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Atmospheric Environment 41 (2007) 3057–3070

www.elsevier.com/locate/atmosenv

Estimates of heterogeneous formation of secondary organicaerosol during a wood smoke episode in Houston, Texas

U. Nopmongcola, W. Khamwichita, M.P. Fraserb, D.T. Allena,�

aCenter for Energy and Environmental Resources, Mail Code R7100, The University of Texas at Austin,

10100 Burnet Road, Austin, TX 78758, USAbCivil and Environmental Engineering Department, Rice University, 6100 Main Houston, TX 77005, USA

Received 12 January 2006; received in revised form 18 September 2006; accepted 29 November 2006

Abstract

Observational data, collected during a wood smoke episode in Houston, Texas, were used to assess the extent to which

acid-catalyzed reactions of carbonyls might contribute to secondary organic aerosol (SOA) formation. The wood smoke

episode was chosen for this analysis because of relatively high concentrations of acidic aerosol, coupled with high

concentrations of SOA precursors during the episode. Photochemical modeling, coupled with ambient measurements,

indicated that acid aerosol-mediated organic aerosol formation reactions, not accounted for in most current

photochemical models, may have led to SOA formation of up to a few mgm�3. In photochemical simulations, acid-

mediated organic aerosol formation was modeled by calculating the rate of impingement of aldehyde molecules on acidic

particles, and then assuming that a fraction of the impingements resulted in reaction. For reaction probabilities on the

order of 0.005–0.0005, the model predicted SOA concentrations were consistent with estimates of SOA based on

observations. In addition, observed concentrations of particulate phase ammonium during the episode were consistent

with high concentrations of the types of organic acids that would be formed through acid-catalyzed reactions of carbonyls.

Although there are substantial uncertainties in the estimates of heterogeneous SOA formation, collectively, these data and

modeling analyses provide evidence for the importance of acid-catalyzed SOA formation reactions.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Biomass combustion; Secondary organic aerosol; Sulfate; Acid; Heterogeneous reactions

1. Introduction

Organic carbon (OC) is a major component ofatmospheric particulate matter (PM) and consists ofprimary organic compounds, emitted directly fromanthropogenic and biogenic sources, as well as

e front matter r 2006 Elsevier Ltd. All rights reserved

mosenv.2006.11.050

ing author. Tel.: +1512 475 7842;

7060.

ess: allen@che.utexas.edu (D.T. Allen).

secondary organic aerosol (SOA), formed viareactions in the atmosphere. SOA is formed throughthe oxidation of gas phase organic compounds;these reactions produce semi-volatile organic com-pounds (SVOC), which partition between gas andaerosol phases. SOA may also be formed via acid-catalyzed heterogeneous reactions of gas phaseorganic compounds with aerosols (Czoschke et al.,2003; Gao et al., 2004; Iinuma et al., 2004; Jangand Kamens 2001; Jang et al., 2002; Tolocka et al.,

.

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Table 1

Average OC, sulfate and total PM2.5 mass concentration (mgm�3,measured using filter samples) for wood smoke (4–8 September)

and non-wood smoke days (21, 27 August, 2 and 14 September)

Site description Wood smoke Non-wood smoke

OC Sulfate PM2.5 OC Sulfate PM2.5

Channelview 6.68 8.74 22.50 2.89 5.87 12.95

Conroe 4.35 7.50 17.45 3.21 5.06 13.18

Galveston Airport 6.60 7.62 22.30 1.70 5.38 10.15

Houston Aldine 8.95 7.75 25.20 3.73 5.43 13.03

Houston

Bayland Park

6.72 7.65 22.93 2.96 5.47 12.48

Houston Deer Park 2 6.87 8.62 24.23 2.67 6.68 13.90

HRM-3 Haden Road 6.54 8.96 22.87 3.51 5.45 14.40

La Porte Airport 6.86 7.71 23.49 2.67 6.03 12.44

Avg. of all sites 6.70 8.07 22.62 2.92 5.67 12.62

Fig. 1. Measurement sites (dots) and domains used in modeling

analyses (red and blue outlined areas); the area in red

corresponds to the red colored domain in Fig. 3.

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–30703058

2004). Specifically, it has been proposed that acid-catalyzed SOA formation involves carbonyl reac-tants undergoing hydration, polymerization, hemi-acetal and acetal formation, aldol condensation,ring opening of terpenoid carbonyls and cross-linking in acidic aerosol media (Jang et al., 2002).Evidence for these heterogeneous reaction pathwayshas largely been based on laboratory experiments.Ambient observational evidence of these surface/heterogeneous pathways has been limited and muchremains uncertain about the mechanisms and rates.This work will examine ambient data and photo-chemical modeling results for a wood smoke episodethat occurred during the Gulf Coast aerosolresearch and characterization experiment, whichprovide evidence for SOA formation via acid-catalyzed reactions.

