Burial Effects of Organic Coatings on the Heterogeneous Reactivity of

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Burial Effects of Organic Coatings on the Heterogeneous Reactivity of Particle-Borne Benzo[a]pyrene (BaP) toward Ozone S. Zhou,* A. K. Y. Lee, R. D. McWhinney, and J. P. D. Abbatt Department of Chemistry, University of Toronto, Ontario, ON M5S 3H6, Canada S Supporting Information *

ABSTRACT: With an aerosol flow tube coupled to an Aerodyne aerosol mass spectrometer (AMS), room temperature (296 ± 3 K) kinetics studies have been performed on the reaction of gas-phase ozone with benzo[a]pyrene (BaP) adsorbed in submonolayer amounts to dry ammonium sulfate (AS) particles. Three organic substances, i.e., bis(2-ethylhexyl)sebacate (BES, liquid), phenylsiloxane oil (PSO, liquid), and eicosane (EC, solid), were used to coat BaP-AS particles to investigate the effects of such organic coatings on the heterogeneous reactivity of PAHs toward ozone. All the reactions of particle-borne BaP with excess ozone exhibit pseudo-first-order kinetics in terms of BaP loss, and reactions with a liquid organic coating proceed by the Langmuir−Hinshelwood (L-H) mechanism. Liquid organic coatings did not significantly affect the kinetics, consistent with the ability of reactants to rapidly diffuse through the organic coating. In contrast, the heterogeneous reactivity of BaP was reduced substantially by a thin (4−8 nm), solid EC coating and entirely suppressed by thick (10−80 nm) coatings, presumably because of slow diffusion through the organic layer. Although the heterogeneous reactivity of surface-bound PAHs is extremely rapid in the atmosphere, this work is the first to experimentally demonstrate a mechanism by which the lifetime of PAHs may be significantly prolonged, permitting them to undergo long-range transport to remote locations.

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studies6−11 have reported a nonlinear dependence of the kinetics on a variety of surfaces, indicating that such reactions proceed via a Langmuir−Hinshelwood (L-H) mechanism that involves two processes: (1) equilibrium partitioning of the gasphase oxidants between the gas phase and the surface and (2) surface reaction between the oxidant and the adsorbed molecule, e.g., the PAHs. Modeling studies have indicated that it is likely that ozone first physisorbs to the particle surface and is then transformed to a reactive intermediate prior to reaction with the PAHs.18,19 Pöschl et al.6 were the first to introduce the experimental technique of aerosol flow tube kinetics to this field, by studying the heterogeneous loss of BaP on soot particles with gas-phase ozone. Importantly, they reported a Langmuir-type dependency of the BaP decay rate constant on ozone concentrations. The LH mechanism has since been observed for a wide range of ozone heterogeneous reactions, e.g., anthracene on water,7,8 PAHs (naphthalene, fluoranthene, anthracene, phenanthrene, pyrene, and BaP) sorbed to 1-octanol,10 as well as anthracene on Pyrex tubes,11 and BaP on azelaic acid9 and 1,1,5,5tetraphenyltetramethylthisiloxane (TTTS) particles.13 Moreover, Kwamena et al.13 explored the kinetics of surface-bound PAHs with ozone and proposed that the ozone partitioning constant (KO3) is a descriptor of the ozone-aerosol surface

INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous air pollutants released into the atmosphere as a byproduct of incomplete combustion processes. Due to their mutagenic and carcinogenic properties extensive efforts have been made to understand their physical-chemical transformations in the atmosphere.1 PAHs containing two and three rings are found predominantly in the gas phase, whereas those containing six or more rings principally adsorb to particles or other surfaces. PAHs with four or five rings partition between both phases to a degree dependent on aerosol and environmental conditions.2 Field measurements and modeling studies have demonstrated that PAHs, including a particularly toxic species, benzo[a]pyrene (BaP), are present in pristine areas, e.g., Arctic and Antarctic regions, as a result of long-range transport from distant combustion sources.3−5 During their transport, PAHs will experience loss by both gas-phase and heterogeneous photo-oxidation processes. Recent laboratory6−14 and modeling studies15 suggest that heterogeneous reactions may be the dominant reactive atmospheric loss process for the larger PAHs, prior to deposition. A number of experimental studies aiming at understanding the heterogeneous reaction of PAHs with atmospheric oxidants have been conducted. Early work of Wu et al.16 and AlebicJuretic et al.17 investigated the heterogeneous reaction of gasphase ozone with surface bound PAHs and found a linear dependence of the pseudo-first-order PAH-loss rate coefficient with gas-phase ozone concentrations. In contrast, more recent © 2012 American Chemical Society

Received: March 30, 2012 Revised: June 7, 2012 Published: June 7, 2012 7050

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Figure 1. Schematic of the experimental setup. The dashed box indicates the setup for EC coating.

