Effect of Hydrophilic Organic Seed Aerosols on Secondary Organic

Jul 26, 2011 - However, a substantial faction of atmospheric aerosol is composed of polar, hydrophilic organic compounds. In this work, we investigate...
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Effect of Hydrophilic Organic Seed Aerosols on Secondary Organic Aerosol Formation from Ozonolysis of r-Pinene Chen 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 ABSTRACT: Gas-particle partitioning theory is widely used in atmospheric models to predict organic aerosol loadings. This theory predicts that secondary organic aerosol (SOA) yield of an oxidized volatile organic compound product will increase as the mass loading of preexisting organic aerosol increases. In a previous work, we showed that the presence of model hydrophobic primary organic aerosol (POA) had no detectable effect on the SOA yields from ozonolysis of R-pinene, suggesting that the condensing SOA compounds form a separate phase from the preexisting POA. However, a substantial faction of atmospheric aerosol is composed of polar, hydrophilic organic compounds. In this work, we investigate the effects of model hydrophilic organic aerosol (OA) species such as fulvic acid, adipic acid, and citric acid on the gas-particle partitioning of SOA from R-pinene ozonolysis. The results show that only citric acid seed significantly enhances the absorption of R-pinene SOA into the particle-phase. The other two seed particles have a negligible effect on the R-pinene SOA yields, suggesting that R-pinene SOA forms a well-mixed organic aerosol phase with citric acid and a separate phase with adipic acid and fulvic acid. This finding highlights the need to improve the thermodynamics treatment of organics in current aerosol models that simply lump all hydrophilic organic species into a single phase, thereby potentially introducing an erroneous sensitivity of SOA mass to emitted OA species.

1. INTRODUCTION Secondary organic aerosols (SOA) are particles formed in the atmosphere from gas-phase oxidation of anthropogenically or naturally emitted volatile organic compounds (VOC), intermediate volatile organic compounds (IVOC) and semivolatile organic compounds (SVOC). Oxidation processes generally lower the vapor pressure of these species, shifting the partitioning of these secondary species from the gas-phase to particle-phase. These species primarily condense on preexisting particles in the atmosphere, although they may also lead to new particle formation under some conditions. The condensation process is often described as an absorptive equilibrium partitioning between the gas-phase and particulate-phase based on Raoult’s Law.1 This partitioning theory is widely employed in general circulation models to predict SOA loadings from emissions inventories with SOA yield parametrizations from specific VOCs determined from smog chamber experiments.2 These models typically assume a well-mixed organic aerosol phase even in the presence of hydrophobic POA, which significantly enhances the modeled SOA yields as additional organic mass is available to absorb greater amounts of oxidized secondary organic gases. However, several thermodynamic studies suggested that more than one organic phases (e.g., hydrophobic vs hydrophilic) should be considered in the ambient particles.3 Recently, Song et al.4 showed that the presence of model hydrophobic primary organic r 2011 American Chemical Society

aerosols such as lubricating oil and dioctyl phthalate (DOP) had no detectable effect on the SOA yields from ozonolysis of Rpinene, suggesting that the SOA species form a separate phase rather than a single well-mixed organic phase with the POA species. Vaden et al. 5 confirmed Song et al.’s conclusion by observing the presence of two separate phases in R-pinene SOA coated dioctylphthalate aerosol using a single particle laser ablation mass spectrometer (SPLAT). Another recent study 6 suggested that SOA from R-pinene ozonolysis and POA from a diesel generator form a well-mixed organic phase after a few hours, but did not investigate the influence of the diesel POA on SOA yield. These studies illustrate the need to further investigate SOA formation in the presence of different types of organic seeds. In the atmosphere, a substantial fraction of the OA is composed of polar, hydrophilic organic compounds emitted from biomass burning or from oxidative aging of hydrophobic POA.7 This poloar, hydrophilic OA may form a well-mixed phase with polar SOA species, thereby enhancing the absorption of SOA into the particle-phase as predicted by Raoult’s Law. Received: April 11, 2011 Accepted: July 26, 2011 Revised: July 14, 2011 Published: July 26, 2011 7323