During August and September 2000, an intensivefine particulate matter measurement campaign wasconducted in southeast Texas as part of the GulfCoast aerosol research and characterization experi-ment (GC-ARCH or the Houston PM Supersite).During part of this period, the region wassignificantly impacted by forest fires. The mostintense fire activity occurred from 30 August to8 September (Junquera et al., 2005), and organiccarbon and sulfate concentrations in PM2.5 wereelevated during this period, as compared to periodsbefore and after the fires (Buzcu et al., 2006;Nopmongcol and Allen, 2006). The relatively highconcentrations of acidic aerosol, described later inthis work, coupled with high concentrations of SOAprecursors, suggest that this episode might haveextensive SOA formation via acid-catalyzed path-ways.

For the analyses of ambient data performed inthis work, 4, 6 and 8 September will be selected torepresent the fire (wood smoke) episode and 21, 27August, 2 and 14 September will represent non-wood smoke episode days (before and after theintensive fire period). 2 September is included as anon-wood smoke day, even though fires occurred onthat day, because prevailing winds advected the fireemissions away from sampling sites (Buzcu et al.,2006). The averaged OC and sulfate concentrations,for wood smoke and non-wood smoke episode days,from nine monitoring stations, are shown inTable 1. The locations of the sites are shown inFig. 1. Sulfate concentrations on wood smoke dayswere, on average, 42% higher than on non-woodsmoke days. The acidity associated with the elevatedsulfate concentrations will be described in later

sections of this paper. OC and total PM2.5

concentrations were 130% and 79% higher onwood smoke episode days than on non-wood smokedays, respectively, as reported in Table 1. Thesedata indicate that OC, sulfate and total PMconcentrations were all elevated during the woodsmoke episode. The processes that lead to theenhanced sulfate concentrations have been exam-ined by Buzcu et al. (2006) and Nopmongcol andAllen (2006), however, the processes that may beresponsible for the elevated OC concentrations havenot yet been quantitatively examined. This workwill first examine whether the elevated OC concen-trations are due to direct emissions from fires. Since,as described in the next section, molecular tracer

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Table 2

Levoglucosan/OC concentrations (ngm�3/mgm�3) during Au-

gust and September 2000

08/21 08/27 09/02 09/06 09/08 9/14

Aldine 91.8/3.9 150.8/2.8 205.3/5.4 593.7/15.2 565.4/5.0 266.1/2.8

HRM-3 52.6/na 20.8/2.3 63.2/5.0 184.9/12.0 460.9/3.9 206.2/3.2

La Porte na/2.6 133.9/1.6 101.9/3.7 393.4/12.9 287.6/3.3 83.2/2.8

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–3070 3059

measurements indicate that a significant amount ofSOA is formed during the wood smoke episode,photochemical modeling will be used to assesswhether that SOA is due to increased partitioningof semi-volatile species into the particulate phase,acid-catalyzed heterogeneous reactions, or somecombination of these phenomena.

2. Estimates of primary OC emissions from fires

One explanation for the elevated OC concentra-tions associated with the wood smoke episode isthat the observed OC is due to primary emissionsfrom the fires. Both molecular and atomic tracershave been used to estimate primary OC emissionsfrom fires. Potassium (K) is sometimes used as amarker for wood combustion emissions, since Ktypically accounts for 0.2–1.8wt% of fine particlemass emitted by fires (Fine et al., 2002; Hildemannet al., 1991). However, other sources, such as meatcooking (Schauer et al., 1999), and refuse incinera-tion (Sheffield et al., 1994) can contribute toambient potassium concentrations, so estimates offire emissions based on K concentrations must beviewed with caution. More recent studies usedlevoglucosan as a tracer for all types of woodsmoke emissions (Fine et al., 2002; Simoneit et al.,1999). Levoglucosan emissions range from 40 to1200mg kg�1 of wood burned (Simoneit et al.,1999), and can account for a few percent to morethan 10% of the fine particle organic mass emittedby fires (Fine et al., 2002). To be used as a tracer offire emissions, levoglucosan must be relatively stablein the atmosphere; a pathway that has beenproposed as a possible sink for levoglucosan is theacid-catalyzed hydrolysis of levoglucosan to formb-D-glucose. However, Fraser and Lakshmanan(2000) found negligible degradation of levoglucosanunder both non-acidic and acidic conditions(sulfuric acid), for up to 10 days. So, levoglucosanis generally regarded as a specific, stable andidentifiable tracer of biomass combustion.

