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Dec 23, 2013 - ... oxidant concentrations, and the identity of the phenol. Our results suggest that 3C* can be the dominant aqueous oxidant of phenols...
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Secondary Organic Aerosol Production from Aqueous Reactions of Atmospheric Phenols with an Organic Triplet Excited State Jeremy D. Smith,†,‡ Vicky Sio,† Lu Yu,‡,§ Qi Zhang,‡,§ and Cort Anastasio†,‡,* †

Department of Land, Air and Water Resources, ‡Agricultural and Environmental Chemistry Graduate Group, and §Department of Environmental Toxicology, University of CaliforniaDavis, 1 Shields Avenue, Davis, California 95616, United States S Supporting Information *

ABSTRACT: Condensed-phase chemistry plays a significant role in the formation and evolution of atmospheric organic aerosols. Past studies of the aqueous photoformation of secondary organic aerosol (SOA) have largely focused on hydroxyl radical oxidation, but here we show that triplet excited states of organic compounds (3C*) can also be important aqueous oxidants. We studied the aqueous photoreactions of three phenols (phenol, guaiacol, and syringol) with the aromatic carbonyl 3,4-dimethoxybenzaldehyde (DMB); all of these species are emitted by biomass burning. Under simulated sunlight, DMB forms a triplet excited state that rapidly oxidizes phenols to form low-volatility SOA. Rate constants for these reactions are fast and increase with decreasing pH and increasing methoxy substitution of the phenols. Mass yields of aqueous SOA are near 100% for all three phenols. For typical ambient conditions in areas with biomass combustion, the aqueous oxidation of phenols by 3C* is faster than by hydroxyl radical, although rates depend strongly on pH, oxidant concentrations, and the identity of the phenol. Our results suggest that 3C* can be the dominant aqueous oxidant of phenols in areas impacted by biomass combustion and that this is a significant pathway for forming SOA.

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INTRODUCTION

As for the volatile organic precursors of aqueous SOA, recent studies have mainly focused on small aliphatic compounds such as glyoxal and the oxidation products of isoprene, α-pinene, acetylene, and other volatile organics.4,7,15−18 In contrast, aqueous reactions of aromatic compounds have not been as well studied. Our interest here is on phenols, which are aromatic alcohols emitted from biomass combustion. Biomass burning is a large atmospheric source of phenols and, more broadly, organic carbon, including nonphenolic aromatic carbonyls.19−22 When exposed to sunlight, aromatic carbonyls are excited to their triplet states, which can abstract a hydrogen atom from phenol, resulting in phenoxy radicals that couple to form oligomers.12,23 Similar reactions initiated by hydroxyl radical oxidation of phenols also produce low volatility, oligomeric SOA,24−27 but aqueous SOA formation by the 3 C*-mediated oxidation of phenols has not been examined. Our goal in this work is to investigate the aqueous reactions of a model 3C* precursor (3,4-dimethoxybenzaldehyde) with three model phenolsphenol, guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol)that represent the major classes of phenols emitted from biomass combustion.19,20 We report the kinetics of these reactions under atmospherically relevant conditions and the mass yields of the low-volatility products formed.

While much of atmospheric organic aerosol (OA) is secondary, traditional models of secondary organic aerosol (SOA) often fail to account for observed mass loadings.1,2 In part, this might be because the models only include gas-phase pathways to form low-volatility products, while more recent work has shown that aqueous reactions can also be an important source of SOA.3−8 In the latter pathway, organic gases partition into the aqueous phase and are then oxidized to lower-volatility products that remain in the particle phase even after the condensed-phase water evaporates. The aqueous formation of SOA can occur through both thermal (dark) and photochemical reactions. For photochemical reactions, the hydroxyl radical (•OH) is typically considered the dominant oxidant for aqueous organics and thus the dominant source of SOA from the aqueous phase.9 However, fog and cloud drops (and, by extension, aqueous aerosol particles) contain a number of other oxidants, including singlet molecular oxygen, peroxyl radicals, peroxides, and triplet excited states of organic compounds (3C*).10−13 These species might play a significant role in the aqueous formation of SOA, but they have received little attention. We are particularly interested in the possible roles of 3C*, which are formed by illumination of light-absorbing organic carbon such as aromatic carbonyls.12−14 There is evidence that triplet excited states are important in the formation of HOOH and HONO, and the loss of phenols in atmospheric drops and particles,12,13,15 but their potential role in SOA formation has not been examined. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1049

