Effect of Hydrophilic Organic Seed Aerosols on Secondary Organic Aerosol Formation from Ozonolysis...

Published: July 26, 2011

r 2011 American Chemical Society 7323 dx.doi.org/10.1021/es201225c | Environ. Sci. Technol. 2011, 45, 7323–7329

ARTICLE

pubs.acs.org/est

Effect of Hydrophilic Organic Seed Aerosols on Secondary OrganicAerosol Formation from Ozonolysis of r-PineneChen Song,†,* Rahul A. Zaveri,† John E. Shilling,† M. Lizabeth Alexander,‡ and Matt Newburn‡

†Atmospheric Sciences & Global Change Division and ‡Environmental Molecular Science Laboratory,Pacific Northwest National Laboratory, Richland, Washington

bS Supporting Information

1. INTRODUCTION

Secondary organic aerosols (SOA) are particles formed in theatmosphere from gas-phase oxidation of anthropogenically ornaturally emitted volatile organic compounds (VOC), inter-mediate volatile organic compounds (IVOC) and semivolatileorganic compounds (SVOC). Oxidation processes generallylower the vapor pressure of these species, shifting the partitioningof these secondary species from the gas-phase to particle-phase.These species primarily condense on preexisting particles in theatmosphere, although they may also lead to new particle forma-tion under some conditions. The condensation process is oftendescribed as an absorptive equilibrium partitioning between thegas-phase and particulate-phase based on Raoult’s Law.1 Thispartitioning theory is widely employed in general circulationmodels to predict SOA loadings from emissions inventories withSOA yield parametrizations from specific VOCs determinedfrom smog chamber experiments.2 These models typicallyassume a well-mixed organic aerosol phase even in the presenceof hydrophobic POA, which significantly enhances the modeledSOA yields as additional organic mass is available to absorbgreater amounts of oxidized secondary organic gases. However,several thermodynamic studies suggested that more than oneorganic phases (e.g., hydrophobic vs hydrophilic) should beconsidered in the ambient particles.3 Recently, Song et al.4

showed that the presence of model hydrophobic primary organic

aerosols such as lubricating oil and dioctyl phthalate (DOP) hadno detectable effect on the SOA yields from ozonolysis of R-pinene, suggesting that the SOA species form a separate phaserather than a single well-mixed organic phase with the POAspecies. Vaden et al. 5 confirmed Song et al.’s conclusion byobserving the presence of two separate phases in R-pinene SOAcoated dioctylphthalate aerosol using a single particle laserablation mass spectrometer (SPLAT). Another recent study 6

suggested that SOA from R-pinene ozonolysis and POA from adiesel generator form a well-mixed organic phase after a few hours,but did not investigate the influence of the diesel POA on SOAyield. These studies illustrate the need to further investigate SOAformation in the presence of different types of organic seeds.

In the atmosphere, a substantial fraction of the OA iscomposed of polar, hydrophilic organic compounds emittedfrom biomass burning or from oxidative aging of hydrophobicPOA.7 This poloar, hydrophilic OAmay form a well-mixed phasewith polar SOA species, thereby enhancing the absorption ofSOA into the particle-phase as predicted by Raoult’s Law.

Received: April 11, 2011Accepted: July 26, 2011Revised: July 14, 2011

ABSTRACT: Gas-particle partitioning theory is widely used in atmosphericmodels to predict organic aerosol loadings. This theory predicts thatsecondary organic aerosol (SOA) yield of an oxidized volatile organiccompound product will increase as the mass loading of preexisting organicaerosol increases. In a previous work, we showed that the presence of modelhydrophobic primary organic aerosol (POA) had no detectable effect on theSOA yields from ozonolysis ofR-pinene, suggesting that the condensing SOAcompounds form a separate phase from the preexisting POA. However, asubstantial faction of atmospheric aerosol is composed of polar, hydrophilicorganic compounds. In this work, we investigate the effects of modelhydrophilic organic aerosol (OA) species such as fulvic acid, adipic acid,and citric acid on the gas-particle partitioning of SOA from R-pineneozonolysis. The results show that only citric acid seed significantly enhancesthe absorption of R-pinene SOA into the particle-phase. The other two seed particles have a negligible effect on the R-pinene SOAyields, suggesting that R-pinene SOA forms a well-mixed organic aerosol phase with citric acid and a separate phase with adipic acidand fulvic acid. This finding highlights the need to improve the thermodynamics treatment of organics in current aerosol models thatsimply lump all hydrophilic organic species into a single phase, thereby potentially introducing an erroneous sensitivity of SOAmassto emitted OA species.

