Atmospheric Environment 40 (200

nice pan

suspended in the chamber. We believe this is due to rapid decomposition of the 3,5-diphenyl-1,2,4-trioxolane and the

tion products with a vapor pressure exceeding theequilibrium vapor pressure established in the atmo-

ARTICLE IN PRESS

Corresponding author. Bourns College of EngineeringCenter for Environmental Research and Technology

(CE-CERT), University of California, Riverside, CA 92521,

sphere may condense onto pre-existing particles orhomogeneously nucleate to form new particles.

1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.atmosenv.2005.10.063

USA. Tel.: +1 909 781 5695; fax: +1 909 781 5790.

E-mail address: dcocker@cert.ucr.edu (D.R. Cocker III).hydroxyl-substituted ester by nucleophilic attack from the ammonia molecule. Additional experiments with

a-methylstyrene/ozone produced SOA that was unaffected by ammonia, suggesting that the addition of a methyl groupled to SOA that stearically hindered nucleophilic attack by the ammonia molecule. The presence of water vapor prior to

styrene oxidation was found to reduce SOA formation, likely due to inhibition of the formation of 3,5-diphenyl-1,2,

4-trioxolane.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: SOA; Styrene; Ozonolysis; Ammonia; Water vapor

1. Introduction

Secondary organic aerosols (SOA) are a largecontributors to ambient PM2.5 mass, accounting for

up to 70% or more of the organic aerosol present inthe urban plume. (Turpin and Huntzicker, 1995;Na et al., 2004). SOA is formed via condensation ofoxidation products of lower volatility than theirprecursor reactive organic gases. Generally, oxida-Kwangsam Naa, Chen Songa,b, David R. Cocker IIIa,b,

aBourns College of Engineering, Center for Environmental Research and Technology (CE-CERT), University of California,

Riverside, CA 92521, USAbDepartment of Chemical and Environmental Engineering, Bourns College of Engineering, University of California,

Riverside, CA 92521, USA

Received 28 May 2005; received in revised form 11 August 2005; accepted 3 October 2005

Abstract

We report on a comprehensive investigation of the inuence of ammonia and water on secondary organic aerosol (SOA)

formation from the styreneozone system. The presence of ammonia and water each affected the gas-phase chemistry

leading to SOA formation, thereby impacting the total aerosol yield for the system. Two lumped products using the classic

semi-empirical gasparticle equilibrium model (a1, K1, a2, and K2 were estimated as 0.0490, 0.3410, 0.1439, and 0.0040,respectively) were sufcient to predict SOA formation in the dry styreneozone system. We propose 3,5-diphenyl-1,2,

4-trioxolane and a hydroxyl-substituted ester as the major aerosol-forming products in the dry, ammonia-free styrene

ozonolysis system. Addition of excess ammonia after SOA formation rapidly and signicantly reduced the aerosol volumeFormation of secondary orgastyrene with ozone in th

ammonia6) 18891900

aerosol from the reaction ofresence and absence ofd water

www.elsevier.com/locate/atmosenv

ARTICLE IN PRESSK. Na et al. / Atmospheric Environment 40 (2006) 188919001890SOA is currently believed to inuence climatechange (Pilinis et al., 1995), human health (Seatonet al., 1995), and visibility degradation (Appel et al.,1985).The production of aerosol is often described by a

semi-empirical gasparticle partitioning approachbased on the work by many researchers (Odumet al., 1996; Pankow, 1994; Takekawa et al., 2003).This useful tool allows for prediction of the aerosol-forming potential of gas-phase hydrocarbons. How-ever, current experimental data have generallyfocused on a single hydrocarbon at xed hydro-carbon: NOx ratios (exceptions include Izumi andFukuyama, 1990; Hurley et al., 2001; Song et al.,2005; Martn-Reviejo and Wirtz, 2005). A numberof environmental chamber studies have been per-formed to investigate the aerosol formation poten-tial (aerosol yield) of small aromatic and naturalhydrocarbon compounds in photochemical reac-tions and ozonolysis. (e.g., Odum et al., 1996, 1997;Grifn et al., 1999; Kleindienst et al., 1999;Takekawa et al., 2003). Recent studies have alsoidentied several organic acids within the aerosolproduced from ozonloysis of both a- and b-pinene(e.g., Hoffmann et al., 1997; Jang and Kamens,1999; Yu et al., 1999, Glasius et al., 1999; Iinuma etal., 2004). However, we are aware of no experi-mental investigations to investigate the possibility ofbaseacid reactions involving ammonia and organicacids that lead to the formation of SOA.Ammonia (NH3) is a ubiquitous trace atmo-

