Photochemical Aging of α-Pinene Secondary Organic Aerosol: Effects

Mar 22, 2012 - The photolysis rates are rapid, a few percent of the NO2 photolysis frequency, and can thus be highly competitive with other aging mech...
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Photochemical Aging of α-Pinene Secondary Organic Aerosol: Effects of OH Radical Sources and Photolysis Kaytlin M. Henry and Neil M. Donahue* Center for Atmospheric Particle Studies, Department of Chemical Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, Pennsylvania, 15213 United States ABSTRACT: This study addresses photochemical aging of secondary organic aerosol (SOA) produced from α-pinene ozonolysis. The SOA is aged via hydroxyl radical (OH) reactions with first-generation vapors and UV photolysis. OH radicals are created through tetramethylethylene ozonolysis, HOOH photolysis, or HONO photolysis, sources that vary in OH concentration and the presence or absence of UV illumination. Aging strongly influences observed SOA mass concentrations, but the behavior is complex. In the dark or with high concentrations of OH, vapors are functionalized, lowering their volatility, resulting in an increase in OA by a factor of 2−3. However, with lower concentrations of OH under UV illumination SOA mass concentrations decrease over time. We attribute this decrease to evaporation driven by photolysis of the highly functionalized second-generation products. The photolysis rates are rapid, a few percent of the NO2 photolysis frequency, and can thus be highly competitive with other aging mechanisms in the atmosphere.

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INTRODUCTION Fine particulate matter (PM) affects visibility, human heath, and climate.1,2 The organic fraction, which can make up 20− 90% of total PM, is still poorly understood as it consists of thousands of compounds from both biogenic and anthropogenic sources.3 Much organic aerosol (OA) formation comes from oxidation of volatile organic compounds (VOCs), which condense to form secondary organic aerosol (SOA). Field observations over the past decade have revealed that ambient OA is highly oxidized,4 with oxidized OA (OOA) dominating in remote areas and making up more than half of OA even in most urban areas. In many cases OOA has a molar oxygen to carbon ratio (O:C) approaching 1:1,5,6 and an average carbon oxidation state of nearly +1.7 Consequently, the role of progressive oxidation, or aging, in OA chemistry is receiving increased attention.7 Historically, aging reactions involving heterogeneous uptake of oxidants, including hydroxyl radicals (OH), to aerosol surfaces have been studied extensively;8−10 however, multigenerational oxidation of vapors by OH has received less attention. Homogeneous gas-phase OH reactions are generally faster than diffusion-limited heterogeneous reactions.11 Furthermore, the initial SOA mass yields for most volatile precursors are typically 0.1−0.2, meaning 10% of the carbon is in the condensed (aerosol) phase and 90% is left in the gas phase.12 A substantial fraction of the OA mass frequently consists of semivolatile compounds, and under typical atmospheric conditions a large fraction of those compounds will be in the gas phase.13 It is virtually certain that those organic vapors will react rapidly in the gas phase.14 The latergeneration oxidation products of those reactions can potentially © 2012 American Chemical Society

form more SOA; the most direct case involves precursors with multiple double bonds such as some terpenes, where sustained oxidation of the less reactive double bond can substantially increase SOA mass yields well after the parent terpene is fully consumed.15,16 However, most first-generation products lack double bonds, so they will be oxidized by OH. That is our focus. Aging by OH can increase SOA mass by converting lowvolatility organic vapors into SOA.17 Most transport models implement SOA formation mechanisms based on parametrizations of smog-chamber data, and most smog-chamber experiments last for only a few hours.12 However, submicrometer particles remain in the atmosphere for on average one week (the typical rain frequency). This gap between the time scales of chamber experiments and atmospheric residence times could result in parametrizations that do not fully represent organic behavior in the atmosphere. Parameterizations that can describe aging have been developed to model semivolatile primary organic emissions,17 and indeed models simulating aging chemistry show improved performance in areas such as Mexico City.18,19 However, models implementing this same aging chemistry in areas dominated by biogenic VOCs, such as the southeastern United States, show a dramatic overprediction of SOA.20 Consequently, the role of aging chemistry for biogenic SOA is in doubt. Special Issue: A: A. R. Ravishankara Festschrift Received: October 26, 2011 Revised: February 24, 2012 Published: March 22, 2012 5932