7324 dx.doi.org/10.1021/es201225c |Environ. Sci. Technol. 2011, 45, 7323–7329

Environmental Science & Technology ARTICLE

In this work, we investigate the effect of pre-existing hydro-philic polar organic seed aerosols on the formation of SOA fromthe ozonolysis of R-pinene. The model organic seed aerosolsused in this work were generated from fulvic acid (FA), adipicacid (AA) and citric acid (CA). FA, which is a complex multi-component mixture of aromatic compounds that bear aliphaticchains with carboxyl, hydroxyl, carbonyl, methyl, or phenolterminal groups, serves as a surrogate for aged substances witholigomeric or polymeric structures found in ambient organicaerosols.8 It should be noted that FA extracted from soil samplesexhibits different physical properties compared to ambientoligomeric or polymeric substances.8b,9 AA and CA, typicallyfound in primarily emitted biomass and aged OA,10 serve assurrogates to the highly oxidized organics detected in the atmo-sphere due to their high O:C atom ratio and low vapor pressures.

Although all of these organic seeds are hydrophilic to somedegree, their hygroscopicities at ambient relative humidity (RH)vary significantly. AA is sparingly soluble in water (2.5 g soluteper 100 g water 11). It was reported that pure AA particles do notabsorb water even at RH up to 95% 12 and its deliquescence pointis over 100% RH.13 CA particles generated by atomization inwater never crystallize and continuously take up water as afunction of RH even at very low RH.14 The hygroscopicity ofFA particle is between those of AA and CA particles. It had beenshown that FA can absorb moderate amounts of water at RH <90%.9

2. EXPERIMENTAL SECTION

Experiments were performed in the indoor 8 m3 Teflonreactor at Pacific Northwest National Laboratory. Temperature(25�27 �C) and RH (2% for dry experiments and ∼60% forhumid experiments) were measured by a Vaisala HMP45A/Dprobe. Seed aerosols are generated by nebulizing aqueoussolutions of FA (Suwannee River Fulvic Acid, IHSS, productnumber 1R101F), AA (Sigma-Aldrich, 99.6%) and CA (Sigma-Aldrich, 99.5%), respectively, with a BGI Collision nebulizer.Before entering the Teflon reactor, seed aerosols were passedthrough two TSI model 3062 diffusion dryers to reduce the RHin the aerosol steam and a TSI 3077 neutralizer. The geometricmean diameters of the seed aerosols are normally 55�65 nm.After nebulization of seed aerosols, a mixture of cyclohexane(Aldrich, 99%) and R-pinene (Aldrich, 98%) was gently warmedin a glass bulb and injected into the Teflon reactor in a stream ofpure air. Cyclohexane, the OH radical scavenger, was introducedinto the reactor at a concentration such that the reaction rate ofOH radicals with cyclohexane exceeds that with R-pinene by afactor of 100. O3 was then generated by passing a flow (1 Lmin�1) of pure dry air through an ultraviolet lamp (JelightModel1000) and into the Teflon reactor. The initial concentration ofO3 was at least three times that of R-pinene, so that the R-pinene

concentration was reduced to a negligible level by the end of eachexperiment (∼3 h). O3 was monitored by a Thermal Environ-mental Instruments Model 49C O3 analyzer. R-Pinene concen-tration was measured in real time using a Proton TransferReaction Mass Spectrometer (PTR-MS, Ionicon Analytik). Sizedistribution and number concentration of aerosols were deter-mined using a ScanningMobility Particle Sizer (SMPS, TSI 3696Series). Aerosol mass and composition were observed using aTime-of-Flight Aerosol Mass Spectrometer (cToF-AMS, Aero-dyne Research Inc.). Before each experiment, the Teflon reactorwas continuously flushed with purified air until the aerosolnumber concentrations were less than 2 #/cm3, NO, NOx, andO3 concentrations were less than 1 ppbv, and the volatile organiccompounds, as observed by the PTR-MS, were similar to thosemeasured directly in purified air.

