbenzyl nitrate 2,3-Dimethylphenol 2,6-Dimethylphenol 2,3-Dimethyl-4-oxo-2-pentenal 3-Methyl-4-oxo-2-pentenal 4-Oxo-2-pentenal

1,2,4-Trimethylbenzene 2,4-Dimethylbenzyl nitrate 2,5-Dimethylbenzyl nitrate 3,4-Dimethylbenzyl nitrate 2,3-Dimethyl-2-butenedial

1,3,5-Trimethylbenzene 3,5-Dimethylbenzyl nitrate 2,2,4-Trimethylpentane 4-Hydroxy-2,2,4-trimethylpentanal Xylenes C8-Di-unsaturated dicarbonylsi

C8-Unsaturated epoxy-1,6-dicarbonylsj

a. Includes products for which there is some evidence of formation.

b. Predicted product, not yet observed, though some evidence exists.

c. Some evidence for formation.

d. Major hydroxy nitrate expected.

e. Expected in much lower yield.

f. Expected in low yield, not observed to date.

g. Evidence exists for these compounds but specific isomers not elucidated. Predicted isomers: 6-oxohepta-2,4dienal; 2-methylhexa-2,4-dienedial; 3-methylhexa-2,4-dienedial.

h. Evidence exists for these compounds but specific isomers not elucidated. Predicted isomers: 6-oxo-4,5-epoxy2-heptenal; 6-oxo-2,3-epoxy-4-heptenal; 2,3-epoxy-2-methyl-4-hexenedial; 2,3-epoxy-5-methyl-4-hexenedial; 2,3-epoxy-3-methyl-4-hexenedial; 2,3-epoxy-4-methyl-4-hexenedial.

i. Potential products, not quantified to date. Potential isomers are: 3,5-octadiene-2,7-dione ; 6-oxo-5methylhepta-2,4-dienal; 2,3-dimethylhexa-2,4-dienedial; 3,4-dimethylhexa-2,4-dienedial; 6-oxo-2-methylhepta2,4-dienal; 6-oxo-4-methylhepta-2,4-dienal; 2,4-dimethylhexa-2,4-dienedial; 6-oxo-3-methylhepta-2,4-dienal; 2,5-dimethylhexa-2,4-dienedial.

j. Tentatively observed. Specific isomers not known; potential products are: 3,4-epoxy-5-octene-2,7-dione; 2,3epoxy-6-oxo-5-methyl-4-heptenal; 4,5-epoxy-6-oxo-5-methyl-2-heptenal; 2,3-epoxy-2,3-dimethyl-4hexenedial; 2,3-epoxy-4,5-dimethyl-4-hexenedial; 2,3-epoxy-3,4-dimethyl-4-hexenedial; 2,3-epoxy-6-oxo-2methyl-4-heptenal; 4,5-epoxy-6-oxo-2-methyl-2-heptenal; 2,3-epoxy-6-oxo-4-methyl-4-heptenal; 4,5-epoxy-6oxo-4-methyl-2-heptenal; 2,3-epoxy-2,4-dimethyl-4-hexenedial; 2,3-epoxy-3,5-dimethyl-4-hexenedial; 2,3epoxy-6-oxo-3-methyl-4-heptenal; 4,5-epoxy-6-oxo-3-methyl-2-heptenal; 2,3-epoxy-2,5-dimethyl-4-hexenedial


Table 7. Atmospheric transformation products of selected PAHs

Parent Observed products

and

Predicted/tentatively identified products

Acenaphthene 3-Nitroacenaphthene 4-Nitroacenaphthene 5-Nitroacenaphthene 2,3-Dihydro-3-oxo-1H-indene-4-carboxaldehydea

2,4-Dihydro-4-(oxoethylidene)-1H-indene-3-carboxaldehydeb

2,3-Dihydro-spiro[1H-indene-1,2'-oxirane]-3',7-dicarboxaldehydec

Acenaphthylene 4-Nitroacenaphthylene 6a,7a-Dihydro-acenaphth[3,4-b]oxirened

1,4-Epoxy-1H,4H-naphtho[1,8-de][1,2]dioxepine

1,8-Naphthalenedicarboxaldehyde 4-(Oxoethylidene)-4H-indene-3-carboxaldehydef

Anthracene 1-Nitroanthracene 2-Nitroanthracene

Benzanthrone 2-Nitrobenzanthrone Biphenyl 2-Hydroxybiphenyl

3-Nitrobiphenyl Fluoranthene 2-Nitrofluoranthene

7-Nitrofluoranthene 8-Nitrofluoranthene

Fluorene Fluorenone 1-Nitrofluorene 2-Nitrofluorene 3-Nitrofluorene 4-Nitrofluorene



Parent Observed products

and

Predicted/tentatively identified products

1-Methylnaphthalene 1-Methyl-2-nitronaphthalene 1-Methyl-3-nitronaphthalene 1-Methyl-4-nitronaphthalene 1-Methyl-5-nitronaphthalene 1-Methyl-6-nitronaphthalene 1-Methyl-7-nitronaphthalene 1-Methyl-8-nitronaphthalene

2-Methylnaphthalene 2-Methyl-1-nitronaphthalene 2-Methyl-3-nitronaphthalene 2-Methyl-4-nitronaphthalene 2-Methyl-5-nitronaphthalene 2-Methyl-6-nitronaphthalene 2-Methyl-7-nitronaphthalene 2-Methyl-8-nitronaphthalene

2-Methyl-1-nitronaphthalene 2-Methyl-1,4-naphthoquinone 2-Methyl-1-naphthol

Naphthalene 1,4-Naphthoquinone 2-Formylcinnamaldehyde 1-Naphthol 2-Naphthol 1-Nitronaphthalene 2-Nitronaphthalene 1-Hydroxy-2-nitronaphthalene 2-Hydroxy-1-nitronaphthalene

1-Nitronaphthalene 1,4-Naphthoquinone 2-Nitro-1-naphthol

Phenanthrene 2-Nitro-6H-dibenzo[b,d]pyran-6-one 4-Nitro-6H-dibenzo[b,d]pyran-6-one

Pyrene 2-Nitropyrene 4-Nitropyrene

a. Chemical Abstracts Service Registry Number (CASRN) is 24257-88-3. b CASRN for (4Z-) isomer is 474688-82-9.

CASRN is 474688-84-1. d. CASRN is 474688-85-2. e. CASRN is 195-15-6. f. CASRN for (4Z-) isomer is 474688-86-3.


c

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