Levoglucosan concentrations in ambient PM thatwere measured during the summer 2000 wood smokeand non-wood smoke episodes have been reportedby Yue and Fraser (2004). Samples were extractedfirst by hexane, then by mixture of 2:1 benzene/isoproponal. The extracts were converted to estersand each organic compound was quantified usingGC–MS. The overall uncertainty in quantificationwas estimated to be 720%. Air samples, collected atthree sites; Aldine, HRM-3, and LaPorte, during 6

and 8 September were used for the wood smokeepisode, and those collected during 21, 27 August, 2and 14 September represented the non-wood smokeepisode. The averaged levoglucosan concentrationsduring the wood smoke and non-wood smokeepisodes were 410 and 124 ngm�3, respectively.

The levoglucosan concentrations together withthe OC concentrations are shown in Table 2. Theselevoglucosan concentrations can be used to estimateOC contributions from wood smoke, if OC tolevoglucosan ratios in the sources are known.Unfortunately, levoglucosan concentrations in PMfrom biomass burning vary significantly, dependingon the fuel type and the combustion conditions,making its use as a tracer species in quantitativesource apportionment challenging. Levoglucosan toOC ratios in the PM emissions from the combus-tion, under various conditions, of typical softwoodsand hardwoods (Oros and Simoneit, 2001a, b) rangefrom less than 0.001 to more than 0.5. Thisvariability could result in a substantial uncertainty(a factor of 2 or more) in the estimates of OC due toprimary emissions, even if fuel type and combustionconditions are well defined.

The variability in estimates of primary OC fromfires can be bounded somewhat by examining datafrom days that are not dominated by fire emissions.As shown in Table 2, the total OC concentration atthree sites in Houston, during non-wood smokeperiods ranged from 1.6 to 3.9 mgm�3. Using themeasured levoglucosan concentrations in the sam-ples from the non-wood smoke period, combinedwith OC to levoglucosan ratios from a variety ofsource apportionment studies, leads to estimates ofwood smoke contributions to OC that range fromless than 1 to more than 10 mgm�3. Details of thesecalculations are reported by Nopmongcol (2005).All of the primary OC estimates, with the exceptionof the estimates based on the data of Schauer et al.(2001), lead to estimated primary emissions that arelarger than the total OC during the non-woodsmoke periods. Since other tracers (e.g., for dieselexhaust and cooking) clearly indicate that biomass

ARTICLE IN PRESSU. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–30703060

burning is not the only source contributing to OCconcentrations, especially on non-wood smoke days(Buzcu et al., 2006), and because use of the Schaueret al. (2001) source profile for wood smoke leads togood mass closure on all sources of OC on non-wood smoke days, the Schauer et al. (2001) sourceprofiles were used in this work.

If the source data of Schauer et al. (2001) are used,the fraction of OC that can be attributed to primarysources can be estimated, albeit with some uncer-tainty. Buzcu et al. (2006) performed this assessmentfor wood smoke and non-wood smoke episodes, asreported in Fig. 2. Secondary organic aerosol andprimary organics from sources that were notattributed in the analysis are referred to as otherorganics. During non-wood smoke episode, otherorganics contributed 33%, 31%, and 36% of theorganic carbon at Aldine, HRM-3 and La Porte,respectively. If SOA represents roughly a third of OCon days with little or no wood smoke, as reported byRussell and Allen (2004), this represents reasonablemass closure. The apportionment of organic carbonin PM2.5 for the wood smoke days showed that thecontribution of the primary wood combustionsources to organic carbon concentrations are higher,but the primary emissions are not sufficient toexplain all of the mass increase. Other organics(SOA and unattributed primary sources) contribute

0

2

4

6

8

10

12

Org

anic

Carb

on (�g

/m3)

O

W

M

V

D

G

Aldine H

Wood smoke

Non-wood

smoke Non-wood

smoke

Fig. 2. Comparison of the source contributions to O

to 50–70% of observed OC mass on the days withfires. These analyses suggest that enhanced SOAformation during the wood smoke episode likelyaccounts for an average of 3–5mgm�3 of OC.

3. Photochemical modeling methods

While some of the additional OC observed duringthe wood smoke episode, as compared to the non-wood smoke episode, is undoubtedly due to primaryemissions, some may be due to additional partition-ing of semi-volatiles into the particulate phase, dueto the additional primary particulate matter avail-able in the atmosphere. In addition, acid-catalyzedorganic condensation reactions may lead to addi-tional organic carbon formation. Both of thesepathways will be examined through photochemicalmodeling.