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This work focuses on aging of SOA formed from α-pinene, the most abundant monoterpene, which is also a model for many biogenic compounds containing an endocyclic double bond.21 As with most SOA experiments, much work has focused on the initial formation of SOA from α-pinene, studying the reaction mechanism, identifying the reaction products, and characterizing the SOA mass yields.22−24 Our objective is to examine the photochemical aging of α-pinene SOA. Photochemical aging includes OH-radical oxidation as well as photolysis of the first-generation oxidation products, some of which constitute “fresh” SOA. We shall explore the effect of several different OH-radical sources as well as the effect of UV-A light on the aging photochemistry. There are already indications that photolysis may serve as a sink for SOA, and this may explain the discrepancies between models and measurements described above: UV (“black”) lights suppress SOA formation from α-pinene + ozone25 and even cause substantial evaporation of SOA formed from isoprene photooxidation.26 If photochemical aging involves both production of new SOA mass via oxidative functionalization and loss of SOA mass via photolysis, the OA lifecycle in the atmosphere may be significantly richer than anticipated by current model descriptions.

(MUCHACHAS), which followed a simple design principle: SOA was generated by complete consumption of a terpene driven by ozonolysis (including associated OH reactions) and allowed to stabilize, and then an OH radical source was turned on to isolate the effects of OH aging. The full MUCHACHAS results are reported in a succession of publications. The combined experiments demonstrated clearly that OH aging alone causes significant SOA mass increases, doubling or tripling the SOA after the equivalent of 1 day of oxidation under typical atmospheric conditions.29 Gas-phase oxidation of cis-pinonic acid by OH generated 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA),30 which has been proposed as a marker for aged biogenic SOA.31 In addition, aging markedly influences SOA hygroscopicity32,33 and volatility.32,34 In addition to MUCHACHAS, we are aware of at least one other study addressing aging of α-pinene SOA using a similar experimental design. Qi et al. reported modest SOA mass increases after adding NO to a mixture of α-pinene SOA and ozone and subjecting the mixture to UV illumination.35 However, the actual OH exposure induced in that experiment is unclear. To illustrate the role of OH oxidation we consider the lowNOx (NO+NO2) pathway of peroxy radical (RO2) chemistry.12 As shown in reaction R1, VOC oxidation via OH can lead to the formation of a hydroperoxide under low-NOx conditions.

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BACKGROUND In addition to its intrinsic importance, α-pinene SOA is an excellent system for isolating the role of biogenic SOA aging. Ozone will only react with the double bond in α-pinene. Once that double bond is oxidized and all α-pinene is consumed, oxidative chemistry should cease and phase partitioning should reach equilibrium. This is what we observe. This permits a clear separation between SOA formation via ozonolysis and its aging by OH. Ozonolysis of alkenes does produce OH radicals; for αpinene ozonolysis the yield of OH can be 0.8−1.27,28 This OH will react with α-pinene through the majority of the initial SOA formation chemistry because the OH reaction with α-pinene is faster than the reaction of OH with first-generation gas-phase products. Because these experiments are conducted with a large excess of ozone, the initial SOA formation chemistry is driven by first-order loss of α-pinene. After several e-folding lifetimes, nearly all of the α-pinene has reacted and all of the OH formed from the ozonolysis has been consumed immediately. Less than 5% of the OH reacts with first-generation products, the rest with α-pinene, consequently almost half of the α-pinene has been oxidized by OH formed via ozonolysis. However, the “fresh” SOA at the end of the formation step is predominantly composed of first-generation α-pinene oxidation products. From a chemical perspective, “aging” refers to second- and later-generation product molecules. Practically, we do not term OH reactions in general as aging, but instead refer to aging as the subsequent OH chemistry (and UV photolysis) after all αpinene is consumed. This chemistry always occurs after allowing the initial SOA to reach equilibrium in the chamber; while the separation between first-generation and latergeneration products is not perfect, it is very pronounced. The separation between formation and aging chemistry is much more difficult in systems where SOA is both formed and aged by OH, for example from aromatic and saturated precursors. In our case, OH radicals can be separately produced during the aging portion of the experiment. The experiments described here were part of a larger campaign, the multiple chamber aerosol chemical aging study