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Table 1. Characteristic Times of Gas-Particle Interactiona process formation of condensable organic gas particle-wall transport (wall deposition) gas-particle transport (condensation)

characteristic time

typical chamber value

τF = (1)/(kO3[O3])

1h

τPW = (1)/(βP)

4h

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)

kO3= 8.7  10 18 cm3/molecule 3 s, first-order ozone reaction rate constant with R-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. a

In this work, we investigate the effect of pre-existing hydrophilic polar organic seed aerosols on the formation of SOA from the ozonolysis of R-pinene. The model organic seed aerosols used in this work were generated from fulvic acid (FA), adipic acid (AA) and citric acid (CA). FA, which is a complex multicomponent mixture of aromatic compounds that bear aliphatic chains with carboxyl, hydroxyl, carbonyl, methyl, or phenol terminal groups, serves as a surrogate for aged substances with oligomeric or polymeric structures found in ambient organic aerosols.8 It should be noted that FA extracted from soil samples exhibits different physical properties compared to ambient oligomeric or polymeric substances.8b,9 AA and CA, typically found in primarily emitted biomass and aged OA,10 serve as surrogates to the highly oxidized organics detected in the atmosphere due to their high O:C atom ratio and low vapor pressures. Although all of these organic seeds are hydrophilic to some degree, their hygroscopicities at ambient relative humidity (RH) vary significantly. AA is sparingly soluble in water (2.5 g solute per 100 g water 11). It was reported that pure AA particles do not absorb water even at RH up to 95% 12 and its deliquescence point is over 100% RH.13 CA particles generated by atomization in water never crystallize and continuously take up water as a function of RH even at very low RH.14 The hygroscopicity of FA particle is between those of AA and CA particles. It had been shown that FA can absorb moderate amounts of water at RH < 90%.9

2. EXPERIMENTAL SECTION Experiments were performed in the indoor 8 m3 Teflon reactor at Pacific Northwest National Laboratory. Temperature (25 27 °C) and RH (2% for dry experiments and ∼60% for humid experiments) were measured by a Vaisala HMP45A/D probe. Seed aerosols are generated by nebulizing aqueous solutions of FA (Suwannee River Fulvic Acid, IHSS, product number 1R101F), AA (Sigma-Aldrich, 99.6%) and CA (SigmaAldrich, 99.5%), respectively, with a BGI Collision nebulizer. Before entering the Teflon reactor, seed aerosols were passed through two TSI model 3062 diffusion dryers to reduce the RH in the aerosol steam and a TSI 3077 neutralizer. The geometric mean 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 warmed in a glass bulb and injected into the Teflon reactor in a stream of pure air. Cyclohexane, the OH radical scavenger, was introduced into the reactor at a concentration such that the reaction rate of OH radicals with cyclohexane exceeds that with R-pinene by a factor of 100. O3 was then generated by passing a flow (1 L min 1) of pure dry air through an ultraviolet lamp (Jelight Model 1000) and into the Teflon reactor. The initial concentration of O3 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 each experiment (∼3 h). O3 was monitored by a Thermal Environmental Instruments Model 49C O3 analyzer. R-Pinene concentration was measured in real time using a Proton Transfer Reaction Mass Spectrometer (PTR-MS, Ionicon Analytik). Size distribution and number concentration of aerosols were determined using a Scanning Mobility Particle Sizer (SMPS, TSI 3696 Series). Aerosol mass and composition were observed using a Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS, Aerodyne Research Inc.). Before each experiment, the Teflon reactor was continuously flushed with purified air until the aerosol number concentrations were less than 2 #/cm3, NO, NOx, and O3 concentrations were less than 1 ppbv, and the volatile organic compounds, as observed by the PTR-MS, were similar to those measured directly in purified air.

