Article pubs.acs.org/est
Role of Aerosol Liquid Water in Secondary Organic Aerosol Formation from Volatile Organic Compounds Jennifer A. Faust,* Jenny P. S. Wong,† Alex K. Y. Lee,‡ and Jonathan P. D. Abbatt Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *
ABSTRACT: A key mechanism for atmospheric secondary organic aerosol (SOA) formation occurs when oxidation products of volatile organic compounds condense onto pre-existing particles. Here, we examine effects of aerosol liquid water (ALW) on relative SOA yield and composition from α-pinene ozonolysis and the photooxidation of toluene and acetylene by OH. Reactions were conducted in a room-temperature flow tube under low-NOx conditions in the presence of equivalent loadings of deliquesced (∼20 μg m−3 ALW) or effloresced (∼0.2 μg m−3 ALW) ammonium sulfate seeds at exactly the same relative humidity (RH = 70%) and state of wall conditioning. We found 13% and 19% enhancements in relative SOA yield for the α-pinene and toluene systems, respectively, when seeds were deliquesced rather than effloresced. The relative yield doubled in the acetylene system, and this enhancement was partially reversible upon drying the prepared SOA, which reduced the yield by 40% within a time scale of seconds. We attribute the high relative yield of acetylene SOA on deliquesced seeds to aqueous partitioning and particle-phase reactions of the photooxidation product glyoxal. The observed range of relative yields for α-pinene, toluene, and acetylene SOA on deliquesced and effloresced seeds suggests that ALW plays a complicated, system-dependent role in SOA formation.
1. INTRODUCTION Aqueous aerosols present a unique reaction medium in the atmosphere because of their small volume and high concentrations of both inorganic and organic solutes.1−3 Secondary organic aerosol (SOA) can form when volatile organic compounds (VOCs) react with OH radicals, ozone, or other atmospheric oxidants to yield semivolatile species.4−6 These products may then condense onto pre-existing seed particles and partition into the aqueous phase.7,8 Individual aerosol particles typically contain 20−80% organics by mass,9 but on a global scale, the total mass of aerosol liquid water (ALW) is 2−3 times greater than the dry particle mass.1 Because aerosols impact both human health and climate, there is great interest in developing models to accurately predict air quality and global climate. Historically, the first models of SOA formation focused exclusively on vapor pressure-driven partitioning of gas-phase molecules into the particle phase and ignored the reactive uptake of volatile compounds.10−12 These types of models consistently underestimated the total mass of SOA formed as well as its degree of oxidation.5,13 Newer models that incorporate aqueous chemistry more successfully reproduce atmospheric observations.1,14−19 A complex combination of mechanisms contributes to SOA formation in the presence of ALW. First, hydrophilic solutes dissolve more favorably into the aqueous phase. This increase in their effective Henry’s law constants is thermodynamically reversible. Second, aqueous-phase reactions can irreversibly drive gas uptake. For example, acid-catalyzed oligomerization reactions generate high-molecular-weight products of low volatility that build SOA mass,20 and aqueous-phase reactions © XXXX American Chemical Society
with dissolved OH and other oxidants are known to form SOA in cloud and fogwater.21,22 Third, particle morphology complicates aqueous-phase chemistry. Organic aerosol becomes less viscous as ALW increases. The organic and aqueous interactions vary from complete mixing to core−shell separation to partially engulfed lens-like structures.23−25 Hydrophilic VOC oxidation products can react in the interfacial region if the aerosol is homogeneous or if a layer of water is adsorbed at the surface. However, if the reaction occurs in the bulk, molecules must diffuse through the particle, a process that can be quite slow in viscous, glassy aerosol.24 The interplay of these various mechanisms from system to system challenges our understanding of the role of ALW in SOA formation. Few field measurements of ALW are available, but recent campaigns are beginning to address this deficit.26−29 Much of what we know about SOA formation from aqueous particles comes from chamber experiments in the laboratory. Although chamber studies capture the evolution of SOA over several hours, they suffer from significant particle and vapor loss to the chamber walls, and the state of wall conditioning can vary widely from experiment to experiment under nominally similar conditions.30−32 As an alternative, flow tubes require high concentrations to compensate for the short residence times, and fast lifetimes do not allow for some radical isomerization processes to occur. Complicated HOx cycling within the flow tube may alter the OH exposure in some photooxidation Received: September 16, 2016 Revised: December 19, 2016 Accepted: January 4, 2017
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Figure 1. Experimental setup involving the TPOT (Toronto Photooxidation Tube). Three-way valves are used to redirect the humidified flow and thereby alter the phase of the seed particles. Both deliquesced (blue) and effloresced (red) sulfate seeds are shown here, but only one type is prepared in the system in any given experiment. The green coating at the particle surface represents organics. Abbreviations are defined in the main text.