October 11, 2013 December 18, 2013 December 23, 2013 December 23, 2013 dx.doi.org/10.1021/es4045715 | Environ. Sci. Technol. 2014, 48, 1049−1057

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EXPERIMENTAL SECTION Materials and Solutions. All chemicals were used as received: phenol (PhOH; 99%), 3,4-dimethoxybenzaldehyde (DMB; 99%), syringol (SYR; 99%), and acetonitrile (HPLC grade) from Aldrich; guaiacol (GUA; 98%) from TCI America; and sodium borate (ACS grade) and sulfuric acid (Trace Metal grade) from Fisher. All solutions were made using purified water (Milli-Q) from a Milli-Q Plus system (Millipore; ≥18.2 MΩ cm) with an upstream Barnstead activated carbon cartridge. Solution pH was adjusted using sulfuric acid for pH ≤ 5 and sodium borate for pH > 5. Aluminum cups were made by pressing a 6-cm diameter piece of foil (0.016 mm thickness) into a custom-built HDPE (high density polyethylene) mold. Cups were cleaned by baking at 500 °C for 8−12 h and weighed using a CAHN 29 electrobalance with a precision of ±1 μg. Solutions contained the triplet precursor (DMB) and either PhOH, GUA, or SYR. We used concentrations of phenol (5− 100 μM) and DMB (1−10 μM) that are expected in cloud or fog drops in areas with significant wood combustion.12,19,21 In this work, we use the abbreviation “PhOH” to refer specifically to phenol (i.e., the compound C6H5OH) and the terms “phenol” and “ArOH” to refer generically to any of the three phenolic species. Illumination. Air-saturated samples were illuminated in stirred, airtight, far-UV quartz cells (1-, 2- or 5-cm path length; Spectrocell) at 20 °C using simulated sunlight from a 1000 W Xe arc lamp with downstream optical filters.11 Online, real-time measurements of SOA yields were performed using airsaturated solutions in stirred, 118 mL Pyrex tubes (thus filtering light below 285 nm) illuminated in a New England Ultraviolet Company RPR-200 photoreactor equipped with 300, 350, and 419 nm bulbs (2, 7, and 7 bulbs, respectively). For each experiment, we also ran a dark control sample wrapped in aluminum foil, but otherwise treated identically to the illuminated sample. Chemical Analysis. Periodically during illumination small aliquots of solution were removed from the illuminated and dark cells to measure the concentrations of ArOH and DMB using an HPLC consisting of the following: Shimadzu LC10AT pump, ThermoScientific BetaBasic-18 C18 column, Shimadzu-10AT UV−vis detector (detection wavelengths of 270, 276, 268, and 307 nm for PhOH, GUA, SYR, and DMB, respectively), degassed eluent of 20:80 acetonitrile/water, and a flow rate of 0.70 mL min−1. We measured the photon flux on each experimental day by determining the photolysis rate constant (j2NB,exp) of aqueous 10 μM 2-nitrobenzaldehyde (2NB) in the same cell used for the phenol illumination.28 Solution pH was measured using an Orion model 420A pH meter. Kinetic Analysis. The measured apparent first-order rate constant for phenol loss (k′LIGHT) was determined as the negative of the slope of a plot of ln([ArOH]t/[ArOH]0) versus illumination time, where [ArOH] is the concentration of phenol (at times t and zero). We used an analogous procedure to determine DMB loss. There was no significant loss (p < 0.05) for phenol or DMB in any dark control. Values of k′LIGHT were normalized to sunlight conditions at midday on the winter solstice at Davis, CA11 (j2NB,win = 0.0070 s−1) and were corrected for the small amount of internal light screening due to DMB:

k′ArOH

⎡ ⎤ k′LIGHT ⎥ ⎢ = ×j ⎢⎣ Sλ × j ⎥ 2NB,win 2NB,exp ⎦

(1)

In this equation, k′ArOH is the normalized first-order decay constant for phenol loss and Sλ is the internal light screening factor,29,30 determined for wavelengths at the peak in the light absorption action spectrum for DMB (310 − 335 nm). For our typical DMB concentration of 5 μM, Sλ ranged from 0.85 for a 5-cm cell to 0.97 for a 1-cm cell, showing that light screening was minor. We fit the pH-dependence of the phenol decay rate constant in illuminated DMB solutions using a sigmoidal regression:12 k′ArOH,Calcd = k′HT +

k′T − k′HT 1+

[H +] Ka

(2)