spheric gas found at widely varying ambientconcentrations depending upon upwind sourcesand land use. Potential removal mechanisms forgaseous ammonia include wet and dry deposition,oxidation by hydroxyl radicals, and reaction withinorganic acidic gases to form secondary aerosol.The calculated half-life for ammonia due to reactionwith hydroxyl radicals is approximately 2 months(Seinfeld and Pandis, 1998). The atmosphericreaction between NH3 and sulfuric acid and nitricacid leading to signicant secondary nitrate andsulfate formation has been well characterized(Seinfeld and Pandis, 1998). However, no workhas been reported in the literature on the potentialrole of ammonia chemistry on SOA formation.Water vapor is another atmospheric species that

may inuence SOA formation. Previous studieshave noted that many atmospheric oxidationproducts are either slightly or moderately hygro-scopic based on tandem differential mobility analy-

sis (e.g., Virkkula et al., 1999, Ansari and Pandis,2000; Cocker et al., 2002). Furthermore, researchershave noted that water can play a signicant role inthe gas-phase chemical pathways for atmospherichydrocarbon oxidation (Hatakeyama et al., 1981;Becker et al., 1993). Therefore, water may play arole in SOA production by altering the nalcomposition and distribution of oxidation productsthrough gas-phase chemistry, organic sorption ofwater, or by water sorption of organic oxidationproducts. Additionally, Cocker et al. (2001) reportsthat the ionic strength of the sorbing particulatemay alter the thermodynamic gasparticle equili-brium.The styreneozone (O3) oxidation system pro-

vides a relatively simple chemical system for whichonly the external double bond is susceptible toozone oxidation, with limited impact on the stablearomatic ring. Tuazon et al. (1993) identiedHCHO and C6H5CHO as the two major productsof the styreneozone oxidation system, with respec-tive molar yields of 3775% and 4175%. CO(772%), CO2 (472%), C6H5COOH (1%), andHCOOH (12%) were observed as minor products.Therefore, the styreneozone system is an idealinitial system to investigate the potential impacts ofgaseous ammonia and water vapor on SOAproduction.The signicant organic PM2.5 aerosol measured in

Western Riverside County near dairy operationsand downwind of the Los Angeles and OrangeCounty air basins motivated this work (Kim et al.,2000; Sawant et al., 2004). The initial hypothesiswas that direct reaction between gaseous NH3 andorganic acids from photochemical oxidation mayenhance SOA production by forming condensablesalts. In this work, we investigate the potential forammonia and water to form additional condensableproducts leading to enhanced SOA formation.

2. Experimental section

2.1. Smog chamber experiments and sample

collection

Experiments were performed in a dark indoor18m3 Teon environmental chamber with a 2.5m1

surface-to-volume ratio. The chamber temperaturewas maintained at 2071 1C. The reactor wasushed a minimum of 10 chamber volumes withpuried compressed laboratory air (Aadco 737series (Cleves, Ohio)). The compressed laboratory

air was further puried by passing through canisters

programmed as follows: 0150 1C at 20 1Cmin ,

the reactor. Ozone was monitored with a Dasibi

ARTICLE IN PRESSK. Na et al. / Atmospheric Environment 40 (2006) 18891900 18911003-AH ozone analyzer (Dasibi EnvironmentalCorporation, Glendale, CA). Ozone measurementswere made at 1-min intervals and have estimateduncertainties of 75%. Ammonia was measured byowing the NOx-free chamber air through a thermaloxidizer set to 980 1C and detecting the oxidizedammonia as NO. The thermal oxidizer was cali-brated using a certied cylinder of ammonia(concentration: 0.3% in N2 with 72% of accuracy;Praxair, Santa Ana, CA) and has an estimatedfollowed by 150 1C for 10min. Estimated uncer-tainty of the hydrocarbon measurement is 75%.Triplicate styrene measurements were obtainedprior to the start of the reaction and repeated every20min thereafter. Experiments commenced byinjecting ozone into the reactor at a rate of1Lmin1 for 40min to produce approximately40075 ppb ozone inside of the chamber. The targetozone concentration was achieved by changing theduration of ozone injection for the xed volume ofof Purals and heated Carulite 300s, followed by alter pack to remove all particulates. The puriedair within the chamber had no detectable non-methane hydrocarbons (o1 ppb), NOx (o15 ppt),no detectable particles (o0.2 particles cm3), and adew-point below 40 1C. Additionally, the chamberwas irradiated for 6 h by 64 blacklights to aid in thecleaning of the chamber. No particles were detectedin the clean chamber air after addition of excessozone or blacklight irradiation.

