Synergy between Secondary Organic Aerosols and Long-Range

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Synergy between Secondary Organic Aerosols and Long-Range Transport of Polycyclic Aromatic Hydrocarbons Alla Zelenyuk,*,† Dan Imre,‡ Josef Beránek,† Evan Abramson,§ Jacqueline Wilson,† and Manish Shrivastava† †

Pacific Northwest National Laboratory, Richland, Washington 99354, United States Imre Consulting, Richland, Washington 99352, United States § University of Washington, Seattle, Washington 98195, United States ‡

S Supporting Information *

ABSTRACT: Polycyclic aromatic hydrocarbons (PAHs), known for their harmful health effects, undergo long-range transport (LRT) when adsorbed on and/or absorbed in atmospheric particles. The association between atmospheric particles, PAHs, and their LRT has been the subject of many studies yet remains poorly understood. Current models assume PAHs instantaneously attain reversible gasparticle equilibrium. In this paradigm, as gas-phase PAH concentrations are depleted due to oxidation and dilution during LRT, particle-bound PAHs rapidly evaporate to re-establish equilibrium leading to severe underpredictions of LRT potential of particle-bound PAHs. Here we present a new, experimentally based picture in which PAHs trapped inside highly viscous semisolid secondary organic aerosol (SOA) particles, during particle formation, are prevented from evaporation and shielded from oxidation. In contrast, surface-adsorbed PAHs rapidly evaporate leaving no trace. We find synergetic effects between hydrophobic organics and SOA - the presence of hydrophobic organics inside SOA particles drastically slows SOA evaporation to the point that it can almost be ignored, and the highly viscous SOA prevents PAH evaporation ensuring efficient LRT. The data show the assumptions of instantaneous reversible gas-particle equilibrium for PAHs and SOA are fundamentally flawed, providing an explanation for the persistent discrepancy between observed and predicted particle-bound PAHs.

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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are among the most common hydrophobic organic pollutants with well documented toxic effects on human health and the ecosystem.1 Appearing on the list of persistent organic pollutants (POPs), they are regulated under the POPs protocol of the convention on longrange transboundary air pollution and by the convention for the protection of the marine environment of the northeast Atlantic (OSPAR convention). Most PAHs are anthropogenic byproducts of energy production and use and of biomass burning. Although the majority of PAH sources are in developing countries, they reach remote regions, including pristine environments, through long-range transport (LRT).2−6 While it is clear that gas-particle partitioning of semivolatile PAHs strongly influences their atmospheric distribution, transport pathways, degradation and environmental fate, the fundamental aspects that determine this partitioning remain unclear. A number of studies have reported an unexpectedly high LRT potential of particle-bound PAHs - for example gas-phase PAH concentrations in the Arctic are orders of magnitude lower than in Europe, while the concentration of particle-bound PAHs is only slightly lower.2 Moreover, based on current understanding of gas-particle partitioning and atmospheric degradation of PAHs some species, like benzo[a]pyrene and fluoranthene, should not undergo LRT at all yet are found in the Arctic at © 2012 American Chemical Society

concentrations similar to those in Europe. In general, existing gas-particle partitioning models severely underpredict observed LRT of particle-bound PAHs, highlighting large knowledge gaps in kinetic partitioning models.2,7,8 A recent review of existing gas-particle partitioning models identified four contrasting descriptions of the relation between PAHs and aerosols that yield very different global fates.2 The earliest modeling efforts assumed that PAHs reside only on the particle surface through surface adsorption. Accordingly, gasparticle partitioning was modeled using the Junge-Pankow parametrization, which follows the Langmuir surface adsorption model.9,10 Compared to measurements, this model significantly underpredicted concentrations of particle-bound PAHs with increased model vs measurement discrepancy for PAHs with higher vapor pressures.7,11 It was therefore suggested PAHs could also absorb to the organic fraction of the aerosol mass.2,10 Harner and Bidleman12 suggested that absorption and activity coefficients of PAH species in aerosol organic matter (OM) are very similar to their absorption in octanol, and as a result the PAH particle-gas partition coefficient, Kp (m3 μg−1), has been Received: Revised: Accepted: Published: 12459