where k′ArOH,Calcd is the calculated value of the normalized firstorder decay constant from the regression, k′HT and k′T are fitted values for the pseudo-first order rate constant for phenol reaction with the protonated (HT) and neutral (T) triplet states of DMB, respectively (e.g., k′HT = kArOH+HT[HT]), and Ka is the fitted acid dissociation constant for the triplet excited state of DMB. Equation 2 is derived from the mole fraction equations (αHT = 1 + [H+]/Ka and αT = 1− αHT) and eq 5.12 The regressions for eq 2 (sigmoidal 3-variable fit), and for eq 6 below (2-variable rational fit), were performed using SigmaPlot Version 11 (Systat Inc.). The initial rate of phenol (RArOH,0) loss under Davis winter sunlight was calculated as follows: RArOH,0 = k′ArOH × [ArOH]0

(3)

Measurement of Secondary Organic Aerosol Yields. SOA mass yields were determined for pH 5 solutions containing 100 μM phenol and 5 μM DMB. Yields are expressed relative to the amount of phenol reacted in the aqueous phase. We used the upper end of expected atmospheric phenol concentrations in order to form enough SOA to measure gravimetrically. SOA yields were measured using two different methods: (1) solution blow-down followed by gravimetric mass measurement, and (2) real-time measurements using an online High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). In the first method, solutions were illuminated with simulated sunlight (with a corresponding dark control) until approximately half of the initial phenol had reacted. The mass of low volatility products were determined by transferring 10 or 12 mL of the light and dark solutions to separate clean, preweighed aluminum cups. The light and dark solutions were then evaporated to dryness using Specialty grade (99.997%) nitrogen gas (Praxair), and the cups were weighed again. The difference in mass before and after evaporation represents the low volatility components in each solution; the difference between the illuminated and dark samples represents the SOA mass. The SOA mass yield, i.e., the mass of SOA formed per mass of phenol reacted during illumination, was calculated using the following: YSOA = (mass of illuminated sample − mass of dark sample) mass of phenol reacted (4)

In the second method to determine YSOA (online HR-ToFAMS), we used an HPLC pump to draw solution (at 1.0 mL 1050

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min−1) from the illuminated Pyrex tubes through a 6-port Teflon valve into the head of a constant output atomizer (TSI, Model 3076). Following atomization by argon, the resulting aerosol was dried by a diffusion dryer and analyzed in real-time by an HR-ToF-AMS. The SOA mass concentrations were determined using the default relative ionization efficiency (RIE = 1.4) for organics. The validity of using this value was supported by the good agreement between the organic carbon concentrations determined by the AMS and by a combustionbased total carbon analyzer. In order to convert the HR-ToFAMS aerosol concentrations (μg m−3) into liquid concentrations (μg L−1), 10.0 μg L−1 ammonium sulfate was added to each solution as an internal standard. We also collected liquid aliquots of the illuminated solution at defined time intervals and analyzed them offline for phenol concentration to calculate the instantaneous mass yield at a given amount of phenol reacted.

concentration is 100 μM. However, because this direct photodegradation is much slower than loss due to the triplet excited state of DMB (SI Figure S1) we did not consider it. Effect of pH and Phenol Concentration on Kinetics. We next focus on the impact of acidity and phenol concentration on phenol loss kinetics in illuminated solutions containing DMB. As shown in Figure 2, the apparent rate

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RESULTS AND DISCUSSION Photochemical Behavior. We start by describing the general photochemical behavior of phenols in the presence of the triplet excited state of 3,4-dimethoxybenzaldehyde (DMB). Figure 1 illustrates the first-order decay kinetics of PhOH in Figure 2. Dependence of PhOH destruction rate constant on pH for illuminated solutions containing 5 μM DMB and either 10 or 100 μM PhOH at 20 °C. Black lines are regression fits to eq 2; fitted parameters are tabulated in SI Table S2. The orange line represents αHT, the fraction of triplet excited DMB that is protonated. There are three control experiments: the gray circle represents a 100 μM PhOH experiment at 5 °C; the gray square represents a 100 μM PhOH experiment at high ionic strength (I = 0.2 M), and the gray triangle represents a 10 μM PhOH experiment with 100 μM NH4Cl added. Error bars represent ±1 SE, propagated from the standard errors of k′LIGHT and j2NB; most error bars are smaller than the symbols.