2.2. Injection

Styrene (purity499.5%; Sigma-Aldrich, St.Louis, MO) was evaporated into a 5Lmin1 N2stream owing through a heated glass manifold.Ozone was injected by owing 1Lmin1 puried airthrough a UV O3 generator. CO was injected from acertied tank (purity: 99.998%; Matheson Tri-Gas,Newark, CA).

2.3. Gas-phase analysis

Styrene measurements were carried out using aHewlett-Packard (Palo Alto, CA) 5890 Series IIPlus gas chromatograph (GC) utilizing a DB-5 60mcolumn (J&W Scientic, Davis, CA) and a ameionization detector (FID). The GC temperature was

1uncertainty of 710%.2.4. Aerosol-phase analysis

Aerosol size distributions and number concentra-tions were obtained every 75 s using a scanningelectrical mobility spectrometer (SEMS). The SEMSwas comprised of a TSI 3077 85Kr neutralizer(St. Paul, MN), a TSI 3081 long-column cylindricaldifferential mobility analyzer (DMA), and a TSI3760A condensation particle counter. The SEMSwas operated with 2.5 Lmin1 sheath and excessow rate, with a 0.25 Lmin1 inlet and classiedaerosol ow rate. The DMA column voltage wasexponentially ramped from 40 to 7000V, allow-ing for measurement of 25730 nm electrical mobi-lity diameter particles. Volumetric ow control towithin 70.5% was achieved by measuring thepressure drop of the gas ow across a laminar owelement and adjusting the ows with MKS 0248Aproportional control valves (MKS, Irvine, CA).These valves were interfaced to LabView software(National Instruments, Houston, TX) using pro-portionalintegraldifferential (PID) control thatcan update the ow at a rate of 100 s1. The datainversion routine accounts for charging and count-ing efciency as well as particle loss. The SEMS wascalibrated at multiple diameters (90 and 200 nm)using polystyrene latex spheres (Duke Scientic,Palo Alto, CA).

2.5. Humidification

The reactor was humidied by owing dry airthrough a sparger immersed in deionized watermaintained at 40 1C by a mantle heater. Thehumidied air was then passed through a conden-sate collector to cool the humidied air to roomtemperature and prevent condensation within thereactor. Humidied water was injected until thetarget relative humidity in the reactor was reached.No particles were detected in the humidied airleaving the humidier system.

3. Results and discussion

3.1. SOA formation

The experimental conditions for all styreneozoneexperiments are summarized in Table 1. Totalorganic aerosol mass concentration was calculatedfrom the observed particle size distribution assum-ing spherical particles and unit density. Measured

particle number and volume concentrations were

corrected for wall loss (Bowman et al., 1997). Thetotal organic aerosol mass concentration produced(DMo) and the total concentration of styreneconsumed (DC8H8) were used to calculate theSOA yield (Y DMo=DC8H8) for each experiment.Fig. 1 shows the SOA yields from the ozonolysis ofstyrene as a function of DMo. The curve t throughthe data in Fig. 1 was generated using Eq. (1), asemi-empirical gasparticle partitioning approach,assuming a hypothetical two-product model basedon Odum et al. (1996):

Y Moa1K1

1 K1Mo a2K21 K2Mo

, (1)

where subscript 1 and 2 designate two lumpedaerosol-forming products, one of relatively highvolatility and the other of low volatility. Mo is theorganic aerosol mass concentration (mgm3), ai themass-based stoichiometric factor of lumped species iformed and Ki (m

3 mg1) the partitioning coefcient

ARTICLE IN PRESS

Table 1

Initial conditions and results obtained from dark experiments

Date Initial conc.

of C8H8(ppb)

NH3 (ppb) RH (%) Temp. (1C) DO3 (ppb) DC8H8(mgm3)

DMo(mgm3)

Yield (%)

24/11/03 48 Dry 19.4 40 191 7.4 3.809/02/04 45 500 Dry 21.2 63 178 5.2 2.915/01/04 46 2000 Dry 20.0 50 192 2.5 1.314/11/03 92 Dry 20.3 90 360 20.3 5.6