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EXPERIMENTAL SECTION Aerosol Preparation and Kinetics Flow Tube. Figure 1 gives a schematic of the experimental setup. The particles were generated by atomizing 0.002−0.005% (w/w) ammonium sulfate (AS) aqueous solution using an atomizer (TSI model 3076). A fraction of the polydisperse AS particle flow was dried by a diffusion drier and then diluted by adding 250−500 standard cubic centimeters per minute (sccm) dry nitrogen using a mass flow controller before entering the benzo[a]pyrene (BaP) coating tube (15-cm-long, 1-cm-i.d.). Particles were coated with BaP by passing the dry AS particles through a heated Pyrex glass tube whose inner wall was coated with solid BaP by depositing BaP as a solution in chloroform and drying the solvent out. Three organics, i.e., bis(2-ethylhexyl)sebacate (BES, liquid at room temperature), phenylsiloxane oil (PSO, a mixture of tetramethyltetraphenyltrisiloxane, tetraphenyldimethyldisiloxane, and alkylphenylsiloxane, liquid at room temperature), or eicosane (EC, solid at room temperature), were used to coat the BaP-AS particles. Depending on the organic vapor pressure, we found it necessary to coat the AS particles via two different ways, shown as dashed arrows in Figure 1. The liquid organics (BES and PSO) with low vapor pressures are coated by adding them into the BaP tube, which was heated to either ∼363 or ∼373 K to arrive at “thin” or “thick” coatings, respectively. The solid organic, i.e., EC, which has a significantly higher vapor pressure, was coated by passing BaP-AS particles through a second heated Pyrex tube (either to ∼333 K or to ∼353 K, for thin or thick coatings, respectively), where the EC was placed. We note that the method used for EC was not appropriate for BES or PSO because we found that most BaP evaporated from the AS particles when passing through the heated tube used for organic coating. Dry nitrogen (1 slpm) was added before sending the resulting particles to the aerosol flow tube to make a total flow of 1.9 slpm. Test experiments using a scanning mobility particle sizer (SMPS) consisting of a differential mobility analyzer (DMA, model 3080, TSI Inc.) and a condensation particle counter (CPC 3025, TSI Inc.), showed that homogeneous nucleation of organic aerosol does not occur

interaction, independent of PAHs adsorbed. On the basis of the higher KO3 values measured for soot6 and TTTS13 compared to other surfaces, such as water,7 self-assembled monolayers,20 octanol thin films,10 aqueous NaCl droplets,21 and azelaic acid aerosols,9 the authors concluded that ozone prefers partitioning to nonpolar surfaces.13 Despite these studies, many open questions on the atmospheric fate of PAHs remain, especially when considering the complex morphology and composition that is characteristic of tropospheric aerosol. Knowing that such particles commonly consist of both inorganic salts and organic materials, it is as yet unclear whether the overall reactivity of PAHs is best characterized by that demonstrated when they are adsorbed to surfaces or, instead, when they are imbedded within a multiphase particle. Previous experimental studies on the heterogeneous reactions of PAHs with ozone, such as those above, have only focused on PAHs adsorbed on singlecomponent surfaces. From early field measurements22,23 and other experimental studies, it is now well recognized that surface-bound PAHs react rapidly under atmospheric oxidant conditions with estimates for the atmospheric lifetime of BaP with respect to loss via reaction with ozone of minutes to hours.13 However, it is quite likely that some degree of burial of the PAHs may occur in the atmosphere, either when other organics condense on primary combustion particles or via secondary organic aerosol (SOA) growth that occurs via photochemical processing. The matrix effects that arise with such multicomponent aerosols are unknown. In particular, does the high degree of surface reactivity remain or is it suppressed by organic coatings that either dissolve or bury the reactant PAHs? In this study, we present the first attempt to study the effect of organic coatings on PAH heterogeneous reactivity. The kinetics on the heterogeneous reaction of ozone with submonolayer amounts of BaP adsorbed on ammonium sulfate (AS) particles are studied as a function of coating thickness and phase, i.e., liquid vs solid. This is the first use of the Aerodyne aerosol mass spectrometer (AMS) to study the heterogeneous reactivity of PAHs on aerosol in an online manner. 7051