constant for loss of PhOH, k′ArOH, is strongly dependent upon pH, increasing by a factor of nearly 20 between pH 6 and pH 2 for 10 μM phenol solutions. To examine whether increasing ionic strength contributed to the increase in k′ArOH with decreasing pH in Figure 2, we performed a control experiment where we adjusted a pH 6, 100 μM PhOH solution to the ionic strength of a pH 2 sample using sodium sulfate. As shown by the gray square in Figure 2, this increase in ionic strength does not affect k′ArOH. Since some triplet states can react with chloride,31 we also tested whether the addition of 100 μM NH4Cl slows the kinetics of phenol loss in a 10 μM PhOH solution. As shown by the gray triangle in Figure 2, k′ArOH in the NH4Cl solution is very similar to the predicted value from solutions without chloride, indicating no significant effect. Finally, we also examined whether temperature affects the kinetics of PhOH loss by 3C*: as seen by the gray circle at pH 1.8 in Figure 2, k′ArOH at 5 °C is within the range of values measured at the default temperature of 20 °C, indicating no significant effect of temperature. As described previously,12 the sigmoidal behavior of k′ArOH with pH suggests that the DMB triplet state is protonated to a more reactive form in acidic solutions. Fitting eq 2 to this Figure 2 data gives an average pKa of 3.3 ± 0.2 for the DMB triplet (SI Table S2), which is similar to the previous value (3.6 ± 0.1) determined by measuring HOOH production during 313 nm illumination.12 Previous work has investigated the protonation of triplet excited states and found that generally the pKA of the excited state is 6−8 pH units higher than that of the

Figure 1. Representative plot of the aqueous oxidation of a phenol by the triplet excited state of DMB; results are shown for pH 2 solutions containing 100 μM PhOH and 5 μM DMB. Open symbols show concentrations of PhOH (squares) and DMB (circles) in illuminated solutions, while filled symbols show concentrations for dark controls.

illuminated solution and the lack of reactivity for PhOH (and DMB) in the dark. In approximately 75% of our experiments, there was no loss of DMB in the illuminated solution, but in the other experiments DMB was photodegraded, with a rate constant that was generally less than 20% of the corresponding phenol value. These general observationsa rapid loss of phenol in solutions illuminated with simulated sunlight but no reactivity in the darkare consistent with a past study performed with 313 nm radiation.12 Our finding of occasional loss of DMB is in contrast to this past work, possibly because we use simulated sunlight here rather than 313 nm radiation. As we describe in Section S1 of the Supporting Information, SI, the triplet excited state of DMB is the oxidant for phenols in our experiments, while contributions from singlet molecular oxygen (1O2*) and hydroxyl radical (•OH) are insignificant. We also examined the direct photodegradation of the three phenols: PhOH and GUA have no direct photodegradation under our conditions, whereas SYR does when the initial 1051

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ground state.12 Figure 2 illustrates that the measured apparent first-order rate constant for ArOH loss (k′ArOH) is a molefraction-weighted sum of the rate constants for phenol with the protonated (HT) and neutral (T) forms of the triplet excited state: k′ArOH = αHTk′HT + αTk′T

Figure 3A,B (SYR > GUA > PhOH) is consistent with the fact that adding electron-donating methoxy groups to phenols decreases the electron oxidation potential, thus increasing the reaction rate.32,33 Figure 3 also demonstrates that the apparent rate constant for phenol loss by triplet excited states decreases with increasing phenol concentration, consistent with the behavior of the acidic 10 and 100 μM PhOH solutions in Figure 2. A similar pattern has been reported in a previous surface water study of methoxy phenols and humic acids.33 We hypothesize that the phenol concentration dependence of k′ArOH in Figure 3 is due to a competition between three fates of the triplet excited state: reaction with phenol, quenching by oxygen to form singlet molecular oxygen (1O2*), and unimolecular decay back to the DMB ground state (Figure 4 and SI Section S3). In

(5)

where αHT and αT are the mole fractions of the two forms of the triplet excited state, and k′HT and k′T are the pseudo firstorder rate constants for ArOH decay. To more simply determine k′HT and k′T, we performed subsequent experiments at pH 2 and 5, since under these conditions the protonated and neutral states of the triplet, respectively, are dominant: αHT is 95% at pH 2 and αT is 98% at pH 5. Since k′HT is much larger than k′T, at pH 2 k′HT ≈ k′ArOH, while at pH 5 we subtract the small contribution of the protonated triplet from the measured value of k′ArOH to determine k′T (SI Section S2). To more fully examine the concentration dependence on the initial phenol we measured kinetics for the three phenols at pH 2 and 5 over a range of phenol concentrations of 1 to 100 μM, approximately the range expected in areas with significant wood combustion.12 As shown in Figure 3, the protonated triplet excited state of DMB is also more reactive than the neutral triplet for guaiacol and syringol: values of k′HT (Figure 3A) are larger than values of k′T (Figure 3B) for all three phenols, although PhOH is most affected. The reactivity order shown in