3. RESULTS AND DISCUSSION

The purpose of this work is to investigate the partitioning ofR-pinene SOA onto hydrophilic polar organic aerosols. Theexperiments were classified into four categories: unseeded ex-periments and seeded experiments using AA, FA, and CA as seedaerosols. Each category includes two conditions: dry (RH≈ 2%)and humid (RH≈ 60%). To ensure reproducibility of the results,we performed two experiments for each condition (except for AAseed aerosol under humid condition, for which only one experi-ment was performed). Initial R-pinene concentrations of theexperiments ranged from 23 to 28 ppbv. Seed aerosol was presentat a sufficient level to reduce, though not entirely suppress, thehomogeneous nucleation of new SOA particles. The experimen-tal conditions, initial seed aerosol mass concentrations, reactedhydrocarbon concentrations, and final SOA mass concentrationsare summerized in Table S1 (Supporting Information).

The effect of hydrophilic polar organic seed particles on theformation of SOA was investigated by comparing the instanta-neous SOA growth under different conditions. The comparisonof instantaneous SOA growth would be valid only if thecharacteristic times for gas-particle mass transfer are muchsmaller than the characteristic times for the formation of con-densable gases from oxidation and the characteristic times for gasand particle losses on the chamber walls, so that a gas-particleequilibrium can be assumed at any given time during theexperiment. We calculate the characteristic times of theseprocesses in our Teflon reactor under the conditions of thiswork. Table 1 gives the characteristic times of the main processesthat are important to the gas-particle interaction as well as theequations that govern these processes. The gas-particle transporttime scales listed in Table 1 were determined in the presence andabsence of seed aerosol respectively. The main uncertainty incalculating the gas-particle transport time scale is the stickingprobability (γ) of SOA from R-pinene ozonolysis. A model

Table 1. Characteristic Times of Gas-Particle Interactiona

process characteristic time typical chamber value

formation of condensable organic gas τF = (1)/(kO3[O3]) 1 h

particle-wall transport (wall deposition) τPW = (1)/(βP) 4 h

gas-particle transport (condensation) τGP = (1)/(∑i(2πNiDp,iλc)/(1 + 8λ/γDp,i)) unseeded: 0.3 min (γ = 1); 2 min (γ = 0.1) seeded:

0.1 min (γ = 1); 1 min (γ = 0.1)a kO3

= 8.7� 10�18 cm3/molecule 3 s, first-order ozone reaction rate constantwithR-pinene; [O3] = 120 ppb, typical concentration of ozone;βP= 3.5� 10�3,typical mass transfer coefficient for particles to chamber surface; λ = 65.1 nm, mean free path; c = 600 m/s, velocity of semivolatile gas molecule;γ, sticking probability; Dp,i, particle diameter of each bin measured by SMPS; Ni, particle number concentration of each bin measured by SMPS.



study 15 showed a value between 0.1�1.0 for the γ for SOA fromm-xylene photooxidation. Using these values, we calculate thatformation of semivolatile products and particle wall depositionoccurred relatively slowly on a time scale of a few hours ascompared to gas-particle transport, which occurred on a timescale of 2 min or less (Table 1). Under these conditions, the gas-particle system is expected to be close to equilibrium at anygiven time.3.1. Comparison of Seeded and Unseeded Experiments