experiments,33 and atypical reactants such as O(1D) and O(3P) may influence chemistry under conditions of high OH consumption and extremely low relative humidity (not applicable here).34 Nevertheless, given sufficient operating time, wall conditioning can reach steady-state much more easily in a flow tube than in a chamber. Accordingly, chambers and flow tubes provide complementary experimental information about the nature of SOA formation processes. Here, we have chosen to employ an aerosol flow tube to monitor SOA composition and relative yield in a unique experimental setup.35 We prepare either deliquesced or effloresced ammonium sulfate (AS) seeds and pass them into the flow reactor, where the relative humidity is always held at exactly 70%. Because RH = 70% falls in between the deliquescence and efflorescence points of ammonium sulfate,36,37 the deliquesced seeds remain liquid in the reactor, with ∼20 μg m−3 ALW, and the effloresced seeds remain dry, with only ∼0.2 μg m−3 ALW. Thus, we are able to observe SOA formation on deliquesced and effloresced seeds at the exact same relative humidity and state of wall conditioning, thereby normalizing wall loss for gas-phase VOC oxidation products. We stress that the critical advantage of our setup is the ability to rapidly and easily switch the phase of the seed particle under controlled conditioning of the flow tube walls. In other words, we are able to isolate the effect of particle phase upon the SOA formation process because we maintain the RH at 70% in the reactor in experiments with both deliquesced and effloresced sulfate seeds. Although chambers nominally have the potential to perform such an experiment, they have not been operated in this manner, nor would one expect them to achieve the precision that can be attained with our new approach in which the flow conditions are exactly the same for experiments conducted with effloresced and deliquesced seeds. We test the hypothesis that ALW promotes SOA growth through the uptake of water-soluble gases in additional experiments in which we dry the deliquesced SOA at the exit of the flow reactor. Upon removal of ALW, do organic gases partition back to the gas phase, or have they irreversibly adsorbed or reacted (or both) into the particle phase? We investigate the rapid reversibility of aqueous-phase processing by passing deliquesced SOA through a diffusion dryer. Particles in the atmosphere persist for hours to days; we receive a glimpse of reversibility over just the first few seconds. This work extends our earlier experiments with isoprene35 to examine SOA formation from three additional VOCs (αpinene, toluene, and acetylene) under low-NOx conditions. αPinene is the most important atmospheric monoterpene and is
emitted from forests and vegetation.5 Its ozonolysis has emerged as a canonical system to study biogenic SOA formation.5 Initial products include carboxylic acids, aldehydes, and hydroperoxides, followed by high-molecular-weight esters and oligomers classified as ELVOCs (extremely low volatility organic compounds).38−40 Toluene and acetylene enter the atmosphere from anthropogenic activities. Toluene is a representative aromatic hydrocarbon found in urban areas; under low-NOx conditions, it is photooxidized to forms cresols and alkenedials.41,42 Acetylene is a tracer for fossil fuel combustion and biomass burning.43 Its primary oxidation product is glyoxal (CHOCHO, >99% branching ratio), which is a small, ubiquitous dicarbonyl that is a canonical system in its own right.16,17,44 Glyoxal chemistry should be particularly sensitive to ALW: its effective Henry’s law constant is 4 × 105 m atm−1 because it is so highly soluble in water and because it undergoes aqueous-phase hydration and oligomerization reactions.45 Some glyoxal forms from toluene