91 379 20.4 5.4

87 357 17.6 4.9

159 789 58.0 7.4

218 1123 94.4 8.4

228 1114 97.1 8.7

222 1109 88.0 8.0

289 1373 131.0 9.5

330 1451 157.1 10.8

340 1400 144.0 10.3

313 1516 118.0 7.8

281 1503 86.2 5.7

305 1630 139.0 8.5

377 1904 219.0 11.5

380 1781 210.0 11.8

375 1973 177.4 9.0

392 2184 204.0 9.3

372 2212 159.5 7.2

375 1997 131.2 6.6

382 2180 131.0 6.0

380 2276 273.0 12.0

376 2304 286.8 12.4

380 2411 227.4 9.4

391 2755 249.7 9.1

Aerosol formed (g m-3)0 50 100 150 200 250 300 350

Yiel

d

0.00

0.03

0.06

0.09

0.12

0.15

In the absence of NH3In the presence of NH3

Fig. 1. Yield of aerosol formed by styrene and ozone reaction in

both the absence and presence of ammonia.

K. Na et al. / Atmospheric Environment 40 (2006) 18891900189211/02/04 94 Dry 23.019/12/03 90 500 Dry 21.523/02/04 203 Dry 19.108/12/03 297 Dry 20.017/05/04 335 Dry 21.024/12/03 308 Dry 20.324/02/04 383 Dry 20.918/02/05 470 Dry 20.209/03/05 487 Dry 20.403/03/04 496 50 20.7

18/03/04 494 60 21.0

18/11/03 504 500 Dry 20.117/02/04 956 Dry 23.114/03/05 966 Dry 21.527/02/04 862 50 19.4

18/02/04 1031 500 Dry 20.421/01/04 911 1000 Dry 19.122/01/04 1008 2000 Dry 21.024/05/04 1088 4000 Dry 23.101/12/03 2057 Dry 21.317/12/03 1930 Dry 22.803/12/03 1952 500 Dry 21.301/03/04 1962 56 19.3

CO (50 ppm) is added before the reaction.Dew point o40 1C.

(analogous to Henrys coefcient) in terms of theorganic mass concentration. Although the organicaerosol phase produced from the atmosphericoxidation of styrene may include a number ofoxidation products, the assumption of two hypothe-tical oxidation products were sufcient to t theaerosol yield data presented in Fig. 1. Usingnonlinear regression, the four parameters of a1,K1, a2, and K2 were estimated to be 0.0490, 0.3410,0.1439, and 0.0040, respectively (R2 0:98).Fig. 2 contains a chemical mechanism for the

formation of condensable species based on themechanism proposed by Tuazon et al. (1993).Briey, ozone can react with the olenic doublebond of styrene to produce an energy-rich ozonide,which subsequently decomposes to form six possibleproducts: benzaldehyde (C6H5CHO), formic acid

ARTICLE IN PRESSK. Na et al. / Atmospheric Environment 40 (2006) 18891900 1893(HCOOH), benzoic acid (C6H5COOH), formalde-hyde (HCHO) and two excited Criegee biradicals(C6H5CHOO and CH2OO ). C6H5CHOO maythen combine with C6H5CHO to form 3,5-diphenyl-1,2,4-trioxolane and a hydroxyl-substituted ester(C6H5CH(OH)OC(O)C6H5). This is analogous to amechanism proposed by Becker et al. (1993), andTobias and Ziemann (2001) who showed that theCriegee biradical can rapidly combine with formal-dehyde. Furthermore, Tuazon et al. (1993) sug-gested that the secondary ozonide may undergopartial conversion into a hydroxyl-substituted esterbased on the detection of a CQO stretch usingFTIR. The vapor pressure of 3,5-diphenyl-1,2,4-trioxolane is estimated as 5.9 108 Torr using theEstimation Programs Interface (EPI) SuiteTM de-veloped by the EPAs Ofce of Pollution Prevention

C

OH

OR

HC

O

R

RCH=CH2 + O3 RCH CH2

OO

O

RCHO + CH2OO RCHOO + HCHO

C

O

O

R

HC

OR

H

Fig. 2. A mechanism for SOA formation from styrene and ozone(proposed by Tuazon et al. (1993)).Toxics and Syracuse Research Corporation (SRC).The equation used is as follows:

ln P 4:4 ln Tb1:803Tb=T 1 0:803 lnTb=T 6:8Tm=T 1, 2

where Tb is the normal boiling point (K), T thetemperature (K) and Tm the melting point (K). Themelting point term is ignored for liquids. However,since 3,5-diphenyl-1,2,4-trioxolane is in crystal format room temperature (NIST Chemistry Webbook,2005), Tm was considered in the estimation. A gas/particle partitioning coefcient (Ki) was calculatedfor 3,5-diphenyl-1,2,4-trioxolane using the esti-mated vapor pressure and Eq. (3) (Pankow (1994)):