VOC + OH → R• + O2 → RO2• + HO2• → ROOH (R1)

The hydroperoxide is a functionalized product with sharply reduced volatility. Hydroperoxides are predicted to be a substantial fraction of SOA;36 they have been identified in limonene SOA37 and also as important gas-phase products during isoprene oxidation at low NOx.38 If an organic compound is functionalized with an −OOH group, the resulting peroxide will be roughly a factor of 300 less volatile than the parent compound;39 consequently this sort of functionalization reaction could cause substantial increases in SOA mass during aging. Organic peroxides have been identified in α-pinene SOA when an OH scavenger was used to isolate the ozonolysis system.40 The technique used by Docherty et al. to measure these peroxides requires large sample sizes, therefore some of the peroxides detected in that study will likely be in the gas phase under atmospheric concentrations. These peroxides will be susceptible to further oxidation by OH in our study, and as the reactions continue to add functionality we expect the aging products to condense to form more SOA. However, the aging chemistry is complex, and we expect later-generation aging products to consist of multiple functional groups and likely multiple conjugations.12 In addition, under high-NOx conditions organic nitrates will form. We use the hydroperoxides as an example of an aging product as they have been identified in many biogenic systems.26,37 In general, when the carbon number is conserved, we refer to this rich chemistry as “functionalization”. If functionalization alone characterized aging, it would cause a progressive increase in SOA mass, as functionalized products are nearly always less volatile than their precursors. However, thermodynamics demand that oxidation eventually cleave C−C bonds to produce smaller carbon-number products, and ultimately CO2.7 These fragmentation reactions will eventually win out over functionalization, and there is considerable evidence that fragmentation becomes more important as organic species become more oxidized.41,42 While OH attack on oxygenated organics can cause them to fragment, oxy5933

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Table 1. Experimental Conditions formation

aging

aging via

[α-pinene] (ppb)

[O3] (ppb)

[OH]t=0a* (*106 cm−3)

[OH]t=1hb* (*106 cm−3)

[O3]t=0 (ppb)

[NOx] (ppb)

base case (no aging) HOOH photolysis HONO photolysis TME ozonolysis

15 38 25 26

880 620 700 400

0 1 11 2

0 1 2 2

840 575 0 2000

400 ppb) monitored by Dasibi 1008-PC. In a typical experiment the seed particles formed a polydisperse mode centered at 200 nm, and subsequent condensation of organics onto these particles caused the mode to shift approximately 20 nm to larger sizes. In many SOA formation experiments a scavenger is used to remove the OH radicals created during α-pinene ozonolysis in order to isolate the ozonolysis system. However, an OH scavenger would interfere with the OH aging and therefore the majority of the experiments presented here were performed without an OH scavenger. The exception was experiments employing HOOH as an OH source, where the HOOH also acted as an OH scavenger before UV illumination.44 5934