photooxidation (∼4% for low NOx)42 and α-pinene ozonolysis (95% saturation. The deliquesced or effloresced seeds were combined with ozone and the VOC of interest in a premixing volume. If the seeds were effloresced, the N2−O2−H2O flow was added to the mixing volume instead of the conditioning tube. In all experiments, the RH in the mixing volume and flow tube was held at 70%, so the particles maintained their original phase. Thus, effloresced AS seeds contained negligible ALW (∼0.2 μg m−3), noting that there are probably about three monolayers of water molecules absorbed to the surfaces of these particles at this relative humidity.47 Deliquesced AS seeds are estimated to contain 19 ± 4 μg m−3 ALW (90% confidence interval, CI, for 10 measurements) at RH = 70%, according to the E-AIM Model II when solid formation is suppressed.48 Once again, we emphasize that simply redirecting the humidified flow allowed us to quickly and easily switch between aerosol phases without altering the RH in the reactor. 2.2. Aerosol Flow Tube. Ozone was introduced to the premixing volume by passing O2 diluted in N2 through a Jelight ozone generator (model 600) at a total flow rate of 160 sccm (toluene experiments) or 130 sccm (α-pinene and acetylene experiments) to achieve an O3 concentration of ∼300 ppb. Toluene was added by bubbling 5.5 sccm N2 gas through liquid toluene (Sigma-Aldrich, anhydrous, 99.8%) at 279 K (Pvap = 9 Torr).49 α-Pinene was delivered by mixing 20 sccm N2 with a 35 sccm flow from a gas cylinder containing 20 ppm α-pinene in N2 (Scott Specialty Gas). Acetylene was delivered by a 55 sccm flow from a gas cylinder containing 20 ppm acetylene in N2 (Linde). The approximate VOC concentrations in the flow tube were 260 ppb for toluene, 280 ppb for α-pinene, and 440 ppb for acetylene. No radical scavengers were added. The gases and inorganic seeds passed from the mixing volume into the Toronto Photooxidation Tube (TPOT) at a total flow rate of 2500 sccm. The TPOT has been characterized elsewhere.35,50 Briefly, this stainless steel flow tube has a volume of 3.2 L and houses two 254 nm lamps. The reactor was operated in the laminar flow regime with a Reynolds number of Re = 25. OH radicals are produced in situ from the photolysis of O3 in the presence of water vapor. The total residence time of the sulfate seeds in the TPOT was 77 s, of which the seeds spent 55 s in the illuminated portion. In the α-pinene ozonolysis experiments, the UV lamps remained off, and the mixing volume contributed an additional 55 s to the reaction time. The high oxidant concentrations in the TPOT simulate both the initial SOA formation and aging processes in the atmosphere. During the toluene and acetylene experiments, the OH exposure in the TPOT is estimated to be ∼2 × 1011 molecules cm−3 s, equivalent to ∼1.5 days in the atmosphere for typical diurnally averaged OH concentrations of 1.5 × 106 C
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Environmental Science & Technology observations indicate that some NH4+ evaporates from the particles as ammonia.35 Prior to measurements, the TPOT was flushed with OH overnight. Particle-free filter measurements were recorded to correct for ambient gases, and background contributions to the mass spectra were removed by subtracting baseline spectra in the absence of the oxidant. The ratio of added organic mass from SOA formation to the background organic mass was 15 for α-pinene, 10 for toluene, and 1.2 for acetylene. The organic background originated from VOC oxidation products that adsorbed to the walls of the flow tube within the first 15 min of an experiment. This background contribution was the same for both the deliquesced and effloresced systems because RH and all other conditions for gas-phase reaction were held constant when the seeds were