3. RESULTS AND DISCUSSION The purpose of this work is to investigate the partitioning of Rpinene SOA onto hydrophilic polar organic aerosols. The experiments were classified into four categories: unseeded experiments and seeded experiments using AA, FA, and CA as seed aerosols. 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 AA seed aerosol under humid condition, for which only one experiment was performed). Initial R-pinene concentrations of the experiments ranged from 23 to 28 ppbv. Seed aerosol was present at a sufficient level to reduce, though not entirely suppress, the homogeneous nucleation of new SOA particles. The experimental conditions, initial seed aerosol mass concentrations, reacted hydrocarbon concentrations, and final SOA mass concentrations are summerized in Table S1 (Supporting Information). The effect of hydrophilic polar organic seed particles on the formation of SOA was investigated by comparing the instantaneous SOA growth under different conditions. The comparison of instantaneous SOA growth would be valid only if the characteristic times for gas-particle mass transfer are much smaller than the characteristic times for the formation of condensable gases from oxidation and the characteristic times for gas and particle losses on the chamber walls, so that a gas-particle equilibrium can be assumed at any given time during the experiment. We calculate the characteristic times of these processes in our Teflon reactor under the conditions of this work. Table 1 gives the characteristic times of the main processes that are important to the gas-particle interaction as well as the equations that govern these processes. The gas-particle transport time scales listed in Table 1 were determined in the presence and absence of seed aerosol respectively. The main uncertainty in calculating the gas-particle transport time scale is the sticking probability (γ) of SOA from R-pinene ozonolysis. A model 7324

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

study 15 showed a value between 0.1 1.0 for the γ for SOA from m-xylene photooxidation. Using these values, we calculate that formation of semivolatile products and particle wall deposition occurred relatively slowly on a time scale of a few hours as compared to gas-particle transport, which occurred on a time scale of 2 min or less (Table 1). Under these conditions, the gasparticle system is expected to be close to equilibrium at any given time. 3.1. Comparison of Seeded and Unseeded Experiments under Dry Conditions (RH ≈ 2%). Figure 1 compares SOA growth curves from the ozonolysis of R-pinene in the absence and presence of polar hydrophilic organic seed aerosols under dry conditions. SOA growth is interpreted by plotting SOA mass as a function of the hydrocarbon reacted over the course of the experiment.16 SOA mass is corrected for wall loss assuming a first-order loss rate calculated by fitting the particle number concentration decay at the end of each experiment.17 For the seeded experiments, SOA mass concentrations were obtained by subtracting the initial seed aerosol mass concentrations from the final wall loss corrected aerosol mass concentrations. A density of 1.23 g/cm3, calculated by comparing the electronic mobility diameter from the SMPS and vacuum aerodynamic diameter from c-ToF AMS,18 was employed to estimate SOA mass concentrations. The reacted R-pinene concentrations were obtained by fitting the PTR-MS measurements to an exponential decay. Figure 1A shows the time-dependent growth curves of two Rpinene ozonolysis experiments in the absence of seed aerosols with similar initial experimental conditions. It can be seen that the 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 absence of 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 curves over the course of the experiments. The SOA growth curves in the presence of FA (Figure 1C) deviate slightly from the baseline curves. The aerosol growth profiles in the presence of FA start slightly slower at the beginning of the oxidation, but increase continuously during the course of the experiments until they surpass those of the unseeded experiments by the end of the experiment. The final SOA yields, however, are very similar. Figure 1B,C show that the addition of AA and FA seeds did not enhance the absorption of gas-phase SOA species into the particle-phase under dry conditions. These data indicate that, similar to dry inorganic seed particles,19 AA and FA seed aerosols act as inert surfaces for SOA species to condense onto and do not mix with SOA species. In contrast, the growth curves in the presence of CA deviate significantly from the baseline growth curves. As shown in Figure 1D, the presence of CA seed aerosols significantly increase the SOA mass (e.g., a 26% increase at 125 μg/m3 of reacted R-pinene), implying that the CA seed aerosols and SOA species could mix together. 3.2. Comparison of Seeded and Unseeded Experiments under Humid Condition (RH ≈ 60%). The hydroscopic growth factors (HGF), which are defined as the ratio of the humidified particle diameter to the dry particle diameter, are 1.0, 1.04, and 1.35 for AA, FA, and CA, respectively, at 60% RH.9,12b,20 The dry diameters of the particles that were used to obtain the HGF were 60 nm for FA and 100 nm for AA and CA. It is likely that FA and CA seed aerosols are present as a mixed aqueous phase while AA seed remains crystalline at 60% RH. The HGF for SOA from 7325