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was no interference at m/z 252 by BaP ozonolysis products, BES or EC. PSO contributed between 10 and 30% to the total m/z 252 signal intensity, and this contribution was found to be proportional to the total organic mass loading. However, it was unaffected by the presence of ozone in the flow tube, and so the contribution of PSO to m/z 252 signal was easily accounted for. Also, it should be noted that with atomizers there is invariably present a small level of AMS-observable organics, arising either from the deionized milli-Q water used in the atomizer or from background laboratory contamination. This signal can be as large as 5−10% of the AS mass loading. However, compared to the total organic mass loading when the organic coatings are applied, the background organic mass is quite small. Chemicals. Benzo[a]pyrene (solid, ≥98%) and eicosane (solid, >99.0%) were purchased from Sigma-Aldrich. Phenylsiloxane oil (DC-704 CAS no. 3982-82-9) was purchased from Dow Corning. Ammonium sulfate (solid, ≥99.5%) and bis(2ethylhexyl)sebacate (liquid, ≥97%) were purchased from Fluka. All chemicals were reagent grade and used without further purification.

when heating the organics in the coating tubes. Under common operating conditions, the aerosol number and AS mass entering the aerosol flow tube were (1−3) × 105/cm3 and 6−30 μg/m3, respectively. Ozone, generated by ultraviolet irradiation of a mixed flow of N2/O2 in a Pyrex glass chamber with a mercury pen-ray lamp (UVP Inc.), was introduced into the flow tube through a movable stainless steel injector used to minimize particle losses. The ozone concentration was controlled by varying the ratio of O2 to N2 passing by the pen-ray lamp and it was measured with a UV photometric O3 analyzer (Thermo model 49i) after passing through the flow tube (Figure 1). A flow of 1.3 slpm dry nitrogen was added to the O3 analyzer to dilute the ozone concentration so that the analyzer samples only 100 sccm from the flow tube. Ozone is in large excess to BaP in the flow tube. The aerosol flow tube, 6 cm i.d. and 96 cm long, was vertically orientated and operated at atmospheric pressure and room temperature (1 atm and 296 ± 3 K) under laminar flow conditions. The relative humidity inside the flow tube was lower than 5%, and the tube was wrapped in aluminum foil to avoid photochemistry. The reaction time between particleborne BaP and ozone was varied between 13 and 66 s by adjusting the injector position along the length of the flow tube. An ozone denuder, which can remove more than 99% of ozone from the flow, was positioned between the flow tube exit and particle detection systems. The above-mentioned SMPS and a compact time-of-flight (C-ToF) AMS were placed behind the ozone denuder to measure particle mobility size distributions and particle chemical composition, respectively. The rest of the flow from the flow tube was removed by a diaphragm pump operating in parallel with the SMPS and AMS (Figure 1). Particle Composition Measurement with the AMS. The chemical composition of the coated particles was analyzed by Aerodyne AMS, which has been described in detail previously in the literature;24−27 only a brief description is given here. In particular, submicrometer aerosol particles are sampled into the AMS (at a flow of 100 sccm) from the aerosol inlet through an aerodynamic lens, forming a narrow particle beam. A chopper located in a particle sizing chamber enables the instrument to operate in either the mass spectrum (MS) or particle time-of-flight (PTOF) modes. During the MS mode, the chopper is removed from the aerosol beam and the particle beam impacts a heated tungsten vaporizer positioned within an electron impact ionizer of a time-of-flight mass spectrometer. During PTOF mode, the chopper is placed in the particle beam, and mass spectra are collected as a function of particle transit time through the sizing chamber, enabling a particle-size dependent mass spectrum to be gathered. The AMS data were analyzed by the Squirrel program (version 1.49B1). The sulfate, PAH and total organic mass loadings were determined using the standard sulfate, PAH, and organic mass fragment assignments, and default relative ionization efficiencies of 1.2 for sulfate and 1.4 for both PAH and organic. Mass loadings are reported using a collection efficiency of unity although we note that all of the kinetics reported arise from relative changes in signal and, as such, are independent of the collection efficiency assumption. BaP was detected by its molecular ion m/z 252, the most intense fragment in the EI mass spectrum28 and normalized by the sulfate mass loading during the kinetics measurements, under the assumption that aerosol sulfate is unreactive. Normalization accounts for any small changes in aerosol loading that may occur during a kinetics run. Test experiments showed that there