under Dry Conditions (RH ≈ 2%). Figure 1 compares SOAgrowth curves from the ozonolysis of R-pinene in the absenceand presence of polar hydrophilic organic seed aerosols underdry conditions. SOA growth is interpreted by plotting SOA massas a function of the hydrocarbon reacted over the course of theexperiment.16 SOA mass is corrected for wall loss assuming afirst-order loss rate calculated by fitting the particle numberconcentration decay at the end of each experiment.17 For theseeded experiments, SOA mass concentrations were obtained bysubtracting the initial seed aerosol mass concentrations from thefinal wall loss corrected aerosol mass concentrations. A density of1.23 g/cm3, calculated by comparing the electronic mobilitydiameter from the SMPS and vacuum aerodynamic diameterfrom c-ToF AMS,18 was employed to estimate SOA massconcentrations. The reacted R-pinene concentrations were ob-tained by fitting the PTR-MS measurements to an exponen-tial decay.Figure 1A shows the time-dependent growth curves of two R-

pinene ozonolysis experiments in the absence of seed aerosolswith similar initial experimental conditions. It can be seen thatthe growth curves of these two experiments overlap well,demonstrating a good reproducibility in our Teflon reactor.

These two growth curves from the experiments in the absenceof seed aerosols serve as the baseline curves.In the presence of AA seed aerosol (Figure 1B), the SOA

growth curves (solid squares) overlap with the baseline curvesover the course of the experiments. The SOA growth curves inthe presence of FA (Figure 1C) deviate slightly from the baselinecurves. The aerosol growth profiles in the presence of FA startslightly slower at the beginning of the oxidation, but increasecontinuously during the course of the experiments until theysurpass those of the unseeded experiments by the end of theexperiment. The final SOA yields, however, are very similar.Figure 1B,C show that the addition of AA and FA seeds did notenhance the absorption of gas-phase SOA species into theparticle-phase under dry conditions. These data indicate that,similar to dry inorganic seed particles,19 AA and FA seed aerosolsact as inert surfaces for SOA species to condense onto and do notmix with SOA species. In contrast, the growth curves in thepresence of CA deviate significantly from the baseline growthcurves. As shown in Figure 1D, the presence of CA seed aerosolssignificantly increase the SOA mass (e.g., a 26% increase at 125μg/m3 of reacted R-pinene), implying that the CA seed aerosolsand SOA species could mix together.3.2. Comparison of Seeded and Unseeded Experiments

under Humid Condition (RH≈ 60%).The hydroscopic growthfactors (HGF), which are defined as the ratio of the humidifiedparticle diameter to the dry particle diameter, are 1.0, 1.04, and1.35 for AA, FA, and CA, respectively, at 60% RH.9,12b,20 The drydiameters of the particles that were used to obtain the HGF were60 nm for FA and 100 nm for AA and CA. It is likely that FA andCA seed aerosols are present as a mixed aqueous phase while AAseed remains crystalline at 60% RH. The HGF for SOA from

Figure 1. Comparison of SOA growth curves in the presence and absence of hydrophilic organic seed aerosols under dry condition. The shaded areasare error computed on the basis of propagation of uncertainties in the ΔMSOA measurements.



R-pinene ozonolysis with cyclohexane OH radical scavenger hasnot been reported. Since the chemical compositions of SOA fromR-pinene ozonolysis are not likely dependent on OH radicalscavengers,21 we estimate a HGF of 1.06 from Cocker et al.’swork 19 in which HGF was measured for SOA generated from R-pinene ozonolysis with 2-butanol as OH radical scavenger. Thewater content associated with SOA is then excluded by applyingthe estimated HGF.Similar to Figure 1, the two SOA growth curves in the absence

of seed aerosols (Figure 2A) serve as the baseline curves.Figure 2B,C displays the SOA growth curves (solid squares) inthe presence of AA and FA, respectively. As seen in Figure 2,under humid conditions, AA and FA do not enhance theabsorption of SOA species into the particle-phase, indicatingthe condensing SOA and the AA and FA seed form a separatephase. According to Figure 2D, in which the SOA growth curvesfor experiments in the presence of CA are shown, the enhance-ment of SOA from CA varies during the course of the experi-ments. CA did not enhance SOA absorption until about 21 μg/m3 SOA was formed, which corresponded to 95 μg/m3 ofreacted R-pinene. The final SOA yield in the presence of CA,however, is clearly higher than observed for the unseededexperiments. We offer an explanation for this result in the nextsection.3.3. Volatility Basis Set Fit. To better understand the mixing

states of the aerosols, we compare the volatility basis set fits(VBSF) of unseeded experiments and seeded experiments in thepresence of CA in Figure 3. Since AA and FA seed aerosols areshown to form a separate phase with R-pinene SOA, their basisset fits are not included. Black solid lines represent the VBSF tothe R-pinene SOA from unseeded experiments that serve as thebaseline (VBSFbaseline), while the blue dotted lines represent the