Ki f om760RT

MWom 106zipoL;i, (3)

where fom is the fraction of organic material, R theideal gas constant (m3 atmmol1K1), T thetemperature (K), MWom the molecular weight ofthe absorbing organic material (gmol1), zi theactivity coefcient (assumed to be 1 for thiscalculation) and PoL;i the sub-cooled liquid vaporpressure. This results in a calculated K of1.356m3 mg1, in general agreement given uncer-tainties in vapor pressure and activity coefcientestimations, to that obtained in Fig. 1(0.341m3 mg1).

3.2. Effect of HCOOH and HCHO addition on SOA

formation

To help demonstrate that 3,5-diphenyl-1,2,4-trioxolane was a major aerosol-forming species,two additional styreneozone oxidation reactionswere conducted with excess HCOOH and HCHO.Winterhalter et al. (2000) has shown that theaddition of HCHO and HCOOH to the b-pinene/ozone system can suppress secondary ozonideformation by stabilizing the energy-rich intermedi-ate biradical species. We thus hypothesized thatexcess HCOOH and HCHO would lead to thebiradical species C6H5CHOO preferentially com-bining with HCOOH and HCHO, thereby reducingthe 3,5-diphenyl-1,2,4-trioxolane produced in thesystem. Therefore, two different mixtures, 2 ppmHCOOH and 500 ppb styrene, and 5 ppm HCHOand 500 ppb styrene were injected independently,followed by injection of 400 ppb O3. In the formercase, no aerosol was produced for this system. In the

3latter case, 10 mgm of aerosol was produced with

ARTICLE IN PRESSC

K. Na et al. / Atmospheric Environment 40 (2006) 188919001894an aerosol yield of 0.8%. This was approximately5% of the aerosol mass formed in the absence ofHCHO.As shown in Fig. 3, more benzaldehyde was

produced in the presence of HCHO as compared tothe reaction in the absence of HCHO. This isattributed to preferential reaction of HCHO withthe biradical (C6H5CHOO ) instead of with ben-

Elapsed time (h)00:00 02:00 04:00 06:00 08:000

100

Fig. 3. Changes of styrene and benzaldehyde concentrations

after the injection of ammonia (styrene: 500 ppb, ozone: 400 ppb,

HCHO: 5 ppm).once

ntra

tion

(ppb)

200

300

400

500Styrene in the presence of HCHOBenzaldehyde in the presence of HCHOStyrene in the absence of HCHOBenzaldehyde in the absence of HCHO

Injecting NH3Injecting NH3zaldehyde. This further supports the hypothesis thatHCOOH and HCHO could act as a scavenger forthe biradical species, C6H5CHOO , and therebyimpeding formation of 3,5-diphenyl-1,2,4-trioxo-lane and the hydroxyl-substituted ester. The reac-tion also implies that the products of the HCOOH/C6H5CHOO and HCHO/C6H5CHOO reactionare too light to condense. Therefore, we concludethat particles are formed when C6H5CHO combineswith C6H5CHOO .

3.3. Effect of NH3 on SOA produced by

styrene ozone system

We hypothesized that nucleophilic NH3 couldreact with acidic oxidation products to formcondensable salts leading to enhanced aerosolformation. Therefore, 500 ppb styrene was mixedwith 400 ppb O3 and allowed to react until aerosolproduction ceased. Next, 1000 ppb NH3 was addedto the reactor, resulting in a signicant decrease ofsuspended aerosol number and volume concentra-tions. Time traces for suspended aerosol numberand volume concentrations for this system aredisplayed in Fig. 4. A clear and negative impacton SOA formation is observed for this system withminimal/no aerosol salt formation.Based on the chemical mechanism presented in