Although any comparable photochemical gridmodel could be used, the comprehensive air qualitymodel with extensions (CAMx) (ENVIRON, 2005)was selected for this study because it is currentlybeing used by the State of Texas for attainmentdemonstrations in areas that violated the NationalAmbient Air Quality Standards for ozone. Theepisode period used in the modeling was 22August—6 September 2000, which included thewood smoke event. The modeling domain was a

ther Organics

ood Combustion

eat Cooking

egetative Detritus

iesel Vehicles

asoline Vehicles

RM3 La Porte

Wood smokeWood smoke

Non-wood

smoke

C in PM2.5 in two cases (Buzcu et al., 2006).

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nested regional/urban scale 36 km/12 km/4 km gridshown in Fig. 3. Meteorological inputs required bythe model were based on results from the mesoscalemeteorological model, version 5, MM5 (TCEQ2004a, b). The volatile organic compound (VOC)and nitrogen oxides (NOx) emission inventoriesused as input for the modeling episode wereprepared by the Texas Commission on Environ-mental Quality (TCEQ) in accordance with US EPAguidance. Details of the VOC and NOx emissioninventory development are available at TCEQ,Houston/Galveston Air Quality Science Evaluation(2004a, b). The sulfur dioxide (SO2) and ammonia(NH3) emission inventories are described elsewhere(Nopmongcol and Allen, 2006).

Because a primary focus of this work is onestimating secondary organic aerosol formation, themodeling approaches used for estimating SOAformation are described in detail here. SOA forma-tion modeling approaches used in regional photo-chemical air quality models have primarily assumeda gas-particle (G/P) equilibrium partitioning (Odumet al., 1996, 1997; Pankow, 1994), in which areactive hydrocarbon (HC) undergoes atmosphericoxidation to produce n semi-volatile (condensable,SVOC) products G1 to Gn as shown

HCþ oxidant! a1G1 þ a2G2 þ � � � þ anGn, (1)

Fig. 3. Modeling domain used in the study: the Regional, East

Texas and Houston–Galveston–Beaumont–Port Arthur nested

domains had 36, 12 and 4 km horizontal resolution, respectively.

where ai is the stoichiometric yield of Gi. In a G/Ppartitioning model, the mass balance for eachSVOC satisfies

Ct;i ¼ Cg;i þ Caer;i, (2)

where Ct,i is the total concentration of species i, Cg,i

is the gas-phase concentration, and Caer,i is theaerosol-phase concentration (all in mgm�3 of air).The total concentration can be expressed as

Ct;i ¼ ai

Mi

MHCDHC; (3)

where Mi is a molecular weight. Assuming that theaerosol species form an ideal organic solution, theirgas-phase concentrations will satisfy

Cg;i ¼ xi

PL;iMi

RT¼ xiC

�i , (4)

where xi is the mole fraction of the species i in thesolution, PL,i is the liquid vapor pressure of pureorganic compound i, and Ci

* is the effectivesaturation concentration in mgm�3 of pure i whichcan be determined from laboratory experiments.

Combining the above equations yields a set of n

equations with n unknowns

Caer;i ¼ aiMi

MHCDHC�

Caer;iC�i =MiP

Caer;i=Mi þ POA=MPOA

for i ¼ 1; n, ð5Þ

where primary organic (POA) (mgm�3) repre-sents any initially present absorbing organic mass.SOA concentrations can then be obtained bysolving this system (Odum et al., 1996, 1997). Thisapproach does not account for heterogeneousreactions.

As shown in Eq. (5), estimates of SOA formationrely on concentrations of POA. Previous work hasshown that primary organic carbon in southeastTexas is substantial (Russell and Allen, 2004) andcan serve as an absorbing medium for SVOCs.Unfortunately, the primary OC inventory availablefor southeast Texas has substantial uncertaintyassociated with it. One approach to dealing withan unknown or uncertain inventory of POA is toassume negligible primary OC as an absorbing massand to assume that condensable products willpartition only to a solution of SOA. This approachresults in under prediction of SOA formation,especially when primary OC is high, such as duringthe wood smoke episode. An alternative approach,that has been used by Russell and Allen (2005),is to use observed values of organic aerosol in

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Fig. 4. Interpolated OC concentrations (mgm�3) at 19:00 for 4

September 2000, within 4 km grid shown in Fig. 3. The

interpolations were based on data from the measurement sites

shown in Fig. 1. These OC concentrations, during the wood

smoke episode, are much higher than the typical concentrations

(�3mgm�3) observed on days without fires; the spatial distribu-

tion of the OC depended on the locations of the fires.

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–30703062

the photochemical modeling calculations. Thisapproach allows a direct and accurate characteriza-tion of the organic aerosol phase available forSVOCs to partition into, and was the approach usedin this work.