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RESULTS AND DISCUSSION Particle Characterization. Under typical operating conditions, the ratio of BaP to sulfate mass loading was 10−3 to 10−4. For a typical mass modal diameter of ∼70 nm and assuming spherical particles, this corresponds to a thickness for the BaP coating of 0.02−0.2 nm (see Supporting Information for a description of how this thickness is determined). With the cross-section diameter of BaP being roughly 1.1 nm × 0.7 nm (length × width), and its thickness through its planar dimension 0.3 nm,29 it is clear that the surface coverage is submonolayer on average. This is all assuming that BaP is adsorbed in a uniform layer and does not adsorb in pockets or islands. To verify this assumption, test experiments were conducted by exposing uncoated BaP-AS particles to high concentrations of ozone in the aerosol flow tube and more than 90% of BaP was found to be consumed. This confirms indirectly that the BaP is coated uniformly on AS particles rather than adsorbed on AS in pockets or islands because if this was the case the total consumption of BaP would not be observed (as with the EC coating experiments). When these particles become coated with solid EC, the BaP should remain on the AS surface and buried under EC. However, for liquid organic coats, because the BaP and BES or PSO were coated simultaneously (Figure 1) we believe that BaP and BES or PSO were either uniformly mixed within the organic coat or else adsorbed to the surface of the organic layer. We note that a similar coating procedure with liquid organic oils on solid salt particles has been demonstrated to form organic coatings of uniform thickness.30 An example of SMPS size distributions of AS particles before and after BES/BaP coating is given in Figure 2. With experimental uncertainties, a change in the size distribution of AS particles is not observable by SMPS when a thin BES coating is applied (triangle symbols). However, the mean mobility diameter of the particles is seen to increase by roughly 25 nm with a thick BES coating (circle symbols). With this thick BES coating, the size increase for smaller AS particles is less than the larger AS particles, indicating that the coating thickness is smaller in absolute terms for smaller particles than for larger particles. 7052

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thick coatings (see Supporting Information). The thin and thick coatings are estimated to be 4−8 and 10−80 nm in thickness, respectively. Interestingly, for the thick coating experiments, the absolute thickness of the coating organics increased with particle size, confirming the observations made from the SMPS system, whereas for the thin coatings, the absolute thickness was largely independent of particle size. It is unclear why this difference arises. Kinetics of BaP Heterogeneous Loss. Figure 4 shows examples of the kinetic plots of the BaP concentration as a Figure 2. Comparison of BaP-AS aerosol distributions with “thin” and “thick” BES coatings. The BES and BaP were heated to ∼353 K for the “thin” coating and to ∼373 K for the “thick” coating. The different initial sizes and concentrations of AS particles are due to the different AS aqueous concentrations in the atomizer.

The particle-size-resolved measurements from the AMS also inform us on the nature of the organic coatings. Figure 3

Figure 4. Example of the kinetic plots for the reaction of ozone with BaP-AS particles coated with thin BES. Symbols represent different ozone concentrations: no ozone (plus symbols), 2.17 × 1014 molecules/cm3 (triangle symbols), and 7.33 × 1014 molecules/cm3 (cross symbols).

function of reaction time for thin BES coatings. The data shown are 15−20 min averages of the normalized BaP concentration measured by the AMS and the uncertainty in each point was taken as the standard error of the measurement. The linearity of the plots illustrates that the heterogeneous loss of BaP with ozone exhibits pseudo-first-order kinetics. The slope of the linear least-squares fit to the data provides the pseudo-first-order rate coefficient (kIobs) at a specific ozone gasphase concentration. Control experiments have been conducted in a manner analogous to the reactive experiments in all ways except that ozone is not present, and the change to the BaP signal as a function of injector position has been observed (Figure 4). The control experiments have been accounted for in final data analysis. Table 1 summarizes all rate coefficients obtained from the present work. The uncertainties of the rate coefficients are calculated from a combination of the least-squares 1-σ uncertainties in the reaction measurements and control experiments. Figure 5 plots these first-order BaP loss rate coefficients as a function of the corresponding ozone concentrations in the flow tube. The data fall into two categories of BaP-AS particles: (i) uncoated and liquid-organic (thin and thick BES, thin PSO) coated, and (ii) thin and thick EC-coated. Considering the first set of data, nonlinear plots with saturation of the reactivity at high ozone concentrations, consistent with a Langmuir−Hinshelwood (L-H) mechanism, are observed for uncoated and liquid organic coated BaP-AS particles. This behavior has been observed for a wide range of ozone reaction systems beyond PAHs, including unsaturated self-assembled monolayers,20 NaCl droplets,21 alumina,31