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

R-pinene ozonolysis with cyclohexane OH radical scavenger has not been reported. Since the chemical compositions of SOA from R-pinene ozonolysis are not likely dependent on OH radical scavengers,21 we estimate a HGF of 1.06 from Cocker et al.’s work 19 in which HGF was measured for SOA generated from Rpinene ozonolysis with 2-butanol as OH radical scavenger. The water content associated with SOA is then excluded by applying the 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) in the presence of AA and FA, respectively. As seen in Figure 2, under humid conditions, AA and FA do not enhance the absorption of SOA species into the particle-phase, indicating the condensing SOA and the AA and FA seed form a separate phase. According to Figure 2D, in which the SOA growth curves for experiments in the presence of CA are shown, the enhancement of SOA from CA varies during the course of the experiments. CA did not enhance SOA absorption until about 21 μg/ m3 SOA was formed, which corresponded to 95 μg/m3 of reacted R-pinene. The final SOA yield in the presence of CA, however, is clearly higher than observed for the unseeded experiments. We offer an explanation for this result in the next section. 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 the presence of CA in Figure 3. Since AA and FA seed aerosols are shown to form a separate phase with R-pinene SOA, their basis set fits are not included. Black solid lines represent the VBSF to the R-pinene SOA from unseeded experiments that serve as the baseline (VBSFbaseline), while the blue dotted lines represent the

VBSF to the R-pinene SOA from seeded experiments with CA seed aerosols (VBSFCA+SOA). These fits are generated using a volatility basis set in which the range of products is specified in terms of volatility bins.22 The volatilities of the products in this work are segmented into four logarithmically spaced volatility bins (expressed as values of C*); C* (0.1, 1, 10, 100 μg/m3). The mass fractions of the total organic mass in each bin (Ri) are the free parameters determined by fitting the experimental data. We list the Ri of VBSFbaseline and VBSFCA+SOA under dry and humid conditions in Table S2 (Supporting Information). The purpose of the basis set fit here is not to provide parameters for using in chemical transport models, but to illustrate the effect of hydrophilic organic seed aerosols on the absorption of SOA in a quantitative manner. Also shown in Figure 3 are the model predictions for SOA absorption to CA seed aerosols (red dashed line) generated using the Ri of VBSFbaseline and including the initial CA seed aerosols mass. We assume that CA and R-pinene SOA form a single organic phase with ideal mixing (i.e., the activity coefficient of SOA in the CA and SOA mixtures is equal to 1). Under dry condition, the CA seed aerosols only contain a small amount of water, thus the initial CA seed aerosol mass concentration is directly calculated from the initial CA seed aerosol volume concentration and its density (1.67 g/cm3). Under humid conditions, the initial CA seed aerosols contain roughly 47% water calculated from its HGF at 60% RH. The initial CA seed aerosol mass concentration is calculated from the corrected CA seed aerosol volume, which is equal to the total aqueous seed aerosol volume minus the volume of water. We restrict the reacted R-pinene to be 130 μg/m3 (the lower limit of reacted R-pinene concentration in this study) for all of the volatility basis set fits to facilitate the comparison among the experiments that have different initial R-pinene concentrations. 7326