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particles, trapped within the bulk particle-phase, or both? (2) How fast do the PAHs evaporate from SOA particles due to changes in gas-phase concentrations? (3) Are the PAHs associated with SOA protected from atmospheric chemical degradation? (4) How much PAHs are incorporated on/in SOA? (5) How fast do the SOA particles themselves evaporate due to decrease in the gas-phase concentrations of organics?

parametrized based on the measured octanol-air partitioning coefficient (KOA). Similar to the Junge-Pankow adsorption model, models that relied only on absorptive PAH partitioning also underpredicted particle-bound PAHs. Dachs and Eisenrich13 developed a dual model that includes PAH absorption into OM and adsorption to black carbon (BC), with adsorption to the aerosol OM being neglected. They suggested that adsorption to BC was the dominant mechanism, and absorption into OM may account for a minor fraction of particle-bound PAHs only. In contrast, Shen et al.14 concluded that absorption into OM, rather than adsorption to soot, dominated. Dachs and Eisenrich13 also stated that to explain the data, they must conclude that particle-bound PAHs do not equilibrate with the gas-phase but were unable to provide justification. Primbs et al.5 reported that their measured gasphase PAH concentrations were not correlated with PAH concentrations in the particulate-phase. To reduce the gap between models and field data, Sehili and Lammel11 modified the Dachs and Eisenrich approach13 by turning off particlephase chemistry to protect particle-bound PAHs from degradation. Similarly, comparing model predictions to field data in China, Wang et al.7 concluded that both absorption to OM and adsorption to BC must be included and particle-bound PAHs need to be protected from atmospheric degradation. This dual adsorption-absorption approach improved agreement between observations and model predictions of average values, but the same is not true when comparing individual data points, as evident from a set of figures presented by Wang et al.7 that show almost no correlation between measured and modelpredicted gas-particle partition coefficients. Current gas-particle partitioning models assume the absorbing OM is liquid and PAHs instantaneously attain equilibrium with the gas-phase by condensation and evaporation. In this paradigm, as the concentrations of gas-phase PAHs decrease due to dilution and degradation, particle-bound PAHs evaporate to re-establish equilibrium. In striking contrast, the observed data show no correlation between PAHs in particulate- and gas-phases,5 with particle-phase PAH loadings being significantly higher than expected, and no correlation between observed and model-predicted loadings.2,8 This persistent discrepancy between model predictions and observations suggests fundamental problems in our understanding of processes governing the atmospheric lifecycle of PAHs. Secondary organic aerosol (SOA) particles, composed of complex mixtures of semivolatile oxygenated organic compounds, comprise the vast majority of atmospheric OM. Until very recently, due to lack of data, SOA particles were assumed to be liquid droplets at equilibrium with the gas-phase through rapid gas-particle partitioning.10,15 Applying this assumption, dilution during LRT results in evaporation of the aerosol OM and with it loss of particle-bound PAHs. However, recent laboratory and field studies, by our group and others, show that SOA particles are highly viscous quasi-solids and evaporate orders of magnitude slower than expected, which means they cannot maintain equilibrium with the diluting and reacting gasphase.16−18 It is therefore not surprising to find that modeling PAH LRT by rapidly evaporating liquid atmospheric OM is significantly flawed. To understand LRT potential of PAHs by SOA particles, we conducted an experimental and modeling study aimed at quantifying the relationship between PAHs and physicochemical properties of SOA particles by answering the following questions: (1) Are PAHs transported on the surfaces of these

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EXPERIMENTAL SECTION Figure 1 presents schematics of the two experimental setups used to quantify room temperature (RT, 23 ± 2 °C)

Figure 1. Schematic of the SOA formation, coating, evaporation, and characterization experiments.