changed. A normal experimental trial would begin after the background organic mass in the AMS reached steady state. We would then sample deliquesced SOA (deliquesced AS + VOC + oxidant) and the organic background (deliquesced AS + VOC), and then effloresced SOA (effloresced AS + VOC + oxidant) and the organic background (effloresced AS + VOC). The procedure was rotated such that effloresced data were measured before deliquesced data in alternate trials, but the order did not appear to affect the relative yields. A total of 5 trials were carried out for α-pinene, 12 for toluene, and 7 for acetylene. In spite of the low signal-to-noise for acetylene, we are confident in our results because of the dramatic and reproducible difference between deliquesced and effloresced relative SOA yields. The reported relative SOA yields were calculated by normalizing the increase in total organic mass (ΔOrg) to the total sulfate mass (SO4), thus accounting for varying collection efficiencies among the different types of particles within the AMS. For example, the relative SOA yield on deliquesced versus effloresced seeds is given by [(ΔOrg)deliquesced/ (SO4)deliquesced]/[(ΔOrg)effloresced/(SO4)effloresced]. Particle timeof-flight (PToF) distributions matched for organics and sulfate, confirming that the SOA is internally mixed and that normalization by total sulfate is appropriate.
Figure 2. Ratio of relative SOA yields from α-pinene, toluene, acetylene, and isoprene on (a) deliquesced and effloresced AS seeds and (b) deliquesced AS seeds with and without passing through a dryer after the flow tube. The individual yields were determined by normalizing the total organic mass produced against the total sulfate mass detected by the AMS. Error bars represent 90% confidence intervals. Isoprene data are taken from ref 35.
then organic molecules that strike the aerosol from the gas phase will encounter the organic surface layer, thereby potentially negating thermodynamic arguments for enhanced Henry’s law partitioning of highly oxygenated molecules into an aqueous environment. Based on literature parametrizations of O:C ratios,25 we determined that the α-pinene and toluene SOA formed in the TPOT experiments is likely to be phaseseparated: the respective O:C ratios are 0.5 and 0.4, calculated with the improved ambient method from Canagaratna et al.55 The value of 1.4 for acetylene SOA is high enough that the aerosol is probably aqueous. The intermediate O:C ratio of 0.6 for isoprene SOA indicates that the aqueous and organic phases are probably partially miscible. Without direct measurements, we do not know whether the α-pinene and toluene SOA adopt a core−shell structure, but a reduced availability of water and sulfate near the particle surface might explain why ALW does not affect relative SOA yields as strongly for α-pinene and toluene as it does for acetylene and, to a lesser extent, for isoprene. We will discuss the role of ALW in the context of multiphase chemistry for α-pinene, toluene, and acetylene SOA below. 3.1. α-Pinene. As illustrated in Figure 2a, SOA production from α-pinene ozonolysis is enhanced by a factor of 1.13 ± 0.06 (90% CI for five measurements) on deliquesced AS seeds relative to effloresced seeds. We expect that organic mass should increase with increasing seed surface area as more ELVOCs condense onto particles, but we could not confidently discern a trend for the three applicable trials over the range of 4.0 × 109 nm2 cm−3 to 9.7 × 109 nm2 cm−3. Yield was weakly sensitive to the total sulfate mass loading for the five relevant trials over the experimental range of 10−40 μg m−3 SO4. The average rate of change of organic mass (ΔOrg) in response to the change in sulfate mass (ΔSO4) was ΔOrg/ΔSO4 = 0.35, as determined from the linear regression fit in Figure S9.