VBSF to the R-pinene SOA from seeded experiments with CAseed aerosols (VBSFCA+SOA). These fits are generated using avolatility basis set in which the range of products is specified interms of volatility bins.22 The volatilities of the products in thiswork are segmented into four logarithmically spaced volatilitybins (expressed as values of C*); C* (0.1, 1, 10, 100 μg/m3). Themass fractions of the total organic mass in each bin (Ri) are thefree parameters determined by fitting the experimental data. Welist the Ri of VBSFbaseline and VBSFCA+SOA under dry and humidconditions in Table S2 (Supporting Information). The purposeof the basis set fit here is not to provide parameters for using inchemical transport models, but to illustrate the effect of hydro-philic organic seed aerosols on the absorption of SOA in aquantitative manner. Also shown in Figure 3 are the modelpredictions for SOA absorption to CA seed aerosols (red dashedline) generated using the Ri of VBSFbaseline and including theinitial CA seed aerosols mass. We assume that CA and R-pineneSOA form a single organic phase with ideal mixing (i.e., theactivity coefficient of SOA in the CA and SOA mixtures is equalto 1). Under dry condition, the CA seed aerosols only contain asmall amount of water, thus the initial CA seed aerosol massconcentration is directly calculated from the initial CA seedaerosol volume concentration and its density (1.67 g/cm3).Under humid conditions, the initial CA seed aerosols containroughly 47% water calculated from its HGF at 60% RH. Theinitial CA seed aerosol mass concentration is calculated from thecorrected CA seed aerosol volume, which is equal to the totalaqueous seed aerosol volume minus the volume of water. Werestrict the reacted R-pinene to be 130 μg/m3 (the lower limit ofreacted R-pinene concentration in this study) for all of thevolatility basis set fits to facilitate the comparison among theexperiments that have different initial R-pinene concentrations.

Figure 2. Comparison of SOA growth curves in the presence and absence of hydrophilic organic seed aerosols under humid condition. The shaded areasare error computed on the basis of propagation of uncertainties in the ΔMSOA measurements.



Under dry conditions, the model (modelCA+SOA) predicts14%more SOA absorption over the baseline (VBSFbaseline) at theend of the experiment. The addition of CA seed (∼33 μg/m3)noticeably enhances the final absorbed SOA mass if we assumeSOA and CA form an ideal solution. For the same amount ofreacted R-pinene, the absorption of SOA to CA seed aerosols(VBSFCA+SOA) is slightly less than that predicted by the model atthe beginning of the experiment, but surpasses the modelprediction quickly. The final SOA absorbed to CA seed aerosolsis 15% higher than what the model predicts. It is possible that theinteractions between CA and SOA molecules in the well-mixedCA+SOA phase reduce the solution activity coefficients as thefraction of SOA in the particle-phase become larger. In addition,the average molecular weight of the particle-phase organics maybe reduced asmore organic compounds with higher volatility andpossibly lower molecular weight partition to the particle phase.These two effects could enhance the partitioning coefficients ofthe SOA species, leading to increased SOA absorption into thecondensed phase.1 Another possibility is that CA may react withthe SOA species bearing alcohol functionality to form oligomersor polymers if the acidity of the particle-phase is high enough.23