evaporation rates of SOA and PAHs associated with SOA. Figure 1a illustrates the setup used to generate pyrene-coated SOA particles, in which monodisperse pure SOA particles generated in a PAH-free reaction chamber and classified with a differential mobility analyzer (DMA1) are passed through a coating chamber containing bulk pyrene and pyrene vapor. When pyrene is kept at RT it adsorbs on the surface of particles to form an approximately monolayer coating, which we estimate to comprise a little under 1% of particle weight. Thicker coatings are produced by heating the pyrene reservoir to create a supersaturated vapor that condenses on the SOA particles. The coating process is controlled by varying the pyrene reservoir temperature and monitoring the coating “thickness” with DMA2, which is also used to select monodisperse pyrene-coated SOA with a well-defined coating thickness. The size-selected pyrene-coated SOA particles are passed through two inline denuders18 and loaded into the evaporation chamber, where particle shape, morphology, vacuum aerodynamic diameter (dva), density, and composition are periodically measured as a function of evaporation time using our single particle mass spectrometer, SPLAT II.18−23 Figure 1b illustrates the procedure followed to generate SOA in the presence of PAH vapors. First, a small amount of bulk PAH sample is placed into the reaction chamber and left for ∼24 h to equilibrate with dry, zero air to ensure that gas-phase PAH concentration is equal to or slightly below PAH saturation vapor pressure. SOA formation is then carried out to completion, at which point the formed particles are classified with DMA1 and transported through the two inline denuders into the evaporation chamber where they are monitored for ∼24 h with SPLAT II. A detailed description of the experimental procedures is provided in the Supporting Information (SI). 12460

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Figure 2. Temporal evolution of the mass spectra of pyrene-coated pure SOA particles (a) and SOA particles formed in the presence of pyrene vapor (b) during evaporation. xn indicates mass-spectral peak intensity magnification by a factor of n.

Figure 3. (a) Evaporation kinetics of pyrene coatings, four different PAHs, and a mix of all four PAHs, incorporated and trapped in SOA particles as the particles form; (b) Temporal evolution of the mass spectra of SOA particles generated in the presence of all four PAHs. xn indicates massspectral peak intensity magnification by a factor of n.

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RESULTS AND DISCUSSIONS

distribution of the DMA classified (dm = 400 nm) pyrenecoated SOA particles peaks at 308 nm and has FWHM of 28%, providing clear evidence that these particles are aspherical with pyrene forming a nodule on top of a spherical SOA core, consistent with our previous findings that pyrene deposited on NaCl particles forms a nodule.22,23 Based on the coatinginduced changes in particle dm and dva, the density of pyrene and SOA, and the measured particle effective density, we calculate a pyrene weight fraction of 85% and a dynamic shape factor of 1.32 for the coated particles, as described in the SI.

We begin by presenting the results of property characterization and evaporation kinetics of SOA particles with a thick pyrene coating. Figures 2a, 3a, 4a, and S1a present results for 175 nm pure SOA core particles coated with pyrene and classified with DMA2 at 400 nm. The dva distribution of the 175 nm pure SOA core particles is shown in Figure S1a as a green shaded peak. The position (dva = 207 nm) and narrow line-width (full width at half-maximum, FWHM, of 6.5%) of this peak indicates these particles are spherical and have a density of 1.18 g/cm3, in excellent agreement with our previous measurements. The dva 12461

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Figure 4. (a) Evaporation of pure SOA particles, pyrene-coated pure SOA particles, and SOA particles with trapped PAHs; (b) The effect of aging on SOA evaporation rates for pure SOA and SOA with trapped PAHs.