3. RESULTS AND DISCUSSION Figure 2 summarizes the relative SOA yields from the oxidation of α-pinene, toluene, and acetylene, along with our earlier studies of isoprene.35 These four systems exhibit a wide spread of responses to particle phase and drying. The mass spectra in Figure 3 present differences in the organic composition of SOA on deliquesced and effloresced AS seeds for each compound. Prior to subtraction, the organic peaks were normalized by total sulfate content in the aerosol. Overlaid mass spectra are available in Figures S6−S11. Figures 4−6 offer an alternate way to compare AMS spectra: by differences in fractional organic composition. Absolute SOA yields are discussed in the Supporting Information. We emphasize that the objective of this work is not to provide quantitative measures of absolute SOA yield, which is better performed in a chamber, but rather to determine the relative influence of seed phase on SOA formation and composition from three different VOC precursors, which a chamber cannot so easily evaluate. AMS measurements do not directly characterize particle morphology, which may affect the extent to which aqueous chemistry participates in SOA formation by directing gas− particle partitioning. If the initial VOC oxidation products form an organic shell surrounding an aqueous and inorganic core, D
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Figure 5. Difference between fractional organic composition of toluene SOA on deliquesced and effloresced AS seeds across 12 trials. See the caption for Figure 4 for more information.
Figure 3. Difference in sulfate-normalized mass spectra for organic components in (a) α-pinene, (b) toluene, and (c) acetylene SOA. The insets show magnified views at high mass-to-charge ratios (m/z). Positive peaks (blue) are present in greater abundance on deliquesced AS seeds, and negative peaks (red) are enhanced on effloresced seeds. Most peaks are positive because the relative SOA yields are greater on deliquesced AS. The gray shading represents ±1 standard deviation (σ) across (a) 5, (b) 12, and (c) 7 trials. Figure 6. Difference in fractional organic composition of acetylene SOA on (a) deliquesced and effloresced AS seeds or (b) deliquesced seeds with and without drying the SOA at the exit of the flow tube. Positive peaks constitute a larger fraction of deliquesced SOA, and negative peaks constitute a larger fraction of (a) effloresced SOA or (b) deliquesced SOA that has been dried. The gray shading represents ±1σ across (a) seven and (b) five trials.
phase diffusion of ELVOCs to AS seeds limited SOA formation in chamber studies by Ehn et al.39 and seems to be ratedetermining for our flow tube as well. Assuming a diffusion constant of 0.05 cm2 s−1,39 ELVOCs require 80 s to traverse the distance halfway between the lamp and the wall of the TPOT, whereas collision times with the particles will be much faster (see section 3.3). These nonvolatile compounds contribute to SOA mass through physical partitioning, and they also undergo irreversible chemical reactions in the particle phase.40,57 At higher aerosol loadings in our experiments and in the atmosphere, semivolatile organic compounds (SVOCs) become increasingly important SOA contributors. Oxygenated hydrophilic molecules, including both ELVOCs and SVOCs, favorably partition into aerosol droplets with high ALW. Although Figure 4 indicates that the oxygenated moieties CHO+ (m/z = 29, a tracer for alcohols and aldehydes) and C2H3O+ (m/z = 43) comprise a greater fraction of α-pinene
Figure 4. Difference between fractional organic composition of αpinene SOA on deliquesced and effloresced AS seeds. Here, “Δ Org. Frac.” is equal to (f m/z)deliquesced − (f m/z)effloresced, where f m/z is the organic signal at any given mass-to-charge ratio (m/z) divided by the total organic signal integrated over all masses. The inset shows the magnified mass spectrum at high molecular weights. Positive peaks (blue) comprise a larger fraction of the SOA on deliquesced seeds and negative peaks (red) on effloresced seeds. The gray shading represents ±1σ across five trials.