Figure 3. Size-resolved mass distributions for chemical species obtained from the PTOF mode of the AMS for the thin BES (a) and thin EC (b) coatings.

provides representative data for thin BES (a) and EC (b) coatings on AS. In particular, we see that either the properties of the organic (e.g., related to its vapor pressure or phase) or the coating procedure affects the character of the coatings. For the solid organic, EC, which is coated onto a BaP-AS particle, the size distributions of the three materials are all similar, indicating a constant ratio of one species to another as a function of particle size. However, for the liquid organic, thin or thick BES, which is coated simultaneously with BaP onto AS, the two organics have a common size distribution, with a mass mode somewhat smaller than that of AS; i.e., the smaller particles are more organic rich than the larger particles. The thickness of the coating organics was estimated using PTOF AMS data by assuming spherical particles and uniformly 7053

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Table 1. Summary of the Pseudo-First-Order Rate Coefficients for the Reaction of Particle-Borne BaP with Different Concentrations of Ozone and Different Coating Organicsa BaP [O3] 0.89 1.03 3.20 4.90 6.35

a

BaP + thin BES

kIobs (s−1)

[O3]

± ± ± ± ±

1.62 2.12 3.81 6.79 7.33 7.55 7.97

0.020 0.018 0.029 0.031 0.028

0.002 0.003 0.003 0.006 0.003

BaP + thick BES

BaP + thin PSO

kIobs (s−1)

[O3]

kIobs (s−1)

[O3]

± ± ± ± ± ± ±

3.92 4.29 4.99

0.032 ± 0.003 0.028 ± 0.002 0.038 ± 0.002

0.89 1.03 3.20 4.90 6.35

0.019 0.024 0.032 0.036 0.040 0.038 0.039

0.002 0.003 0.004 0.006 0.006 0.005 0.005

BaP + thin EC

kIobs (s−1)

[O3]

± ± ± ± ±

1.62 2.12 3.81 6.79 7.33 7.55 7.97

0.020 0.018 0.029 0.031 0.028

0.002 0.003 0.003 0.006 0.003

BaP + thick EC

kIobs (s−1)

[O3]

kIobs (s−1)

± ± ± ± ± ± ±

3.92 4.29 4.99

0.032 ± 0.003 0.028 ± 0.002 0.038 ± 0.002

0.019 0.024 0.032 0.036 0.040 0.038 0.039

0.002 0.003 0.004 0.006 0.006 0.005 0.005

O3 concentrations are in units of 1014 molecules/cm3. The uncertainties of the rate coefficients represent 1-σ standard deviations.

Table 2. Comparison of Literature Studies on the Heterogeneous Reaction of Ozone with BaP or Anthracene on Different Aerosol Substrates with the Present Work aerosol substrate

KO3 (10−15 cm3)

kImax (s−1)

BaP BaP

soot azelaic acid

280 ± 20 1.2 ± 0.4

0.015 ± 0.001 0.048 ± 0.008

anthracene

TTTS

100 ± 40

0.010 ± 0.003

anthracene

azelaic acid

2.2 ± 0.9

0.057 ± 0.009

BaP BaP BaP

AS AS-BES AS-PSO

14 ± 4 4.1 ± 0.1 13 ± 4

0.034 ± 0.002 0.051 ± 0.001 0.047 ± 0.004

PAH 6

Pöschl et al. Kwamena et al.9 Kwamena et al.13 Kwamena et al.13 this work this work this work

Figure 5. Pseudo-first-order rate coefficients kIobs as a function of gasphase ozone concentrations for the reaction of particle-borne BaP and ozone with different coating organics. The plots for the liquid and uncoated BaP particles were fitted using a nonlinear least-squares fit of eq 1. The fitting parameters (KO3 and kImax) are listed in Table 2.