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noise of the Q-AMS data (∼1−2% of total organic signal) does not allow us to observe small changes in composition. While most of the results presented here rely on the ratios just discussed, accurate organic-aerosol aging mechanisms require a mass balance that accounts for both particle deposition to walls and subsequent condensation of organic vapors to those deposited particles.45,50 To account for particle losses to the walls, we determined a first-order mass loss rate constant for each experiment, based on observed exponential decay during periods when chemistry was quiescent and particle modal diameters were constant. In the instance where there is a large nucleation mode of organic particles, it is necessary to correct both the organic and sulfate traces before calculating the O:S. An exponential decay was fit, individually, to both these traces during the period after SOA formation was complete. However, when the majority of organics condense directly onto the seed particles, there is no need for this correction to examine O:S. Ultimately as we examine the quantitative organic mass changes in these aging experiments, wall loss corrections must be applied to the suspended organic mass observed with the QAMS. The above organic decay rate was also used to perform organic wall-loss corrections for scenarios in which only the suspended particles grow (ω = 0) and where vapors may condense onto all particles (suspended and deposited, ω = 1). In the ω=1 case, we cannot use the correction defined by Hildebrandt et al.45 in the event that organics and sulfate do not have the same wall-loss rate (due to a nucleation mode of organic particles). Instead, as vapors may condense onto all particles (suspended and deposited), the exponential decay of the suspended particles is taken in its mathematical representation as the percent of the total organic mass (suspended plus wall-deposited) that remains suspended. The difference in these methods is ∼10% when the mass of nucleated particles is small. Finally, AMS data was compared to SMPS data to check for consistency in the organic traces and the OA yield agreement from these instruments is within 50%.

experiment. Dry ammonium sulfate particles may bounce off the vaporizer, but as they become coated with organics these particles will begin to stick, being collected more efficiently; however, if all particles are a mixture of organics and sulfate, the CE will not influence the measured O:S. Wall losses are a substantial challenge in long time scale experiments such as these, but we can still use O:S as an unambiguous measure of whether a given process (i.e., aging) causes a net condensation or evaporation of organics, as the sulfate seeds are effectively nonvolatile. The aerodynamic size distributions of the sulfate and organic mass at the end of a typical SOA formation stage are shown in Figure 1. This is what we inevitably observe in OA coating

Figure 1. Particle time-of-flight aerodynamic size distribution after completed formation of first-generation SOA from α-pinene + ozone, as measured with an aerosol mass spectrometer. All of the sulfate seed particles are coated with organics as shown by the green organic trace completely overlapping the red sulfate trace. However, the organic trace is shifted to smaller particle sizes compared to the sulfate seeds and in addition has a second smaller mode. This smaller mode is due to new-particle nucleation during α-pinene ozonolysis while the offset in the larger modes exists because the SOA condenses onto the seed condensational surface area and does not mix with the seeds.

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RESULTS AND DISCUSSION Figure 2 shows the organic to sulfate (O:S) ratio for all aging scenarios, normalized to the pre-aging value to facilitate comparison of the different cases. The sulfate in the chamber is nonvolatile and therefore changes in O:S indicate an increase or decrease in organic aerosol. In the base case, after initial formation chemistry is complete, there are no OH radicals to oxidize the first-generation gas-phase products and therefore there should be no change in the organic mass. This is confirmed by the constant O:S seen in this case. This stability of SOA after complete α-pinene consumption strongly indicates that the SOA has reached a phase-partitioning equilibrium. Because the SOA growth is driven by a supersaturation of vapors,51 any residual disequilibrium would drive continued growth. There was no sign of such growth over several hours, even though previous experiments in the same chamber have shown that α-pinene SOA particles do shrink substantially after isothermal dilution drives the equilibrium toward evaporation.52 We use the base case in a dark chamber to emphasize both that equilibrium is reached and that there is a clear distinction between formation chemistry and secondary aging chemistry. Cases in which UV lights were turned on without an OH source are not shown in Figure 2. Changes in SOA mass for these cases were small and variable, depending on initial SOA loading (initial SOA loading affects the