These observations are consistent with recent laboratory investigations revealing rapid production of ELVOCs from autoxidation of α-pinene SOA on a second-time scale.56 GasE
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Lengthier chamber experiments are needed to test this kinetic limitation hypothesis. Furthermore, solute concentrations are more dilute in the deliquesced aerosol, and Turpin and coworkers have shown that the oligomerization of glyoxal strongly depends on precursor concentrations.61 As the concentration of glyoxal decreases, it is more likely to form simple organic compounds by aqueous OH oxidation rather than large molecules by aqueous oligomerization. Finally, liquid−liquid phase separation could also play a role. Nakao et al. observed low uptake of glyoxal into toluene SOA, which they attributed to the high concentration of organics.62 If the toluene SOA in the TPOT separates into a core−shell morphology, then uptake of glyoxal into the bulk aqueous phase may be limited. 3.3. Acetylene. We were intrigued by the moderate effect of aerosol water on toluene SOA relative yield even though one of the many oxidation products is glyoxal, which should partition much more readily into deliquesced AS than into effloresced AS. In spite of its volatility, glyoxal has a large effective Henry’s law constant in water due to aqueous-phase hydration and oligomerization reactions (KH,eff = 4.19 × 105 m atm−1),45 and that value increases to ∼1 × 108 m atm−1 in concentrated AS solutions because of the “salting-in” effect.17,63 We explored the OH photooxidation of acetylene, which is dominated by glyoxal chemistry (>99%),44,64 to more explicitly probe the contributions of glyoxal to deliquesced and effloresced SOA. Indeed, aerosol water dramatically increased the relative production of acetylene SOA by a factor of 2.0 ± 0.1 (90% CI for seven measurements). We attribute the 100% enhancement plotted in Figure 2a in part to glyoxal uptake into the aqueous phase, in which hydration frees the aldehyde group to react,17,65 and also to favorable partitioning of higher-order acetylene oxidation products into solution. The aqueous and organic phases in acetylene SOA are most likely miscible, in contrast to the liquid−liquid phase separation we predict for α-pinene and toluene SOA. As a result, water is available at the surface of acetylene SOA to facilitate multiphase gas and aqueous chemistry. In the TPOT, uptake of gas-phase glyoxal into the aerosol competes with further reactions in the gas phase. The uptake rate constant is given by 1/4γ < v > A, where γ is the uptake coefficient, < v > is the average gas-phase velocity of glyoxal, and A is the total surface area of the sulfate seeds. The predicted rate constants range from 8 × 10−4 s−1 to 0.3 s−1 for γ = 2.4 × 10−3 (estimated value for glyoxal into AS at 70% RH)66 to γ = 1 (maximum value). We note that the uptake coefficient is uncertain having not been measured on ammonium sulfate solutions free of organics and at room temperature, but the full range of associated time constants is much shorter than the residence time in the flow tube. However, the gas-phase reaction frequency of glyoxal with OH is 0.03 s−1, assuming that the OH concentration in the TPOT is 3 × 109 molecules cm−3 and the rate constant for reaction with glyoxal is k = 1.1 × 10−11 cm3 molecules−1 s−1.67 Thus, the higher-order components of acetylene SOA likely form through a combination of gas-phase processes in addition to aqueous-phase chemistry, encompassing both closed-shell reactions and continued photochemistry68 with OH uptake. The mass spectrum in Figure 6a indicates that deliquesced acetylene SOA contains a greater fraction of the oxidized fragments CO+ (m/z = 28), CHO+ (m/z = 29), and CO2+ (m/ z = 44), but oligomer content is not enhanced relative to effloresced SOA. As Turpin and co-workers observed, glyoxal
SOA on effloresced versus deliquesced AS, the peak intensities are higher in the mass spectrum of deliquesced SOA, as expected, when the organic signals are normalized by total sulfate mass (Figure 3a). For m/z > 80, Figure 4 indicates that large molecules and oligomers constitute a greater fraction of the α-pinene SOA formed on deliquesced seeds vs effloresced seeds. The sulfate-normalized mass spectra in Figure S3 do reveal organic oligomers in the effloresced particles, although at lower concentrations, which is in agreement with a recent thermodynamic model predicting that ELVOCs with as many as seven oxygens can partition to either the aqueous phase or the organic phase.58 It is unclear whether the