Figure 4. Scheme for the formation and reactions of a triplet excited state. The ground state of a generic chromophore (C) absorbs light (with rate constant jhv,abs) to form an excited singlet state (1C*), which can either return to the ground state or undergo intersystem crossing (ISC) to the triplet state (3C*). The triplet state pool contains both protonated (HT) and neutral (T) molecules. The triplet has three sinks: reaction with O2, unimolecular relaxation to the ground state, and reaction with a phenol (or other reactant), which can either lead to reactive loss of the phenol (kAROH+3C*) or nondestructive physical quenching (kQ).

natural water systems, the O2 pathway is the dominant sink for 3 C* at low concentrations of phenol.23,34−36 However, as phenol concentrations approach 100 μM, phenol becomes an important sink, thus suppressing the triplet steady-state concentration, [3C*], which decreases k′ArOH since k′ArOH = kArOH+3C*[3C*] (where kArOH+3C* is the second-order rate constant for reaction of 3C* with phenol). This effect is illustrated for all three phenols, at both pH values, in Figure 3. Viewed in terms of the rate of phenol loss, RArOH, the rate approaches a plateau at higher phenol concentrations as phenol becomes a major sink (SI Figure S2). At a concentration of 100 μM, reaction with phenols accounts for approximately 40−60% of the loss of the DMB triplet, with the exception of PhOH at pH 5, which is an insignificant reactive sink for the DMB triplet (SI Figure S5), although it can physically quench the triplet (SI Table S3). Using the model of 3C* formation and loss shown in Figure 4, we can derive an expression for the observed concentration dependence of k′ArOH (SI Section S3; eq S10):

Figure 3. Dependence of the phenol destruction rate constant for the protonated and unprotonated triplet excited states (k′HT and k′T, respectively) on initial phenol concentration at pH 2 (panel A) and pH 5 (panel B) in illuminated solutions containing 5 μM DMB. The pH 5 PhOH rate constants in Panel B are multiplied by a factor of 10 for clarity. Lines are nonlinear regression fits to eq 8; the resulting parameters are listed in Table 1. Error bars represent ±1 SE, propagated from the standard errors of k′LIGHT and j2NB; most error bars are smaller than their corresponding symbols. 1052

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Table 1. Rate Constants for Phenol Destruction by the Protonated (HT) and Neutral (T) Triplet Excited States of DMB k′HTa −1

PhOH GUA SYR

kArOH+3C*b(109 M−1 s−1)

k′Ta −1

−1

−1

a (min )

b (min μM )

a (min )

b (min μM )

kArOH+HT

kArOH+T

123 (±14) 79 (±5) 37 (±2)

0.8 (±0.3) 0.6 (±0.2) 0.6 (±0.1)

3190 (±420) 99 (±9) 59 (±20)

30 (±10) 0.6 (±0.2) 0.5 (±0.2)

3.4 (±1.2) 5.3 (±1.9) 11 (±3)

0.13 (±0.09) 4.2 (±3.0) 5.8 (±4.1)

Parameters for regression fits determined using Figure 3 data in a regression of eq 8. bSecond-order rate constants for reaction of the DMB triplet state with phenols (±1 SE), determined by using the a term from the eq 8 regression, the pKa in SI Table S2, and eq 6.