Twenty-four-hour average organic carbon con-centrations in PM2.5 were available at the monitor-ing sites shown in Fig. 1. It was assumed that thetotal amount of OC in PM2.5 was a goodapproximation of total organic absorbing mass.Absorbing mass for each grid cell during each hourwas then obtained by interpolating time resolvedPM2.5 data within the 4 km domain. Details of themethodology have been described by Russell andAllen (2005). Briefly, hourly measurements of totalPM2.5 using tapered element oscillating microba-lances (TEOM) were used to distribute averagedaily OC mass, at specific sites into specific hours.For example, if the TEOM mass for a specific hourwas twice the daily average TEOM mass, then theOC concentration for that hour was assigned avalue twice the daily average. The kriging inter-polation method was chosen for the interpolation(point kriging, linear variogram, slope ¼ 1, nodrift). These spatially resolved hourly OC concen-trations were assumed to uniformly distribute overthe planetary boundary layer (PBL). An example ofan OC interpolation is shown in Fig. 4.

Note that the interpolated OC mass does notrepresent primary organic aerosol, but total absorb-ing mass. It is, however, the POA that is required inthe model formulation described by Eq. (5). CAMxcodes were modified to read the interpolated OCmass and internally estimate POA mass for eachhour. POA was approximated as a mass differencebetween interpolated OC and predicted SOA fromthe last hour. For example, if the interpolated OC at900 h was 16 mgm�3 and the predicted SOA at 800 hwas 9 mgm�3 the POA at 900 h would be 7 mgm�3.

To model acid-catalyzed SOA formation, animpingement/reactive uptake calculation approachwas used. An impingement model, rather than anexplicit mechanism, was used because a variety ofmechanisms have been proposed for the acid-catalyzed organic condensation reactions on acidicaerosol, and because quantitative information onthe rates and extent of these reactions are not yetavailable. In this approach, the SOA formation rateis defined as the product of the total number ofcollisions per unit time between catalytic surfacesand gas phase reactants, multiplied by the prob-ability that a collision results in a condensation

reaction:

d½SOA�i;het=dt ¼ gZi, (6)

Zi ¼ 1=4A½Ci�ni, (7)

where i represents the reacting species, ni is the mean

molecular velocity, calculated asffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8RT=pMi

pand A

is the surface area potentially associated with acid-catalyzed reactions. Available acidic particle surfacewas calculated by assuming a particle density of1.5 g cm�3 and an average acidic particle diameter of0.25mm. The mass of the primary smoke emissionswas drawn from the work of Junquera et al. (2005),and the smoke particle diameter was based on thework of Reid (2004), who listed aerosol volumemedian diameters for fresh smoke in the 0.25–0.3mmrange. The degree of neutralization was representedby hydrogen ion concentration, [H+].

Hydrogen ion concentration was estimated byperforming a cation–anion balance. In order toconvert this degree of neutralization into anestimate of available acidic aerosol surface arearequired in Eq. (7), several assumptions weremade. Specifically, it was assumed that for eachmole of [H+], there would be 0.5mol of sulfate

ARTICLE IN PRESSU. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–3070 3063

(48 mg/mmol�1 of [H+]). Since sulfate accountsfor 30%, of aerosol mass, on average, there wouldbe (1/0.3)� 48 mg of total aerosol per mmol of[H+] (Russell et al., 2004; Tropp et al., 1998). Thus,1 mmol of [H+] would be associated with 160 mgof PM mass. Further assuming an average particledensity of 1.5 g cm�3 and an average acidicparticle diameter of 0.25 mm, allows for calcula-tion of acidic aerosol mass and surface area. While,this estimate of acidic surface area has signifi-cant assumptions associated with it, the estimateshould be suitable for estimating the order ofmagnitude of SOA formation due to acid-catalyzedreactions.

The tools used to estimate aerosol sulfate shouldalso be noted. Observational data indicated thatduring the wood smoke episode sulfate could formvia heterogeneous sulfate formation on carbonac-eous surfaces with reaction probabilities on theorder of 0.01 (Nopmongcol and Allen, 2006). So, inaddition to conventional sulfate formation mechan-isms, in this work, heterogeneous mechanisms forsulfate formation were considered as described byNopmongcol and Allen (2006).

Once the available acidic surface has beencalculated, the concentrations of gas phase reactantsparticipating in acid-catalyzed reactions must bedetermined. The gas-phase chemical mechanismused within CAMx in this work was the carbonbond-IV mechanism (CBIV). The aldehydes(ALD2) species in the CBIV mechanism representscarbonyl groups with two or more C atoms, and sowas chosen as the potential reactant. For each moleof ALD2 that was assumed to react via an acid-catalyzed mechanism, it was assumed that 32 g (theassumed molecular weight of ALD2) was added toSOA. CAMx codes were modified to account forthese acid-catalyzed heterogeneous reactions.