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

Under dry conditions, the model (modelCA+SOA) predicts 14% more SOA absorption over the baseline (VBSFbaseline) at the end of the experiment. The addition of CA seed (∼33 μg/m3) noticeably enhances the final absorbed SOA mass if we assume SOA and CA form an ideal solution. For the same amount of reacted R-pinene, the absorption of SOA to CA seed aerosols (VBSFCA+SOA) is slightly less than that predicted by the model at the beginning of the experiment, but surpasses the model prediction quickly. The final SOA absorbed to CA seed aerosols is 15% higher than what the model predicts. It is possible that the interactions between CA and SOA molecules in the well-mixed CA+SOA phase reduce the solution activity coefficients as the fraction of SOA in the particle-phase become larger. In addition, the average molecular weight of the particle-phase organics may be reduced as more organic compounds with higher volatility and possibly lower molecular weight partition to the particle phase. These two effects could enhance the partitioning coefficients of the SOA species, leading to increased SOA absorption into the condensed phase.1 Another possibility is that CA may react with the SOA species bearing alcohol functionality to form oligomers or polymers if the acidity of the particle-phase is high enough.23 The c-ToF AMS mass spectra of SOA generated in the presence of CA do not support the latter hypothesis, i.e., no obvious changes are observed when comparing the mass spectra of SOA, CA, and SOA + CA. However, the extensive molecular fragmentation produced by electron impact in the AMS makes it difficult to 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 two distinct regimes. In the first regime, the presence of CA seed

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aerosol has no noticeable effect on SOA yields, which are similar to that from the unseeded experiments considering experimental uncertainties. In the second regime, the amount of SOA that partitions into CA seed aerosol gradually exceeds both the baseline and the model prediction. The final absorptions of SOA to CA seed aerosols are 11% and 17% higher than the model prediction and baseline at 130 μg/m3 of reacted R-pinene, respectively. It is noted that the enhancement of SOA absorption due to CA seed aerosols under humid condition is considerably smaller than that under dry condition (17% vs 31%), which apparently cannot be explained by the small difference of the initial CA seed aerosol mass concentrations (28 vs 33 μg/m3). It has been shown that SOA from R-pinene ozonolysis has only limited solubility in water.19,24 Prisle et al. 25 observed R-pinene SOA stays largely in a separate organic phase in the presence of aqueous (NH4)2SO4 seed at a RH higher than 85%. The first part of the VBSFCA+SOA in which no enhancement of SOA absorption is detected is consistent with Prisle et al.’s observation, indicating that R-pinene SOA resides mostly in a separate phase rather than dissolved into the aqueous CA phase. The second part of the VBSFCA+SOA, in which SOA absorption is obviously enhanced, suggests that a well-mixed SOA + CA organic phase forms later in the experiment. However, the significantly smaller enhancement of overall SOA absorption under humid conditions may indicate a significant mass of CA seed aerosols still resides in the aqueous phase. Therefore, we hypothesize that an organic phase containing SOA and CA and an aqueous phase containing mostly CA, water and a small amount of SOA coexist in the particle phase. To calculate the distributions of CA and SOA between the aqueous phase and the organic phase, one would need to acquire information about the detail chemical compositions of R-pinene SOA and its thermodynamic properties, which are still poorly understood.

4. IMPLICATIONS Previous model studies 26 typically assumed a hydrophobic and a hydrophilic phase in the particle-phase, where the hydrophilic phase is usually considered as a well-mixed aqueous-phase of water, water-soluble inorganics and organics, and the hydrophobic phase is composed of only water-insoluble organics. Gasphase SOA species can partition into either phase depending on their polarities. A recent work 27 proposed a separate water phase in addition to the hydrophobic and hydrophilic phases. The primary emitted least polar SVOCs partition only to the hydrophobic phase; the secondary SVOCs formed from oxidation of the primary SVOCs and further generations of oxidation partition only to the hydrophilic phase; water-soluble organics, such as glyoxal partition only to the water phase. However, the phase states of ambient organic aerosols could be far more complicated than assumed in these models. The results presented here suggest that ambient aerosols may be composed of multiple hydrophilic organic phases instead of only one. By lumping all of the particle-phase hydrophilic polar organics together and assuming only one well-mixed hydrophilic organic phase, these models may overestimate the ambient SOA mass. The mixture comprising R-pinene SOA and a hydrophilic organic seed considered in this work may not be directly applicable to authentic ambient aerosols that are significantly more complex. Nevertheless, this work underscores the need to systematically investigate phase state behavior of organic and mixed organic inorganic aerosols under carefully controlled, atmospherically relevant conditions. 7327