Fig. 2 and the experiments presented in Section 3.2,the likely major condensable species for this systemare 3,5-diphenyl-1,2,4-trioxolane and the hydroxyl-substituted ester. We propose two additionalmechanisms (Figs. 5a,b) for the removal of 3,5-diphenyl-1,2,4-trioxolane and the hydroxyl-substi-tuted ester by direct reaction with NH3, respectively.In the case of 3,5-diphenyl-1,2,4-trioxolane(Fig. 5a), the electronegativity of the neighboringoxygens will lead to a net positive charge induced onthe a-carbon (Fig. 5a). NH3, a nucleophile, may addto the a-carbon and cleave 3,5-diphenyl-1,2,4-trioxolane yielding benzaldehyde, hydrogen perox-ide (H2O2), and phenylmethanimine (C7H7N).The possible attack by nucleophilic NH3 on the

a-carbon was further explored by reaction of500 ppb a-methylstyrene (C6H5(CCH3)QCH2)with ozone to form a secondary ozonide with theidentical chemical structure as 3,5-diphenyl-1,2,4-trioxolane except for the methyl group attached tothe a-carbon. About 1 ppm of NH3 was added tothe a-methylstyrene/ozone system after aerosolproduction was completed. Contrary to the styr-ene/ozone system, addition of NH3 produced a 10%increase in aerosol volume concentration. Thisresult is consistent with our hypothesis that themethyl group would sterically hinder nucleophilicattack at the a-carbon. The increase in aerosolvolume concentration with ammonia addition maybe attributable to organic acidbase chemistryleading to organic acidammonia salt formation.We hypothesize that the acid produced from theozonolysis of a-methylstyrene leading to organicacidammonia salt formation after ammonia addi-tion is benzoic acid, which is formed through thecarbonyl oxide hydroxyl radical generation channel(Orzechowska and Paulson, 2005).It is further proposed that the other expected

product of styrene ozonolysis, the hydroxyl-sub-stituted ester, can be decomposed by nucleophilicNH3 attack on the hydrogen atom attached to theoxygen shown in Fig. 5b. Finally, the particle-phasehydroxyl-substituted ester can be broken down intobenzaldehyde and an ammonium salt (ammoniumbenzoate, a combination of benzoic acid and

ammonia). The vapor pressure of ammonium

ARTICLE IN PRESS

ectedonc.d forrrect concted f

3

K. Na et al. / Atmospheric Environment 40 (2006) 18891900 1895Num

ber c

once

ntra

tion

(cm-3 )

3.0e+4

6.0e+4

9.0e+4

1.2e+5

1.5e+5

# conc. not corrwall loss for # c# conc. correctevol. conc. not cowall loss for vol.vol. conc. correc

Injecting NHbenzoate was estimated to be 2.2 107 Torr usingEq. (2). The proposed mechanisms are furthersupported by the increased rate of C6H5CHOformation relative to styrene consumption afterNH3 injection, as shown in Fig. 3.Further, 2 ppm NH3 was injected into the system

lled with 5 ppm of HCOOH. However, noaerosol formation was observed, showing that thereaction of HCOOH with NH3 does not generateappreciable quantities of aerosol. This experimenthelps to support our hypothesis that 3,5-diphenyl-1,2,4-trioxolane and the hydroxyl-substituted esterare the major aerosol-forming species.A nal set of experiments was conducted with

ammonia present during the oxidation of styrene.For these experiments, 1000 ppb styrene was oxi-dized with 400 ppb ozone with ve differentconcentrations of ammonia (500, 1000, 2000, and4000 ppb) added to the reactor prior to the additionof 400 ppb ozone. These results are shown in Fig. 6.The presence of ammonia clearly altered thechemical mechanism for SOA formation. Based onFig. 5, very little SOA formation was expected as

Elapsed tim00:00 02:00 04:0

0.0

Fig. 4. Changes in number and volume concentrations after the injecVolu

me

conc

entra

tion

(m3

cm-3 )

50

100

150

200

for wall loss

wall lossed for wall loss.or wall lossNH3 reacts with the signicant aerosol-formingspecies 3,5-diphenyl-1,2,4-trioxolane and the hydro-xyl-substituted ester. Therefore, an additional un-known condensable product was produced whenammonia was present during styrene oxidation. Theextent of SOA formation is still a function ofammonia present in the reactor.

3.4. Effect of water vapor on aerosol formation

Water vapor is a signicant gaseous speciespresent in the atmosphere. Its average compositionof the atmosphere varies from 0 to 4 104 ppm(Pidwirny, 2004). It has been well documented thatthe oxidation of hydrocarbons can lead to polaroxidation products that subsequently absorb water(Hoffmann et al., 1997; Jang and Kamens, 1999; Yuet al., 1999, Glasius et al., 1999; Iinuma et al., 2004).However, water may also play a direct role in thegas-phase chemistry leading to the formation ofoxidation products in the styrene/O3 system. Wepropose a gas-phase mechanism in reaction (a)based on work by Hatakeyama et al. (1981),

e (h)0 06:00 08:00

0

tion of NH3 (styrene: 500 ppb, ozone: 400 ppb, NH3: 1000 ppb).