La Porte Supersite AL

0

5

10

15

20

25

0 12

A22

0 12

A23

0 12

A24

0 12

A25

0 12

A26

0 12

A27

0 12

A28

0 12

A29

0

A

Initial Base Ca

Fig. 5. Modeled and observed surface layer aldehydes (ALD2) conce

September 2000 (TCEQ, 2005c).

4. Results and discussion

4.1. Model performance

Model performance for this episode has beenevaluated by comparing observed and predictedozone, sulfate, aldehyde, and VOC concentrations.Ozone modeling performance statistics were calcu-lated by the TCEQ (TCEQ, 2004c). Although thereare discrepancies between observed and modeledpeak ozone concentrations on some days, theoverall performance was judged acceptable for airquality planning by the State of Texas. Modelperformance in predicting VOC concentrations wasalso evaluated by TCEQ, by examining predictedand observed diurnal patterns in VOC concentra-tion at specific sites. Time series of observed andmodeled aldehyde (ALD2) concentrations during 22August—6 September 2000, at the La Porte site areshown in Fig. 5. Observed species were transformedto CB-IV species for this purpose. Discrepanciesbetween observed and modeled peak aldehydeconcentrations exist on some days, however, theoverall aldehyde predictions shown in Fig. 5 werejudged to be acceptable for estimating the rates ofheterogeneous carbonyl reactions.

Finally, previous work demonstrated that themodel could accurately predict a fraction of SO2

conversion to sulfate that is consistent with ob-servations when wood smoke is not present andexcess sulfate observed with the presence of woodsmoke could be explained by heterogeneous reac-tion pathways (Nopmongcol and Allen, 2006).

4.2. Modeling results

CAMx was used to simulate the extent to whichSOA formation would be enhanced due to acid-

D2 Concentrations

12

30

0 12

A31

0 12

S1

0 12

S2

0 12

S3

0 12

S4

0 12

S5

0 12

S6

0

se Observed

ntration at the La Porte monitoring site from 22 August to 6

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catalyzed carbonyl condensation reactions and dueto the availability of additional primary OCparticulate emissions as a condensation medium.Three model simulations were performed:

Case 1: Basecase—there is no modification toCAMx (no acid-catalyzed reactions) and no pri-mary particulate OC (POA).

Case 2: CAMx codes were modified to take intoaccount additional primary particulate OC (POA)emissions as a condensation medium but the acid-catalyzed reactions are not included.

Case 3: CAMx codes were modified to take intoaccount additional POA emissions as a condensa-tion medium and the acid-catalyzed reactions areincluded.

To examine whether the availability of additionalprimary particulate OC emissions affected SOAformation by providing additional volume ofcondensation media, simulations with and withoutthis additional absorbing mass due to the woodsmoke (Cases 2 and 1) were compared. Thesesimulations did not include any acid-catalyzedcarbonyl condensation reactions. The results, shownin Fig. 6, indicate that the maximum enhancementof SOA formation due to additional condensationvolume was 0.24 mgm�3, which is less than 10% ofthe predicted enhancement in SOA due to the woodsmoke, based on ambient data.

A next step in evaluating the potential importanceof SOA formation via acid-catalyzed routes is toestimate the extent of acidic aerosol surface. In this

Fig. 6. (a) Ground level SOA concentrations (mgm�3) predicted with the

between SOA concentrations in Cases 1 and 2 (Case 2–1) predicted by C

two cases is observed) on 6 September. Note that different scales are a

work, the degree of neutralization is presented ashydrogen ion concentration, estimated by perform-ing a cation–anion balance (molar basis)[H+] ¼ 2[SO4

2�]+[NO3�]�[NH4

+]. Concentrationsof other cations and anions were generally negli-gible. Fig. 7 shows the spatial and temporaldistribution of the acidic aerosol (as hydrogen ionin mmolm�3) for one of the modeled days. Thesimulations indicate that the fire plumes impact theLaPorte monitoring site on 6 September and thatthe aerosol in the plumes is initially acidic.

CAMx simulations with various reaction prob-abilities were performed. The reaction probabilityrepresents the fraction of carbonyl acid aerosolcollisions that result in SOA formation. Thesimulation results for 6 September are depicted inFig. 8. Also shown in Fig. 8 are measurements ofaerosol carbon made using an aerosol mass spectro-meter at the LaPorte site, which indicates amaximum in carbonaceous aerosol of more that25 mgm�3 on 6 September. The reaction probabilityg was assumed constant throughout the modelingdomain, at all times of day. Values of 0.0001,0.0005, 0.005 and 0.001 were assumed for g.Relatively small amounts of SOA (less than2 mgm�3) are predicted via acid-catalyzed reactionsfor values of g less than 0.0005, but at values near orabove 0.0005, the magnitude of SOA formation ismore consistent with observations.