The aspherical shape of coated particles provides unequivocal evidence that pyrene and SOA are not miscible, consistent with the findings that SOA is not in a liquid phase and does not form a solution, even with liquid hydrophobic organics like lubricating oil, fuel, and dioctyl phthalate.19,24,25 We next load these size-selected particles, via two denuders, into the evaporation chamber and monitor changes in their dva, composition, density, morphology, and shape as a function of evaporation time. Figures 2a, 3a, 4a, and S1a show the temporal evolution of mass spectra, pyrene mass-spectral intensity (gray filled circles), particle volume, and dva distributions, respectively. The mass spectra in Figure 2a illustrate the changes in particle composition with evaporation. At t = 0 the mass spectrum of pyrene-coated particles is dominated by pyrene. However, as the pyrene coating evaporates rapidly, relative to the SOA, after 182 min of evaporation pyrene becomes undetectable, and the mass spectra are nearly indistinguishable from those of pure SOA, shown for reference in the lower panel. Figure 3a provides a clear illustration of this transformation in a plot of the fraction of remaining PAHs relative to t = 0, defined as [IPAH(t)/I(t)]/[IPAH(0)/I(0)], where IPAH(t)/I(t) and IPAH(0)/I(0) represent the relative intensities in PAHs mass spectral peaks measured at time t and at the beginning of the experiment, i.e. before passing the particles through the two denuders, respectively. Figure S1a shows that particle dva rapidly decreases and, with it, the line-width of the dva distribution, indicating a return to spherical particle shape. After 182 min of evaporation, the particle dva distribution is as narrow as that of spherical particles and peaks at 159 nm, which is smaller than the size of the original SOA core particles (dva= 207 nm). This suggests that some of the SOA core surface must have remained uncoated, allowing for a fraction of the SOA core to evaporate. A comparison between the long-term evaporation kinetics of the SOA core of the coated particles and that for pure SOA presented in Figure S2 shows them to be almost identical.

An additional experiment was conducted on pure SOA particles coated with a thinner pyrene coating. If pyrene remains on the particle surface, the thinner coating should evaporate faster. In contrast, the evaporation of dissolved pyrene should be faster from SOA particles with higher pyrene content. Figures 3a and 4a show the data for pure SOA particles exposed to RT pyrene at saturation vapor pressure (open black circles), for which we estimate a pyrene coating thickness of about a monolayer, or ∼1% by weight. Figure 3a shows that the evaporation of pyrene from thinly coated pure SOA particles is much faster than that from SOA particles with a thick pyrene coating: after 60 min in the evaporation chamber no trace of pyrene was detected in the particles’ mass spectra. Figure 4a shows that in this case the evaporation rate of the entire particle is virtually indistinguishable from that of pure SOA. The data clearly show that the evaporation kinetics of pyrene and SOA is in agreement with pyrene residing on the particle surface. In both cases pure SOA particles were exposed to pyrene vapors after SOA formation was complete, and the pyrene coating evaporated rapidly, leaving no detectable trace behind. Absence of pyrene in the mass spectra after relatively short evaporation times, combined with the submonolayer detection capability of pyrene by SPLAT II, provides direct evidence that pyrene does not diffuse into the bulk of SOA particles and that PAHs cannot undergo long-range transport on the surface of SOA particles. These experimental findings demonstrating that PAHs can adsorb to the surface of SOA suggest that when SOA particles form and grow in the presence of gas-phase PAHs, the PAHs can adsorb to the surfaces of SOA throughout the formation process during which oxidized products can condense on the PAHs, trap them, and prevent their evaporation. Due to lack of experimental data, SOA particles were assumed to be in a low viscosity, liquid-like phase from which more volatile PAHs would rapidly escape by diffusion and evaporation. However, recent experimental data from a number of groups show that SOA is in a highly viscous semisolid phase 12462