organic phase, the aqueous and inorganic phases, or both are exposed at the surface of α-pinene SOA in the TPOT. Oligomerization reactions in the aqueous phase, either at the surface or in the bulk aqueous core, could contribute to the increase in highmolecular-weight species present in α-pinene SOA at high ALW content. We cannot definitively conclude whether physical partitioning or chemical reactions are responsible for the 13% increase in α-pinene SOA yield on deliquesced seeds. When deliquesced SOA passes through the diffusion dryer after the TPOT, the overall change in organic mass is not statistically significant: the ratio of [(Org)dried SOA/(SO4)dried SOA] to [(Org)deliquesced SOA/ (SO4)deliquesced SOA] in Figure 2b is 0.97 ± 0.06 (90% CI for four measurements). Thus, most of the organic matter that condenses onto deliquesced AS does not desorb immediately upon water removal, either because the vapor pressures of the α-pinene oxidation products are sufficiently low that they do not evaporate within the residence time in the dryer, or because the oxidation products react in the particle phase to make nonvolatile species. 3.2. Toluene. The relative yield of SOA generated from toluene photooxidation behaves similarly to SOA from αpinene ozonolysis: deliquesced seeds increase the yield relative to effloresced seeds by a factor of 1.19 ± 0.05 (90% CI for 12 measurements), and drying the deliquesced toluene SOA does not provoke statistically significant changes in organic mass (relative SOA yield =1.0 ± 0.1, 90% CI for four measurements). The enhancement in SOA yield in the presence of ALW is likely caused by uptake of oxygenated aliphatics and by aqueous reactions of glyoxal, one of the toluene oxidation products.59 Figure 5 shows that m/z = 29 (CHO+, prominent mass spectral fragment of glyoxal and its oxidation product glyoxylic acid) and m/z = 47 (CH3O2+, a glyoxal tracer)60 comprise a greater fraction of the organics formed from toluene photooxidation in the presence of deliquesced seeds versus effloresced seeds. When the absolute signals are normalized by total sulfate content (Figure 3b), the CHO+ peak in the mass spectrum is ∼10% higher for deliquesced SOA than for effloresced SOA. Because drying the deliquesced SOA does not appreciably remove organic mass (see Figure 2b), we tentatively conclude that fast reversible absorption of photooxidation products is not responsible for the higher yield on deliquesced seeds. Particle-phase reactions are generally posited to make significant contributions to toluene SOA. However, more high-molecular-weight oligomers formed on effloresced AS seeds rather than on deliquesced seeds, both in terms of fractional organic mass (Figure 5) and sulfate-normalized mass (Figure 3b). The limited reaction time in the TPOT could partially explain why aqueous-phase oligomerization reactions are not noticeably enhanced in deliquesced toluene SOA. F
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high levels of isoprene, glyoxal, and sulfate, in keeping with findings from Carlton and Turpin.27 Isoprene is directly emitted to the atmosphere from biogenic sources, and glyoxal is a secondary product of both biogenic and anthropogenic precursors, including toluene, acetylene, and isoprene itself. Whereas long-term, worldwide measurements of VOCs are readily available, field studies have only recently begun to characterize ALW,27−29 which depends on the ambient relative humidity as well as the concentration and hygroscopicity of existing aerosol. Nguyen et al. considered field campaigns from 21 sites around the globe and consistently calculated the highest concentrations of ALW in areas where anthropogenic sulfate drives water uptake.29 Therefore, aqueous-driven SOA formation should be most important when isoprene and glyoxal emissions converge on air masses influenced by anthropogenic pollution. Finally, we note that although particle-phase water may not cause marked changes in SOA yield in α-pinenedominated zones, aqueous chemistry nevertheless influences the SOA composition. In summary, the laboratory flow tube studies reported here capture a complex snapshot of multiphase SOA formation. The mix of VOCs, seeds, and oxidants present in the atmosphere further complicates attempts to develop a universal model of the effects of ALW on SOA mass and composition. More laboratory experiments and field measurements are needed to understand particle-phase water and its impacts on air quality and the global radiation budget, especially as a changing climate increases the concentration of water vapor in the atmosphere.