A comparison between predicted and observedsecondary organic aerosol concentrations, at the

presence of additional absorbing mass (Case 2) and (b) difference

AMx at 10 a.m. (the time when the maximum difference between

pplied.

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Fig. 7. Degree of neutralization, [H+] (mmolm�3), on 6 September at: (a) 03:00, (b) 06:00, (c) 12:00 and (d) 15:00 h on 6 September at the

La Porte site (LPT).

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–3070 3065

La Porte site, for various reactive coefficient valuesis shown in Tables 3(a–c). The observed SOAconcentrations are based on the source apportion-ment calculations of Buzcu et al. (2006), shown inFig. 2. The acid-catalyzed reactions included in themodel contributed to a significant portion of theaverage SOA concentration predicted by the model(up to 1.6, 2.6, and 5.8 mgm�3 for g ¼ 0.0005, 0.001,0.005, respectively), averaged over the area outlinedin blue in Fig. 1. The temporal evolution ofpredicted SOA corresponded well to the observedtemporal evolution of OC concentrations.

It is difficult to pinpoint the value of the reactionprobability parameter g that leads to results mostconsistent with observations due to limitations inestimating the observed secondary organic concen-trations. Nevertheless, the overall indications arethat the acid-catalyzed heterogeneous reactions ofaldehydes did influence organic carbon concentra-tions during this episode. A reaction probability of0.005 can contribute up to 5–10 mgm�3 of addi-tional SOA.

While this modeling analysis indicates thataldehydes were available, acidic aerosol was avail-

able and that acid-catalyzed reactions could pro-duce the magnitude of OC unaccounted for insource allocations, measurements of the oligiomericreaction products of acid-catalyzed carbonyl reac-tions would provide more direct evidence of thesepathways. Measurements of the oligiomeric reactionproducts are not available, however, measurementsof high levels of cations in the aerosol, attributed tohigh concentrations of organic acids, provideindirect confirmation of the presence of oligiomericcompounds. Fig. 9 shows inorganic ion concentra-tions measured at the La Porte site on 28 August(no fire) and 6 September (intensive fires). On theday without fire activity, Fig. 9(a) indicates thatsulfate and ammonium concentrations, expressed inmolar equivalents, were comparable and that theinorganic portion of the aerosol is neutral. Incontrast, Fig. 9(b) indicates that ammonium con-centrations (in molar equivalents) were in excess ofthe inorganic anion concentrations on 6 September,a day with high fire activity.

Comparing the temporal distribution of OCobserved on 6 September (Fig. 10) with the temporalpattern of the excess ammonium concentrations

ARTICLE IN PRESS

Fig. 8. Predicted ground level SOA concentrations (mgm�3) on 6 September at 03:00 h (left) and at 06:00 h (right) due to impingement of

ALD2, from sources within the domain, onto acidic particles, with a reactive uptake coefficient of: (a) 0.0001 (b) 0.0005, (c) 0.001 and (d)

0.005. Ambient measurements of OC at the LaPorte site (LPT) are also listed.

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–30703066

(ammonium unneutralized by inorganic anions)suggest that the observed excess ammonium is dueto organic acids. Comparisons of the mass of OC to

the mass of ammonium suggest that the organic acidsare likely formed by acid-catalyzed condensation oflow molecular weight species. Specifically, the ratios

ARTICLE IN PRESS

Table 3

Comparison of SOA (mgm�3) formed with and without inclusion of heterogeneous reactions using various reactive uptake coefficients (g),and observations of total OC (mgm�3) and estimations of secondary organic aerosol concentrations at La Porte

Time Observations

total OC

Estimated

secondary

organicsb

Domain-averagea Domain-maximuma

With

heterogeneous

reactions

Without

heterogeneous

reactions

With

heterogeneous

reactions

Without

heterogeneous

reactions

(a) g ¼ 0.0005

00:00 17.2 11.5 3.5 3.1 5.4 4.7

05:00 27.2 18.2 4.4 3.1 7.8 5.6

10:00 17 11.4 4.0 2.4 5.2 2.7

15:00 11.4 7.6 2.4 1.8 3.6 2.4

(b) g ¼ 0.001

00:00 17.2 11.5 3.8 3.1 5.9 4.7

05:00 27.2 18.2 5.3 3.1 9.2 5.6

10:00 17 11.4 5.0 2.4 6.9 2.7

15:00 11.4 7.6 2.9 1.8 4.8 2.4

(c) g ¼ 0.005

00:00 17.2 11.5 5.2 3.1 10.5 4.7

05:00 27.2 18.2 7.9 3.1 13.6 5.7

10:00 17 11.4 8.2 2.4 11.3 2.7

15:00 11.4 7.6 4.8 1.8 9.5 2.4

aDomain is the area outlined in blue in Fig. 1.bEstimated SOA is based on total OC measured for the hour multiplied by the average fraction of OC that was attributed to SOA for

that period by Buzcu et al. (2006).