experiments; the vast majority of the organics lie in the same size range as the sulfate, but the organics are shifted toward smaller sizes than the sulfate. This shift is characteristic of coating because the organic condensational flux depends on the surface area of the seed particles (modified for diffusion for larger particles when the Knudsen number, Kn ≲ 1). The second, smaller mode of organics visible in Figure 1 indicates a small amount of new-particle formation (50% increase in organic mass. Finally, as the particles start to evaporate a combination of this effect and coagulation keeps the median diameter constant. (c) OH aging via TME ozonolysis under dark conditions, followed by illumination with UV light. After initial SOA formation, aging is initiated in a dark chamber by generating OH radicals via TME ozonolysis (light gray box). After two hours of dark aging, the UV lights are turned on (white box) while TME ozonolysis continues to produce ∼106 OH cm−3. For this experiment two limiting wall-loss corrections are shown: ω = 1 where vapors condense to both suspended and wall-deposited particles and ω = 0 where vapors condense only to suspended particles (dashed green line). Using either correction, an increase in aerosol mass is seen during the dark aging period. The divergence between the two wallloss corrections emphasizes the importance of this correction for long time experiments as either a doubling or tripling of mass is seen in just two hours of aging. However, with the use of UV lights that organic growth stops. The halt in organic growth suggests that fragmentation reactions are occurring under the UV light. In addition, f43 (gray, right axis) increases during dark aging with the addition of organic mass. When the evaporation takes over under UV illumination, the f43 stays constant, which is similar to the composition results seen in the other evaporation cases.

evaporation and coagulation. During the course of the experiment, f44 stays constant but f43 decreases during both SOA formation and aging. Finally, the third aging case was conducted in a dark chamber using TME ozonolysis to produce OH radicals. As mentioned above, the O:S shows substantial organic growth during aging. This is confirmed by the organic mass concentration data in Figure 3c. In the dark chamber, the OH aging caused the organic mass to double or triple (depending on the wall-loss correction) over the course of 2 h. Most of this increase was in the form of relatively volatile secondary products, as shown by a coincident increase in f43. This fragment is often associated with fresh, semivolatile OA,6 and this increase is consistent with volatility data during other MUCHACHAS experiments showing that the average volatility of the SOA increased during aging.32,34 The f44 stayed essentially constant throughout aging. Figure 3c also shows the effect of photolysis. To isolate this effect, the UV lights were turned on after 2 h of aging while OH production by TME ozonolysis continued. The lights halted or even reversed the SOA growth, depending on the wall-loss correction. The increase in f43 halted at the same time. Again, with the relatively modest OH production by TME ozonolysis, photolysis evidently roughly counterbalanced the formation of additional SOA mass via OH oxidation chemistry; under these conditions the later-generation products appear to have achieved a rough steady state, and thus no net SOA production or loss was observed. When comparing these three aging scenarios, it is important to assess the initial conditions prior to aging. A distribution of both gas- and particle-phase first-generation products is formed during the initial α-pinene ozonolysis, and differences in this initial distribution for different experiments may have affects on aging chemistry that are not due to the variation in OH sources. In the HOOH aging case, due to evaporation it was necessary to begin with a higher SOA concentration to maintain sufficient signal-to-noise as the aging progressed. Therefore, as shown in Table 1, the initial α-pinene concentration was higher in the

Figure 3. OH aging of organic aerosol (OA) via three OH sources. In all panels the organic mass (solid green, left axis) has been wall-loss corrected assuming that the organic vapors may condense onto both suspended and wall-deposited particles (ω = 1). In each experiment, initial OA formation (dark gray box) coincides with an increase in f43. (a) OH Aging via HOOH photolysis. During the organic formation here HOOH is present and acts as an OH scavenger.44 At the onset of aging, initiated by turning the UV lights on, the organic mass immediately decreases. With OH concentrations on the order of 106 molecules cm−3, this time axis can be viewed as real-time atmospheric aging. The aging chemistry causes OA evaporation as gas-phase reactions shift products to higher volatility, forcing some aerosol to evaporate to maintain equilibrium. During this interval the oxygen content of the OA remains constant, as demonstrated by the unchanged m/z 44:43 (pink, right axis). (b) OH aging via HONO photolysis. Here when the UV lights are turned on, HONO photolysis produces ∼107 OH cm−3, resulting in an increase in organic mass. However, most of the HONO is consumed within this first hour, and subsequently OH is near 106 cm−3. The condensation of oxidized vapors is able to overcome evaporation during fast OH chemistry, however when the conditions become similar to those in the HOOH 5937