The c-ToF AMS mass spectra of SOA generated in the presenceof CA do not support the latter hypothesis, i.e., no obviouschanges are observed when comparing the mass spectra of SOA,CA, and SOA + CA. However, the extensive molecular fragmen-tation produced by electron impact in the AMS makes it difficultto definitively rule out oligomer formation.Under humid conditions (Figure 3B), the model predicts only a

modest increase (up to 5%) of SOA formation over the baseline.The absorption of SOA to CA seed aerosols is composed of twodistinct regimes. In the first regime, the presence of CA seed

aerosol has no noticeable effect on SOA yields, which are similar tothat from the unseeded experiments considering experimentaluncertainties. In the second regime, the amount of SOA thatpartitions intoCA seed aerosol gradually exceeds both the baselineand the model prediction. The final absorptions of SOA to CAseed aerosols are 11% and 17% higher than the model predictionand baseline at 130 μg/m3 of reacted R-pinene, respectively. It isnoted that the enhancement of SOA absorption due to CA seedaerosols under humid condition is considerably smaller than thatunder dry condition (17% vs 31%), which apparently cannot beexplained by the small difference of the initial CA seed aerosolmass concentrations (28 vs 33 μg/m3). It has been shown thatSOA from R-pinene ozonolysis has only limited solubility inwater.19,24 Prisle et al. 25 observed R-pinene SOA stays largely ina separate organic phase in the presence of aqueous (NH4)2SO4

seed at a RH higher than 85%. The first part of the VBSFCA+SOA inwhich no enhancement of SOA absorption is detected is consistentwith Prisle et al.’s observation, indicating that R-pinene SOAresides mostly in a separate phase rather than dissolved into theaqueous CA phase. The second part of the VBSFCA+SOA, in whichSOA absorption is obviously enhanced, suggests that a well-mixedSOA + CA organic phase forms later in the experiment. However,the significantly smaller enhancement of overall SOA absorptionunder humid conditionsmay indicate a significantmass ofCA seedaerosols still resides in the aqueous phase. Therefore, we hypothe-size that an organic phase containing SOA andCA and an aqueousphase containing mostly CA, water and a small amount of SOAcoexist in the particle phase. To calculate the distributions of CAand SOA between the aqueous phase and the organic phase, onewould need to acquire information about the detail chemicalcompositions ofR-pinene SOA and its thermodynamic properties,which are still poorly understood.

4. IMPLICATIONS

Previous model studies 26 typically assumed a hydrophobicand a hydrophilic phase in the particle-phase, where the hydro-philic phase is usually considered as a well-mixed aqueous-phaseof water, water-soluble inorganics and organics, and the hydro-phobic phase is composed of only water-insoluble organics. Gas-phase SOA species can partition into either phase depending ontheir polarities. A recent work 27 proposed a separate water phasein addition to the hydrophobic and hydrophilic phases. Theprimary emitted least polar SVOCs partition only to the hydro-phobic phase; the secondary SVOCs formed from oxidation ofthe primary SVOCs and further generations of oxidation parti-tion only to the hydrophilic phase; water-soluble organics, suchas glyoxal partition only to the water phase. However, the phasestates of ambient organic aerosols could be far more complicatedthan assumed in these models. The results presented here sug-gest that ambient aerosols may be composed of multiple hydro-philic organic phases instead of only one. By lumping all of theparticle-phase hydrophilic polar organics together and assumingonly one well-mixed hydrophilic organic phase, these modelsmay overestimate the ambient SOA mass. The mixture compris-ing R-pinene SOA and a hydrophilic organic seed considered inthis work may not be directly applicable to authentic ambientaerosols that are significantly more complex. Nevertheless, thiswork underscores the need to systematically investigate phasestate behavior of organic and mixed organic�inorganic aerosolsunder carefully controlled, atmospherically relevant conditions.

Figure 3. Comparison of volatility basic set fits to the SOA growthcurves in the presence and absence of CA seed aerosols under dry (3A)and humid conditions (3B).



’ASSOCIATED CONTENT

bS Supporting Information. Two supplementary tables. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

This research was supported by the U.S. Department ofEnergy’s (DOE) Atmospheric System Research (ASR) programand by the Environmental Molecular Sciences Laboratory(EMSL), a national scientific user facility sponsored by DOE’sOffice of Biological and Environmental Research and located atPNNL. Pacific Northwest National Laboratory is operated forthe U.S. Department of Energy by Battelle Memorial Instituteunder Contract DE-AC06-76RLO 1830.

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