of BaP on solid azelaic acid aerosols.13 Agreement with past studies lends confidence that the experimental approach, including the first use of the AMS to monitor PAH concentrations online, is providing valid data. Further inspection of these data leads to the conclusion that the liquid organic coatings, whether thick or thin, has remarkably little effect on the kinetics compared to the uncoated reference. This can be interpreted in a number of ways that are impossible to confirm given lack of detailed knowledge of the morphology of the particles. Electron microscopic examination of these particles was attempted but due to their volatile nature under vacuum and with exposure to the electron beam, no conclusive results were obtained. In general, though, we believe that the rate-determining step of the reaction between O3 and BaP takes place at the gas-particle interface and the bulk reaction of BaP with dissolved O3 is of minor importance, because L-H kinetic plots would not arise if the bulk reaction dominates. If BaP is ultimately consumed by reactive oxygen intermediates, e.g., O atoms, formed from ozone on the surface, then it is possible that their bulk reaction might contribute to the total loss of BaP at a non-negligible rate. However, the extent to which such bulk chemistry is occurring with such intermediates cannot be quantified. Also, we note that because the AS particles contain some degree of organic impurity, it is possible that the change in the kinetics with application of an organic coating is not pronounced because the BaP was already adsorbed in the AS particles to the organic impurity. However, if this is the case, the organic impurity is likely to be hydrophilic and soluble, because it is present in the atomizer. Thus, it would be expected that this gives rise to very different kinetics than would prevail with hydrophobic oils such as PSO and BES.

mineral dust,32 and 1-hexadecene.33 Note that when we examined the BaP decays as a function of particle size on the liquid coated particles, we saw the same kinetics as for the overall loss rate. For that reason we focus on only the total BaP loss kinetics. Two parameters, i.e., kImax and KO3, can be obtained by fitting the data for the uncoated and liquid-organic-coated BaP-AS particles in Figure 5 using the following L-H equation: I kobs =

I k max K O3[O3]

1 + K O3[O3]

(1)

where is the maximum first-order rate coefficient for BaP loss, KO3 is the ozone gas-to-surface partition coefficient, and [O3] is the gas-phase ozone concentration. The fitting parameters from the present work are listed in Table 2, where they are compared with recent literature results on the heterogeneous reactions of ozone with BaP or anthracene on different aerosol substrates. It has been noted previously that the values of kImax exhibit surprisingly little variability across a very wide range of ozone heterogeneous systems, with values ranging between roughly 10−3 and 10−2 s−1 in most cases. This is suggestive of a common rate-determining step in these processes, likely involving the conversion of ozone to a reactive oxygen intermediate.19 Likewise, for the liquid organic-coated particles that display L-H behavior in this study, the values of kImax are between 0.034 and 0.057 s−1, in excellent agreement with the value of 0.048 s−1 previously measured by our group for the loss kImax

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These conclusions, however, are subject to assumptions of particle morphology and coating uniformity that cannot be uniquely tested using kinetics data alone. That being said, the results provide an appropriate starting point for subsequent studies, some of which are currently underway in our laboratory. In particular, with the recent interest in the phase of atmospheric SOA, i.e., whether it is highly viscous and more like a glassy solid or more liquid-like instead,40−42 it will be interesting to repeat these experiments where a SOA coating is applied to the particles. Given the dependence of the diffusion coefficients on relative humidity, it is possible that the diffusion model presented above could be tested by examining how the loss rate of PAH is affected by relative humidity. The effects of relative humidity on the multiphase oxidation kinetics of protein substrates have been observed previously.39 From an atmospheric perspective, observations from ambient air measurements, supported by a few laboratory and modeling studies, suggest that the reactivity of PAHs may be reduced when they are adsorbed/absorbed to aerosol.43−46 This stands in contrast to the bulk of laboratory studies, described in the Introduction, that have shown particle-associated PAHs to react extremely rapidly with atmospheric oxidants, e.g., the BaP heterogeneous lifetime with ozone is on the order of minutes to hours.6 Indeed, using a global aerosol chemistry-transportmodel, Lammel et al.44,45 suggest that PAHs absorbed in the organic matrix of particulate matter are shielded from the gasphase oxidants, which makes their long-range transport possible. For example, it is well documented that PAHs, such as BaP, can make their way to the Arctic, far removed from combustion sources.3−5 The present work provides direct evidence from a laboratory study that the burial of PAHs by solid organic substances is a viable mechanism to suppress the heterogeneous/multiphase reaction during the days to weeks required for long-range atmospheric transport. Subsequent laboratory studies with coating organics more closely resembling SOA material will make this conclusion stronger.