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in which diffusion is extremely slow.18,26−29 It is therefore reasonable to expect that condensation of SOA on top of these surface-adsorbed hydrophobic organics, during SOA formation and growth, can trap the PAHs inside the highly viscous SOA particles. Indeed, in recent publications we have demonstrated, for the first time, that when SOA particles form in the presence of PAHs and other hydrophobic organic vapors, these organics are incorporated into the SOA particles, where they become trapped.18,19 The concurrent processes of SOA condensation and PAH adsorption has the potential to incorporate and trap significant amounts of PAHs into the bulk of these highly viscous particles. Below we will show that in the case of αpinene SOA formed in the presence of pyrene vapor the trapped pyrene concentration constitutes ∼5% of particle mass. To generate SOA particles with trapped PAHs we conducted a set of experiments in which a small amount of different PAHs, or a mix of several PAHs, was placed at the bottom of a Teflon bag filled with particle-free zero air. Approximately 24 h later SOA formation was carried out in the presence of these PAHs vapors. Once reaction was complete, the SOA particles are sizeselected with a DMA, and their composition, size, shape, density, and evaporative kinetics characterized with SPLAT II. Note that on the time scale of the SOA formation, the gas phase reaction between PAHs and ozone is negligible. We find that when SOA particles form in the presence of PAH vapors (or other hydrophobic compounds) the formed particles are spherical, and they acquire and trap a significant amount of hydrophobic organics. In addition to α-pinene, we generated SOA particles from isoprene, cyclooctene, and nhexene and found that in all cases, when pyrene vapor is present during particle formation, a significant amount of pyrene is incorporated within the SOA particles. The mass spectra, relative pyrene mass-spectral intensity, remaining particle volume fraction, and dva distributions of SOA particles formed in the presence of pyrene vapor are shown in Figures 2b, 3a, 4a, and S1b, respectively. The mass spectra in Figure 2b indicate a significant amount of pyrene in these particles and relatively small changes in the pyrene fraction as a function of evaporation time, as is clearly illustrated in Figure 3a (open red circles). The dva distribution of these particles is narrow (Figure S1b), demonstrating particle sphericity, and after ∼24 h of evaporation the particle diameter remains almost constant. Figure 4a shows that in ∼24 h only 20% of the particle volume fraction evaporates. These data clearly demonstrate that SOA particles uptake pyrene during their formation and that the properties of these particles, and the associated pyrene, are significantly different from those of pure SOA particles exposed to pyrene after SOA formation. Comparing pyrene evaporation from pyrene-coated particles and from SOA particles formed in the presence of pyrene vapor justifies describing pyrene as being trapped in these particles. Figure 3 presents results of experiments conducted on SOA particles formed in the presence of four individual PAHs: pyrene, fluoranthene, phenanthrene, and benz(a)anthracene, which have RT saturation vapor pressures of 6·10−4, 1.2·10−3, 1.6·10−2, and 2.8·10−5 Pa, respectively, and a mix of all four PAHs. Note that the evaporation rate of individual PAHs does not correlate with their vapor pressure. Figure 3b shows, as an example, how the mass spectra of SOA particles formed in the presence of the mix of all four PAHs change with evaporation time. The parent ion peaks at m/z 202, 202, 178, and 228 for pyrene, fluoranthene, phenanthrene, and benz(a)anthracene,

respectively, are clearly visible. A comparison of Figures 2a, 2b, and 3b points to dramatic differences between the evaporation kinetics of PAHs associated with SOA as surface coating and PAHs incorporated into the bulk of SOA particles during SOA formation. In Figure 2a we start with particles composed mostly of pyrene, with an SOA core representing only 15% of particle mass, to find that within ∼2 h of evaporation nearly all the pyrene has evaporated, leaving behind a nearly pure SOA particle. In contrast, Figures 2b and 3 show that even after ∼24 h of evaporation the SOA particles still contain between 50% and 80% of the original PAH acquired during SOA formation. Figure 3a shows that in each case there is a sharp drop in PAH content during the initial evaporation process, which we attribute to rapid evaporation of PAHs residing on or near the particle surface. This rapid evaporation is followed by a much slower decrease in particle PAH content and is attributed to PAHs trapped inside the highly viscous SOA particle-phase. The data show that because of diffusion limitations it takes PAH molecules, even those with rather high vapor pressures, a long time to diffuse to the surface of an SOA particle and evaporate. As noted above, the most successful modeling approaches of PAH LRT assume a combination of PAHs’ adsorption on soot and absorption in OM, with the latter assumed to be similar to their partitioning in an octanol−water solution. However, absorption and evaporation of PAHs into/from liquid octanol are expected to be drastically different from PAH gas-particle partitioning in the highly viscous SOA. As the data presented here show that PAHs on the SOA particle surface cannot penetrate into the particle therefore rapidly evaporate, while PAHs incorporated into the SOA bulk are trapped. To illustrate the effect of evaporation on the temporal evolution of particle-bound PAHs under the assumption of gasparticle equilibration we developed a box model, described in detail in the SI. This model utilizes commonly used partitioning coefficients of pyrene for dual adsorption-absorption on total suspended particulate (TSP), assumes that pyrene maintains instantaneous thermodynamic equilibrium between its particlephase and the gas-phase, and that TSP is nonvolatile and nonreactive. As Figure S3 shows, the model predicts almost complete loss of particle-phase pyrene within a day, which is in sharp contrast to ambient measurements that indicate barely changing concentrations of particle-bound PAHs during their LRT.3,30 The data presented above indicate that when PAHs are incorporated into bulk SOA during particle formation and growth they become trapped inside the highly viscous SOA particles and evaporate at least an order of magnitude slower than predicted by all partitioning models. Moreover, the highly viscous SOA drastically reduces the rates of heterogeneous reactions, limiting them to the layers close to particle surface, thus serving as a protective shield for the particle-bound PAHs. To quantify the amount of PAHs incorporated inside the SOA particles generated in the presence of PAHs vapors, in addition to SPLAT II, we use the Aerosol Mass Spectrometer (AMS, Aerodyne Research Inc.). The mass spectra of mixed SOA/PAH particles were quantified based on the measured mass-spectral signal intensities of size-selected pure PAH particles and pure SOA particles. Applying this approach to SOA formed in the presence of pyrene vapor yields ∼5% for the pyrene weight percent in the SOA. Extending this approach to the other four PAHs presented in Figures 3 and 4 yields similar values (a few percent of SOA mass). Based on the 12463