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is spread fairly evenly from 330 to 400 nm, while in the chamber it is more narrowly confined to a 20 nm peak near 350 nm. Consequently, the peak black-light intensity is greater than typical ambient conditions but not dramatically so. When the OH levels are ∼106 molecules cm−3, both the HOOH and HONO photolysis experiments shown in Figure 2 exhibit SOA losses of approximately 0.2 h−1. To compare the UV effect in Figure 3c to panels a and b, it is important to look at the change in slope as a decrease in expected production for each of these cases when UV light is introduced. The interrupted growth shown in Figure 3c is at least as significant a loss when UV illumination is added to the TME + ozone OH source. These observations thus support a photolytic loss of SOA in these experiments of approximately 6 × 10−5 s−1 (an average of both particle and gas-phase photolysis). This is a modest value equal to 2% of the NO2 photolysis frequency, but this still implies a photolysis lifetime of one day for later-generation SOA oxidation products, which is much less than the typical 1week residence time of SOA in the atmosphere. Photolysis rates of a few percent of the NO2 photolysis rate are not at all unusual for oxidized organic molecules, and so it is not surprising in retrospect that we observe these effects. The rates we observe are very consistent with the 5−9 h photolysis lifetimes (3−5 × 10−5 s−1) of SOA in aqueous solution reported by Bateman et al.43 In our experiments the particles were dry, so photolysis was occurring in either the vapor phase or the dry condensed phase (or both phases) at a comparable rate to that reported by Bateman et al. Those authors focused on changes to the SOA composition, while we address principally the effect of photolysis (photodegradation in their terms) on the overall aerosol mass. The rates are highly competitive with particle residence times in the atmosphere, which is the most important gauge of importance for a process; because the deposition loss frequency of submicrometer particles is of order 2 × 10−6 s−1 (a 1-week lifetime), photolysis at the rate we observe or even substantially slower will still play a major role in the evolution of organic aerosols in the atmosphere.

HOOH aging case than in the other aging scenarios. In Figure 3, the larger initial α-pinene concentration accounts for the majority of the higher initial SOA concentration in panel a (as compared to panels b and c). In addition, the product distribution following α-pinene ozonolysis with HOOH present as an OH scavenger (in panel a) is slightly different from the product distribution following ozonolysis in the absence of a scavenger, where the OH radicals react with α-pinene. However, ozonolysis and OH oxidation of α-pinene form broadly similar products, and the most significant issue is the volatility range where products will reside substantially in both the particle and gas phases. In all three cases we are concerned with the C* = 100 μg m−3 bin and its adjacent bins, which hold a substantial mass fraction of the first-generation products.13,44,56 The differences we see in the various aging cases can be attributed largely to the aging conditions (e.g., UV light and OH concentration) and not the slight changes in initial product distributions. Wall losses complicate mass balances in experiments with time scales longer than the wall-loss time scale. In the case of simple α-pinene ozonolysis (with fairly high ozone, as in our experiments), the chemistry occurs so rapidly that there are few particles on the wall when SOA formation is complete. This means that there is little concern for whether particles on the chamber walls interact with the gas-phase components and to what extent. However, in aging experiments the time scale is necessarily much longer and wall losses become significant. We correct for wall losses using two limiting cases, shown in Figure 3c. In the first case (the solid line) suspended and walldeposited particles are presumed to grow at the same rate (ω = 1). This is the case shown in all panels of Figure 3. In the second case (the dashed line), once particles deposit to the walls they are assumed to stop growing due to condensation (ω = 0). We regard these as limiting cases, and they diverge as more particles are lost to the walls and the ratio of suspended to wall-deposited particles decreases. The wall-loss correction introduces much less error in the O:S shown in Figure 2 (while these ratios have been corrected for wall losses, it is a secondorder correction). Regardless of the wall-loss correction, it is clear that photochemical aging affects the organic concentration in the chamber; however, the sign of the change is subject to other variables. When aging is confined to OH radicals formed in the dark, there is aerosol growth, consistent with the proposed radical chemistry. It is expected from well-established gas-phase organic oxidation mechanisms that many multifunctional latergeneration products will form.57 Functionalization chemistry likely also occurs when the UV lights are on. In the TME + ozone case presented above, the TME flow continued when the UV lights were illuminated. Consequently, the OH production and subsequent functionalization reactions continued. We must consider, however, the balance between functionalization chemistry and UV intensity. The functionalization reactions dominate even under UV light in the early stages of the HONO aging case, which shows an initial increase in aerosol mass during the period of high OH concentrations. However, the introduction of UV lights brings photolysis into the equation. The UV lights in the CMU smog chamber have a distinct peak at 350 nm.25 This enables the use of HOOH as an OH source because there is sufficient overlap with the HOOH cross section in this spectral region. The NO2 photolysis rate in the chamber of 3 × 10−3 s−1 is comparable to ambient values,45 but under solar illumination the overlap integral for NO2 photolysis