The similarity of the results could, therefore, arise because either the liquid organics are not coating the ammonium sulfate in a uniform manner, leaving the BaP as exposed as it was on the uncoated particles, or the liquid organics are uniformly coating the particles and BaP is freely available to react with ozone at the surface of the organics. Using a similar coating approach and a secondary electron yield measurement technique, Ziemann and McMurry30 reported uniformly coated dioctyl sebacate on salt particles. The above-mentioned first case (i.e., the liquid organics are not coating the ammonium sulfate in a uniform manner, leaving the BaP as exposed as it was on the uncoated particles) is therefore unlikely to prevail because the liquid organics and BaP are coated simultaneously and solubility of the BaP in both BES and PSO is high. We note that the diffusion times for BaP or O3 through a liquid organic coat are extremely rapid. Typical diffusion coefficients for BaP/O3 in liquid organics are 10−5 to 10−6 cm2 s−1,34−37 corresponding to short diffusion time scales on the order of 10−9 and 10−7 s for thin (4−8 nm) and thick BES (10−80 nm) coats, respectively, using the following expression:38

x2 =

4Dt π

(2)

where x is the coating thickness, D is the BaP/O3 diffusion coefficient, and t is the time scale of the interaction between O3 and BaP. With such a short diffusion time scale, the liquid coat may not affect the kinetics given that the interaction time between O3 and BaP in the flow tube is in the range of tens seconds to more than 1 min. In other words, the reaction with the liquid-coated particles is not expected to be diffusioncontrolled because the BaP is readily able to migrate to the surface to replenish any reactant lost through heterogeneous reaction with ozone. On the other hand, as can been seen in Table 1 and Figure 5, with a thin EC coating of only a few nanometers, the kinetics are noticeably suppressed below the values prevailing for uncoated particles. When that coating is made thicker, to tens of nanometers, the reaction is shut off entirely over our experimental time scales. Given the very much slower diffusion constants for BaP/O3 in a solid organic, it is likely that the reaction with EC-coated particles is rate limited by solid-phase diffusion. Given that some reactivity is observed with coats of 4−8 nm thickness, we can estimate that the diffusion constant of BaP/O3 through solid EC, is on the order of 10−15 cm2 s−1 using eq 2. This is consistent with typical values for diffusion coefficients (10−15−10−20 cm2 s−1) that have been reported for chemical diffusion in bulk solids.39 With such diffusion coefficients, reaction time scales would be on the order of a few minutes to a year. Conclusions and Atmospheric Implications. This paper presents the first kinetics results for the multiphase ozone oxidation of a PAH when present in a mixed-inorganic/organic aerosol particle. Whereas the heterogeneous oxidation of PAHs has been studied previously on pure organic and inorganic particles, this study attempted to systematically determine whether the addition of organic material to a largely inorganic aerosol particle core leads to changes in PAH lifetime. The results are consistent with a simple reactivity model whereby diffusion of the PAH through liquid organic coatings is so fast that the kinetics are not significantly affected, whereas solid organic coatings can entirely shut down the multiphase oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

Estimation of the thickness of the coating materials. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 416 946 7359. Fax: +1 416 946 7359. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by NSERC (Canada) is gratefully acknowledged. S.Z. thanks Jay Slowik and Rachel Chang for their assistance in AMS setting-up and operation.

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REFERENCES

(1) Boffetta, P.; Jourenkova, N.; Gustavsson, P. Cancer Causes & Control 1997, 8, 444−472. (2) Finlayson-Pitts, B. J.; Pitts, J. N. Chemistry of the Upper and Lower Atmosphere; Academic Press: San Diego, CA, 2000. (3) MacDonald, R. W.; Barrie, L. A.; Bidleman, T. F.; Diamond, M. L.; Gregor, D. J.; Semkin, R. G.; Strachan, W. M. J.; Li, Y. F.; Wania, F.; Alaee, M.; et al. Sci. Total Environ. 2000, 254, 93−234.

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dx.doi.org/10.1021/jp3030705 | J. Phys. Chem. A 2012, 116, 7050−7056