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evaporation kinetics data we estimate that ∼10% of the particlebound pyrene is on or near the particle surface, with the remaining pyrene trapped inside the SOA particles. To compare our measured concentration of pyrene trapped within SOA particles during SOA formation in the presence of pyrene vapors to that predicted by partitioning models, we calculate pyrene content within the bulk SOA phase assuming absorptive partitioning of pyrene to OM and adsorption of pyrene to the surface of OM using the Junge-Pankow model (see the SI for details).9,31 The predicted amount of pyrene represents 8% of the particle mass, half of which is absorbed in bulk SOA by the absorptive KOA model. This is in good agreement with our measured pyrene content in α-pinene SOA of 5%. The Junge-Pankow model, however, predicts that the other 4% of the particle mass represents pyrene adsorbed on the SOA surface, which is about a factor of 8 higher than observed. So far, we provided direct experimental evidence that PAHs are incorporated and trapped inside SOA particles, evaluated the amount of SOA-bound PAHs, and provided a simple explanation of how the viscous SOA phase protects PAHs from atmospheric oxidation and evaporation. The final step is to provide a quantitative answer to the question of how fast do the SOA particles themselves evaporate during their transport as the corresponding gas-phase dilutes by mixing and chemistry. Until very recently, SOA particles were assumed to evaporate sufficiently fast as to maintain equilibrium with the gas-phase at all times. This means that because of dilution and gas-phase chemistry these particles would evaporate, making LRT of particle-bound PAHs impossible. However, recent data, from our group and others, indicate that SOA particles evaporate orders of magnitude slower than expected.18,32 For example, our data show that after 24 h of evaporation, in an organics vapor-free environment, ∼30% of α-pinene SOA particle mass or ∼70% of particle size remains. Translating this finding to the real atmosphere, where organic vapor concentrations decrease slowly, means that evaporation is orders of magnitude slower than assumed.18 Most importantly, we find that the presence of hydrophobic organics, incorporated into SOA particles during particle formation, reduces the rate of SOA evaporation. Figure 4a shows the measured evaporation behavior of pure SOA particles, SOA particles formed in the presence of the vapors of four individual PAHs, and a mix of all four PAHs. In all cases, the data show a relatively fast evaporation phase lasting about two hours followed by a significantly slower evaporation that we followed for ∼24 h. Comparison of the evaporation kinetics of pure SOA and SOA generated in the presence of gas-phase PAHs indicates that the presence of trapped PAHs consistently slows evaporation. For example, the diameter of SOA particles formed in the presence of pyrene vapor decreases by only ∼7% after 24 h of evaporation in an organics vapor-free environment. For LRT it is also important to quantify the effect of aging on SOA evaporation. In these experiments, particles were stored in the Teflon bag in which they were generated for an additional 12−24 h, before being size-selected and transferred to the evaporation chamber. Figure 4b shows the results of measured evaporation rates of fresh (solid lines) and aged (dashed lines) particles composed of pure SOA, SOA formed in the presence of gas-phase pyrene, and SOA formed in the presence of a mix of PAHs. In all cases the volume fraction evaporating during the fast evaporation phase decreases after aging. The change for pure SOA particles is relatively small, but the changes for SOA