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

Hour

[um

ol/m

3 ]

[NH4]

Excess NH4

[SO4]

[NO3]

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

Hour

[um

ol/m

3 ]

[NH4]

Excess NH4

[SO4]

[NO3]

a b

Fig. 9. Observed inorganic (SO42�, NO3

� NH4+) concentrations at La Porte site on: (a) 28 August and (b) 6 September 2000 (Source:

Canagaratna et al., 2005). Excess ammonium is the measured ammonium concentration minus the ammonium required to neutralize

sulfate and nitrate.

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–3070 3067

ARTICLE IN PRESS

0

5

10

15

20

25

30

35

40

45

0:00 2:00 4:00 6:00 8:00 10:0012:0014:0016:0018:0020:0022:00 0:00

Hour

[ug/m

3]

-0.4

-0.2

0

0.2

0.4

0.6

0.8

[umol/m3]

Total Organics

Excess ammonium

Fig. 10. Observed total organic concentrations (mgm�3, left axis) and estimated excess ammonium concentrations (mmolm�3, right axis)

at La Porte site on 6 September 2000 (Source: Canagaratna et al., 2005).

Table 4

Ratios of total organics and ammonium concentrations observed

at La Porte site on 6 September 2000

Total

OC/NH4

(mgmg�1)

Total

OC/NH4

(mgmmol�1)

C atom/NH4

(mgmmol�1)

Minimum 2.5 43 2.4

Median 4.7 80 4.4

Average 6.8 116 6.4

Maximum 30.6 520 28.9

U. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–30703068

of total OC concentrations to molar excess ammo-nium (mgmmol�1) on 6 September range from 43 to520 with a median of 80 (see Table 4). Assuming ageneric ratio of total organic/organic carbon of 1.5,the median value corresponds to about four carbonmolecules neutralizing each ammonium molecule.This implies that the excess ammonium is mostprobably associated with organic molecules thatpossess extensive acid functionality.

Conventional SOA formation pathways are notable to explain this high degree of organic acidity.Russell and Allen (2005) reported that the majorityof SOA formed in southeast Texas on non-woodsmoke days during this episode was due to biogenicprecursors, especially a-pinene; a-pinene (C10H16)reacts with O3/OH/NOx to produce norpinonalde-hyde (C9H16O2), pinonic acid (C10H16O3), pinic acid(C9H14O4) and other products. Each of these photo-oxidation products possess two carbonyl groupsand the organic mass per acidic functionality ofthese products are 154 or more grams of organic permole of acid. These values are much higher than themedian value of 80, required to neutralize observed

ammonium concentrations. In contrast, if glyoxal(C2H2O2) undergoes acid-catalyzed heterogeneousreactions via hydration and polymerization pro-cesses (Jang et al., 2002; Jang et al., 2003;Kleindienst et al., 2004; Kalberer et al., 2004), eachmolecule of glyoxal hydration product (MW 94) willcontain four acidic OH groups. The observed OC toparticle bound ammonium ratios, summarized inTable 4, can thus be explained by a combination ofpinene oxidation products and acid-catalyzed car-bonyl condensation products.

5. Conclusion

Observational data, collected during a woodsmoke episode in Houston, Texas, indicated thatwood smoke enhanced SOA formation in the area.A 3-D photochemical grid model was used toexamine whether that SOA was due to increasedpartitioning of semi-volatile species into the parti-culate phase, or acid-catalyzed heterogeneous reac-tions. The simulations indicated that increasedpartitioning of semi-volatile species into the parti-culate phase was relatively negligible (less than 10%of the enhancements in SOA formation). Incontrast, calculations of potential SOA formationdue to acid-catalyzed reactions of carbonyls, weregenerally consistent with observed SOA concentra-tions. Further, observed concentrations of ammo-nium were consistent with high concentrations ofthe types of organic acids that would be formedthrough acid-catalyzed reactions of carbonyls.Although there are substantial uncertainties in theestimates of heterogeneous SOA formation, collec-tively, these data and modeling analyses provide

ARTICLE IN PRESSU. Nopmongcol et al. / Atmospheric Environment 41 (2007) 3057–3070 3069

evidence for the importance of acid-catalyzed SOAformation reactions.

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