oxidation favors simple acids over large oligomers at low concentrations.61 Small changes in composition at high masses are obscured in our experiments because the absolute acetylene SOA yield is low, as discussed next, compounding the preexisting challenge of detecting oligomers by electron impact ionization. Upon drying deliquesced acetylene SOA (Figure 6b), the CHO+ signal is lost to a greater extent than CO+ and CO2+, probably because the aldehydes are more volatile than the acids, and some high-molecular-weight products desorb after the brief residence time in the dryer. From a quantitative perspective, Figure 2b indicates that drying reduces the integrated organicto-sulfate mass ratio of deliquesced SOA by 40% ± 10% (90% CI for five measurements) within just a few seconds. Thus, acetylene SOA formation appears to be partially reversible, which matches the results of Kroll et al., Galloway et al., and others who have measured glyoxal uptake on long time scales.17,69,70 Volkamer et al. tentatively suggest that acetylene SOA formation may also be reversible, but they lack convincing evidence for the relative importance of reversible Henry’s law partitioning and irreversible particle-phase photochemistry.44 We speculate that a combination of both processes likely occurs. 3.4. Atmospheric Implications. When the relative SOA yields presented here for α-pinene, toluene, and acetylene SOA are combined with our earlier results for isoprene,35 a unified picture of the role of ALW on SOA formation does not emerge. We found a 13% ± 6% and 19% ± 5% enhancement in SOA yield on deliquesced versus effloresced AS seeds for α-pinene ozonolysis and toluene + OH photooxidation, respectively; a large 100% ± 10% enhancement for acetylene + OH photooxidation; and an intermediate 60% ± 20% enhancement for isoprene + OH photooxidation (see Figure 2a). We assessed the reversibility of aqueous-driven chemistry by passing deliquesced SOA through a dryer, simulating rapid RH cycling in the atmosphere. The total organic mass was not significantly affected for α-pinene or toluene SOA within the brief, 3 s residence time in the dryer. Drying removed 40% ± 10% of the mass of deliquesced acetylene SOA and 20% ± 10% of the mass of deliquesced isoprene SOA on this short time scale (see Figure 2b). The SOA composition changed after drying for all systems, suggesting that ALW affects the chemical makeup of aerosols not only when they form but also as they age. The overall influence of ALW on SOA formation is difficult to generalize because the effects are entangled in the chemistry and morphology specific to each system, and we have not even tested particle acidity. On the whole, ALW appears to most strongly affect SOA precursors that produce more simple acids and carbonyls. On the microscopic scale, particle structure may control the participation of aerosol water in SOA formation. When a gasphase oxidation product reaches the surface of an aerosol, does it strike the aqueous phase, the organic phase, or some mixture of the two? Can reactants diffuse rapidly enough through the aerosol, especially if they must traverse a hydrophobic outer shell to reach an inner aqueous core? How do acidic conditions alter partitioning between the gas, aqueous, and organic phases? To answer these questions and improve atmospheric models, thermodynamic predictions must be combined with imaging and single-particle techniques71 that experimentally probe particle morphology. Based on the relative SOA yields from our laboratory studies, we expect that ALW will impact SOA formation in regions with
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b04700. Table of relative yields and uncertainties; discussion of absolute yields; figures showing organic mass spectra and dependence of relative SOA yield on sulfate loading. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 416-946-7359; e-mail:
[email protected]. ORCID
Jennifer A. Faust: 0000-0002-2574-7579 Jonathan P. D. Abbatt: 0000-0002-3372-334X Present Addresses †
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340, United States (J.P.S.W.). ‡ Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Republic of Singapore (A.K.Y.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council for funding. REFERENCES
(1) Ervens, B.; Turpin, B. J.; Weber, R. J. Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): A review
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DOI: 10.1021/acs.est.6b04700 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