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’ ASSOCIATED CONTENT

bS

Supporting Information. Two supplementary tables. This material 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 of Energy’s (DOE) Atmospheric System Research (ASR) program and by the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. ’ REFERENCES (1) Pankow, J. F. An Absorption-Model of Gas-Particle Partitioning of Organic-Compounds in the Atmosphere. Atmos. Environ. 1994, 28 (2), 185–188. (2) (a) Kanakidou, M.; Tsigaridis, K.; Dentener, F. J.; Crutzen, P. J. Human-activity-enhanced formation of organic aerosols by biogenic hydrocarbon oxidation. J. Geophy. Res.-Atmos. 2000, 105 (D7), 9243–9254. (b) Chung, S. H.; Seinfeld, J. H., Global distribution and climate forcing of carbonaceous aerosols. J. Geophys. Res.-Atmos. 2002, 107 (D19); (c) Tsigaridis, K.; Krol, M.; Dentener, F. J.; Balkanski, Y.; Lathiere, J.; Metzger, S.; Hauglustaine, D. A.; Kanakidou, M. Change in global aerosol composition since preindustrial times. Atmos. Chem. Phys. 2006, 6, 5143–5162. (3) (a) Chang, E. I.; Pankow, J. F. Organic particulate matter formation at varying relative humidity using surrogate secondary and primary organic compounds with activity corrections in the condensed phase obtained using a method based on the Wilson equation. Atmos. Chem. Phys. 2010, 10 (12), 5475–5490. (b) Saleh, R.; Khlystov, A. Determination of Activity Coefficients of Semi-Volatile Organic Aerosols Using the Integrated Volume Method. Aerosol Sci. Technol. 2009, 43 (8), 838–846. (c) Erdakos, G. B.; Pankow, J. F. Gas/particle partitioning of neutral and ionizing compounds to single- and multi-phase aerosol particles. 2. Phase separation in liquid particulate matter containing both polar and low-polarity organic compounds. Atmos. Environ. 2004, 38 (7), 1005–1013. (d) Bowman, F. M.; Melton, J. A. Effect of activity coefficient models on predictions of secondary organic aerosol partitioning. J. Aerosol Sci. 2004, 35 (12), 1415–1438. (e) Pankow, J. F. Gas/ particle partitioning of neutral and ionizing compounds to single and multi-phase aerosol particles. 1. Unified modeling framework. Atmos. Environ. 2003, 37 (24), 3323–3333. (4) Song, C.; Zaveri, R. A.; Alexander, M. L.; Thornton, J. A.; Madronich, S.; Ortega, J. V.; Zelenyuk, A.; Yu, X. Y.; Laskin, A.; Maughan, D. A., Effect of hydrophobic primary organic aerosols on secondary organic aerosol formation from ozonolysis of alpha-pinene. Geophys. Res. Lett. 2007, 34 (20), L20803, DOI 10.1029/2007gl030720. (5) Vaden, T. D.; Song, C.; Zaveri, R. A.; Imre, D.; Zelenyuk, A. Morphology of mixed primary and secondary organic particles and the adsorption of spectator organic gases during aerosol formation. Proc. Natl. Acad. Sci. U.S.A. 2010, 107 (15), 6658–6663. (6) Asa-Awuku, A.; Miracolo, M. A.; Kroll, J. H.; Robinson, A. L.; Donahue, N. M. Mixing and phase partitioning of primary and secondary organic aerosols. Geophys. Res. Lett. 2009, 36, 5. (7) (a) Sullivan, A. P.; Weber, R. J., Chemical characterization of the ambient organic aerosol soluble in water: 2. Isolation of acid, neutral, and

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