particles with incorporated PAHs are significantly more pronounced. For example, the diameter of aged SOA with trapped pyrene decreases by less than 4% in 24 h of evaporation. Experiments on α-pinene SOA particles containing other hydrophobic organics consistently show that the presence of hydrophobic compounds magnifies the aging effect, i.e. reduction in evaporation rate due to aging is more significant for SOA with trapped hydrophobic PAHs than for pure SOA. In addition, evaporation kinetics of SOA particles from several different precursors exhibit very similar behavior. Moreover, RT evaporation rates of ambient atmospheric SOA particles internally mixed with small amounts of sulfate are found to be as almost identical to α-pinene SOA with trapped pyrene. In other words, their evaporation under organic vaporfree environment is extremely slow, which means that under atmospheric conditions their evaporation would be even slower. Based on all the SOA evaporation studies conducted by us thus far, we conclude that the evaporation rates of SOA particles mixed with hydrophobic organics are so slow that SOA cannot be at equilibrium with the gas-phase and can be approximated as nonvolatile.16 Our data show that when SOA particles form in the presence of the vapors of hydrophobic organics, including PAHs, the hydrophobic organics adsorb to the particle surface during the formation process. As the particles grow by condensation and coagulation, these organics become trapped inside the highly viscous SOA particles. The SOA matrix shields the hydrophobic organics from the oxidizing atmosphere and prevents them from evaporation to restore equilibrium with gas-phase PAHs. The data indicate that PAHs can be efficiently transported by SOA particles only if they are embedded within SOA particles, for which PAHs must be present in the gas-phase during SOA formation. In contrast, PAHs adsorbed on the SOA surface do not penetrate the viscous SOA and rapidly evaporate. Urban locations, in which a significant fraction of SOA forms by the oxidation of organic vapors emitted from the same location and/or sources as those emitting PAHs, including biomass burning emissions, create a situation that guarantees efficient LRT of PAHs and other hydrophobic organic compounds, like POPs, that can have important public health implications. In addition, as biogenic SOA precursors are transported to urban locations forming SOA, they would also incorporate locally emitted anthropogenic PAHs. Perhaps the most surprising finding is the observed synergetic relationship between PAHs and SOA. The presence of even a small amount of hydrophobic organics inside SOA significantly decreases the SOA evaporation rate and amplifies the effect of aging, thus creating conditions that ensure efficient LRT of both SOA particles and PAHs, consistent with observations. This synergy between PAHs and SOA particles has important implications not only for human health but also for climate change. It is important to point out that to develop comprehensive understanding of the PAHs’ atmospheric stability and their content in ambient OM there is a clear need for additional studies to quantify partitioning of PAHs in SOA under a wide range of atmospherically relevant conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental setup, temporal evolution of dva distribution for pyrene-coated pure SOA particles and SOA formed in the 12464

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presence of pyrene vapor, comparison between evaporation kinetics of the SOA core of the coated particles and that for pure SOA, gas-particle partitioning scheme, box model calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Bioscience Division and Office of Biological and Environmental Research (Atmospheric Research Program). Additional support was provided by the Laboratory Directed Research and Development program (Chemical Imaging Initiative). This research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL). PNNL is operated by the US Department of Energy by Battelle Memorial Institute under contract No. DE-AC06-76RL0 1830.

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