ARTICLE IN PRESS

C

O O

HR O

C HR

NH3

CHR O

C HR

NH2O OH

CHR

C HR

NH

+

OH2O2 +

K. Na et al. / Atmospheric Environment 40 (2006) 188919001896C

O O

HR O

C HR

NH3

C

O OH

HR O

C HR

NH2

O OH

CHR OH

C HR

NH

+

NH3

(a)Atkinson (1990) and Becker et al. (1993):

R CHOO H2O! HORCHOOH! RCOOHH2O; (a)

where R represents a phenyl group. Atkinson (1990)hypothesized that the dominant loss process for thebiradical species CH2OO was the formation ofcarboxylic acids through reaction with water asshown in reaction (b):

CH2OO H2O! HOCH2OOH! HCOOHH2O: (b)

Additionally, Neeb and Moortgat (1999) identi-ed from FTIR spectra that when H2O was addedto the stabilized Criegee intermediate (CH2OO ,termed carbonyl oxide) formed from ozonolysis ofisobutene, an increase in HCOOH was observed.This was explained by a proposed reaction of waterand the stabilized Criegee intermediate CH2OO

C

OH

OR

HC

O

R C

O

OR

HC

O

R NH4+

C

O

OR C

O

RNH4

+H C

O

OR C

O

R+H 4HN(b)

Fig. 5. Two possible reaction pathways for particle-to-gas conversion after the addition of ammonia.

Ammonia added (ppb)0 1000 2000 3000 4000 5000

Yiel

d (%

)

0

2

4

6

8

10

12

14

Fig. 6. Changes in aerosol yield for varying amounts of added

ammonia in experiments with 1 ppm of styrene.

(Neeb et al., 1997). Likewise, we suggest that thebiradical RCHOO could react with H2O to formbenzoic acid as shown in Fig. 7. Therefore, weperformed a series of styreneO3 oxidation experi-ments at 50% relative humidity (RH) to ascertainthe impact of H2O. Fig. 8 shows the evolution ofnumber and volume concentrations (corrected forwall loss) for 1000 ppb styrene ozonolysis in thepresence and absence of water vapor. Fig. 9 displaysthe aerosol yields for both the humid and dry

systems. Reduced aerosol production is clearly seenin the systems containing water vapor. The decreas-ing formation of SOA in the presence of watervapor might be due to the two reaction sequencesshown in Fig. 7 We postulate that H2O can stabilize

ARTICLE IN PRESS

C OH

+ H2O

O

CO

HOH

OH

O

CH O

+ H2O2

C O H

+ H2O

(a)

(b)

Fig. 7. Reaction of an energy-rich biradical species with water

vapor.

ed tim02:0

r con

cent

ratio

n (cm

-3 )

6.0e+4

9.0e+4

1.2e+5

1.5e+5Vo

lum

e co

ncen

tratio

n (m

3 cm

-3 )

50

100

150

200

250

300

. conc. with RH=50%

. conc. with RH

the energy-rich biradical species to form benzoicacid (Fig. 7a), thereby reducing the availability ofthe biradical species necessary to produce 3,5-diphenyl-1,2,4-trioxolane (Fig 2). In the otherreaction scheme (Fig. 7b), the interaction of anenergy-rich biradical species with H2O leads tohydroperoxy(phenyl)methanol, which decomposes toform H2O2 and benzaldehyde. However, when H2O(RH 50%) is added to this reaction, benzaldehydeproduction was not signicantly increased comparedto the reaction in the absence of H2O. Therefore, weconclude that Fig 7a is the most likely route to reduceSOA formation in this system by inhibiting thebiradical species reaction with C6H5CHO to form3,5-diphenyl-1,2,4-trioxolane.

3.5. Hydroxyl radical effects

It is noted that alkeneO3 chemistry can producehydroxyl radicals (OH) in the dark. Orzechowska

substituted ester were proposed to be the majoraerosol-forming products from the reaction.