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CONCLUSIONS As we begin to understand that OA evolves after its initial formation, smog-chamber experiments aim to discover aging effects on specific systems, in this case biogenic SOA derived from α-pinene. The vapors in this system do age, something that is not yet well represented in chemical transport models, and the effect can be either aerosol growth or evaporation. The direction of change depends on two variables examined in this study: OH concentration and photolysis. In dark conditions OH oxidation increases the OA mass, consistent with expectations as well as coordinated experiments in other laboratories conducted during the MUCHACHAS campaign. This growth is slowed and even reversed under intense UV illumination. On the basis of this, and also earlier findings for α-pinene SOA,25 limonene SOA,43 and isoprene SOA,26 we conclude that photolysis of the highly functionalized, oxidized organic molecules associated with aged SOA is an important process that must be constrained and incorporated in chemical transport models in addition to the effects of aging by OH oxidation. The photolysis from intense UV light in this work must be scaled down to atmospheric conditions to understand the balance between OH functionalization chemistry and photolysis. With respect to atmospheric conditions, the overall result of aging biogenic SOA is an increase in OA 5938

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mass, as has been demonstrated throughout the MUCHACHAS campaign. However, photolysis is an important process during the lifetime of a particle, and the photolysis results presented here may help add a sink process to the typical functionalization pathway used in aging schemes thus far. A critical question is how general these conclusions are, and how they might extrapolate to other SOA sources. Photolysis requires chromophores, and thus specific organic precursors may favor photolabile products. Indirect evidence suggests that photolysis may be less important in the high-NOx toluene SOA system, where SOA mass yields increase significantly when the UV intensity is increased by a factor of 3 (though in that case the overall rate of photochemical oxidation likely changed as well).45 Interestingly, to our knowledge the sensitivity to UV has so far been reported largely under low-NOx conditions. Even in the HONO case presented above, the first-generation ozonolysis products were produced under low-NOx conditions, so the later-generation products are at most hybrids of low- and high-NOx chemistry. If the low-NOx products or moieties (for example multifunctional peroxides) are especially vulnerable to photolysis, this may explain why SOA appears to be tied to human activity, even though the carbon in OA is often modern, suggesting a biogenic origin.58−60 It could be that additional OA production is not the issue, but rather additional OA loss is important. If SOA formed under low-NOx conditions not only grows due to OH oxidation but shrinks due to photolysis, it is possible that low-NOx aging could have little overall effect on the total OA mass, or even deplete it. If, on the other hand, SOA formed under high-NOx conditions resists photolysis, SOA formed in urban plumes could persist and even grow with aging, whether it was formed from anthropogenic or biogenic precursors. Thus, while chamber experiments focused largely on first-generation SOA have often found higher SOA production under low-NOx conditions,15,16,61 after consideration of the full, multigenerational lifecycle of SOA in the atmosphere, higher NOx may ultimately be associated with higher organic aerosol levels.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (412)-268-4415. Fax: (412)-268-7139. E-mail: nmd@ cmu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the EPA STAR program through the National Center for Environmental Research (NCER). This paper has not been subject to EPA’s required peer and policy review and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred.

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NOTE ADDED AFTER ASAP PUBLICATION This article posted ASAP on March 22, 2012. Figure 3c has been revised. The correct version posted on April 5, 2012.

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