ARTICLE IN PRESS

Table 2

Simplied gas-phase styrene reaction with ozone

Reaction Reaction rate constant

(cm3molecule1 s1, at298K)

(1) Styrene+O3-P1+0.07OH 1.2 1017(2) Styrene+OH-P2 5.8 1011(3) O3+HO2-OH+2O2 2.0 1015

14

3 )K. Na et al. / Atmospheric Environment 40 (2006) 188919001898Time (s)0 3000 6000 9000 12000 15000

Conc

entra

tions

of p

rodu

cts

form

edby

sty

rene

ozo

nolys

is (m

olecu

les cm

-

0.0

2.0e+12

4.0e+12

6.0e+12

8.0e+12

1.0e+13Styrene + O3Styrene + OH

Fig. 10. Comparison of reaction rates between styreneozone(4) O3+OH-HO2+O2 6.8 10(5) OH+HO2-H2O+O2 1.1 1010(6) HO2+HO2-H2O2+O2 1.7 1012

1.2e+13and styreneOH (2 ppm styrene, 400 ppb ozone, OH yield of 7%).When ammonia was added to the chamberafter the completion of aerosol formation, thenumber and size of the particles decreasedrapidly. We therefore propose a mechanism forgaseous ammonia to inuence SOA formation inthe system. Experiments conducted at elevatedrelative humidity led to a reduction in SOAformation. This reduction was attributed tostabilization of the biradical intermediatereducing the extent of secondary ozonide formationand thereby leading to form products withhigher vapor pressure. We believe that the ndingsin this paper are the rst to indicate that ammoniamay directly inuence the chemistry leading to SOAformation. Further study of the interaction ofammonia with other hydrocarbon systems as wellas the propensity of ammonia to form organic saltsof lower volatility is required so that one can assessthe overall impact of ammonia on atmospheric SOAand Paulson (2002) report a 774% yield for OHradicals during styrene ozonolysis. To assess theimpact of hydroxyl radical on styrene consumption,a simplied gas-phase chemical kinetic reactionmodel was applied using the reaction mechanismas shown in Table 2. The relatively high reactionrate coefcient of styrene with hydroxyl versusozone leads to some styrene consumption by theOH byproduct of styrene ozonolysis. This is shownin Fig. 10 with a net consumption by ozone versusOH remaining at 93% and 7%, respectively.Therefore, the effect of OH radicals on styreneconsumption and SOA formation was determinedto be minor. However, it is important to recognizethat a small fraction of the reaction is occurringthrough the hydroxyl route for which we have notprovided a chemical mechanism that may have aslight inuence on overall SOA formation in thesystem.

4. Summary and conclusion

Aerosol formation from styrene ozonolysiswas studied in a concentration range of 502 ppmto investigate the effect of ammonia and watervapor on the formation of secondary organicaerosol (SOA). SOA yields of up to 12% wereseen for the experimental conditions studied.3,5-diphenyl-1,2,4-trioxolane and a hydroxyl-formation.

Hoffmann, T., Odum, J.R., Bowman, F., Collins, D., Klockow,

D., Flagan, R.C., Seinfeld, J.H., 1997. Formation of organic

62696278.

ARTICLE IN PRESSK. Na et al. / Atmospheric Environment 40 (2006) 18891900 1899aerosols from the oxidation of biogenic hydrocarbons.

Journal of Atmospheric Chemistry 26, 189222.

Hurley, M.D., Sokolov, O., Wallington, T.J., Takekawa, H.,

Karasawa, M., Klotz, B., Barnes, I., Becker, K.H., 2001.

Organic aerosol formation during the atmospheric degrada-

tion of toluene. Environment Science and Technology 35,

13581366.

Iinuma, Y., Boge, O., Gnauk, T., Herrmann, H., 2004. Aerosol-

chamber study of the a-pinene/O3 reaction: inuence ofparticle acidity on aerosol yields and products. AtmosphericAcknowledgements

We gratefully acknowledge funding support fromNational Science Foundation grant ATM-0234111.The authors would like to thank Dr. Paul Ziemann,Dr. Roger Atkinson, Kurt Bumiller, CameronSwitzer, and Jean Wang for their help withexperiment setup and measurement.

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ARTICLE IN PRESSK. Na et al. / Atmospheric Environment 40 (2006) 188919001900

Formation of secondary organic aerosol from the reaction of styrene with ozone in the presence and absence of ammonia and waterIntroductionExperimental sectionSmog chamber experiments and sample collectionInjectionGas-phase analysisAerosol-phase analysisHumidification

Results and discussionSOA formationEffect of HCOOH and HCHO addition on SOA formationEffect of NH3 on SOA produced by styrene-ozone systemEffect of water vapor on aerosol formationHydroxyl radical effects

Summary and conclusionAcknowledgementsReferences