a

interaction as nonreactive physical quenching, it has been suggested that the interaction initially forms a phenoxy radical, which is then reduced by superoxide to regenerate the parent PhOH, leading to no apparent reaction.23 In contrast to the dominant role of quenching for PhOH at pH 5, for all of the other conditions in our experiments the ArOH-DMB interaction is dominated by chemical reaction, with little evidence of quenching (SI Table S3). On the basis of our mechanistic understanding of the competition between dissolved oxygen and phenols for the triplet excited states of DMB (eq 6) we can predict the firstorder rate constant for phenol loss (k′ArOH) in our illuminated solutions for any pH or phenol concentration As shown in SI Figure S7, the loss of all three of the phenols depends on phenol concentration and pH, with PhOH being especially sensitive to pH. Thus the environmental lifetimes of aqueous phenols due to reactions with 3C* depend both upon the concentration of triplet precursors as well as the concentrations of triplet sinks (e.g., phenols) and acidity. Extending this framework to quantify rates of phenol loss by triplet excited states in atmospheric waters requires information about the rate of triplet formation and the pseudo-first order rate constant for loss of triplets by phenols and other reactants. Unfortunately, we cannot currently constrain either of these parameters because of a lack of data. Mass Yields of Secondary Organic Aerosol. Recent work has shown that the oxidation of phenols by aqueous •OH produces significant amounts of low-volatility products.27 To understand whether oxidation by aqueous triplet excited states also efficiently produces SOA, we measured SOA mass yields in illuminated pH 5 solutions containing 5 μM DMB and 100 μM phenol. Our first method of determining SOA yields uses gravimetric measurements and so we must first understand our “background” mass. To do this, we measured the mass of lowvolatility material remaining in a series of dark control solutions blown down with N2 in the same manner as our SOA samples. As shown in SI Figure S8, mass concentrations average 1.4 ± 0.1 μg mL−1 for solutions that contained H2SO4, 5 μM DMB, and 100 μM ArOH. In contrast, in our illuminated solutions after approximately half of the initial ArOH has reacted, the average blown-down masses are 7.0, 6.9, and 12.8 μg mL−1 for PhOH, GUA, and SYR, respectively (SI Figure S9). Using these illuminated solution masses, with a correction for the corresponding dark masses, we calculate with eq 4 that SOA mass yields in our solutions are near 100% for all three phenols, with average (±1 σ) values of (113 ± 14)%, (94 ± 19)%, and (114 ± 19)% for PhOH, GUA, and SYR, respectively (Figure 5). These yields are very similar to values from aqueous PhOH, GUA, and SYR solutions with •OH as the oxidant (Anastasio and Sun; in preparation). SOA mass yields greater than 100% indicate incorporation of other atoms into the products, most likely oxygen, probably via hydrox-

k′ArOH = 1 ⎛ kO2+3C*[O2] + k ′3C* ⎜ + ⎝ jhv,abs φISC[C] × kArOH +3C*

kArOH + 3C * + k Q jhv,abs φISC[C ] × kArOH + 3C *

⎞ [ArOH]⎟ ⎠ (6)

Here, kO2+3C* is the second-order rate constant for reaction of 3 C* with molecular oxygen, k′3C* is the first-order rate constant for unimolecular decay of 3C* to the ground state C, jhv,abs is the rate constant for light absorption by the chromophore C, φISC is the intersystem crossing quantum yield of the excited singlet (1C*) to triplet (3C*), [C] is the chromophore concentration (DMB in our experiments), and kQ is the second-order rate constant for nontransformative interaction between the 3C* and a phenol. The apparent second-order rate constant kArOH+3C* is a mole-fraction-weighted combination of the second-order phenol rate constants with the protonated and neutral triplet states (kArOH+HT and kArOH+T, respectively): kArOH + 3C * = αHTkArOH + HT + αTkArOH + T

(7)

We fit eq 6 to our data using a two-parameter equation: k′ArOH =

1 a + b[ArOH]

(8)

The results of these regressions are listed in Table 1 for all three phenols and are shown graphically as the lines in Figure 3. As described in SI Section S3, from these fits we determined the second-order rate constants for reaction of the DMB triplet excited state (3DMB*) with phenol (i.e., kArOH+3C*) and the second-order rate constant for nonreactive quenching of the triplet state by phenol (kQ). On the basis of our Figure 3 regression fits, values of kArOH+3C* for the triplet state of DMB range from 0.13 × 109 M−1 s−1 for phenol at pH 5 to 11 × 109 M−1 s−1 for syringol at pH 2 (Table 1; SI Section S3). We obtain similar values from regressions of phenol destruction rate versus concentration (Figure S2 and Table S4); however, we recommend the values reported in Table 1. The fast rate constants for phenols with the neutral DMB triplet are similar to values previously reported for three other triplets with a suite of phenols at near neutral pH.23 We find that the rate constants for phenols with the protonated triplet state of DMB are even faster, by factors of 1.3, 1.9, and 31 for GUA, SYR, and PhOH, respectively (Table 1). Overall, the slowest rate constant for chemical reaction is for PhOH with the neutral DMB triplet state. However, under these conditions, PhOH rapidly physically quenches 3DMB*: from SI eq S14 (Section S3), the quenching rate constant (kQ) is 6.4 (±6.3) × 109 M−1s−1 (SI Table S3) and only 2.0 (±1.0) % of the PhOH-triplet interactions result in reaction (i.e., kArOH+3C*/(kQ + kArOH+3C*) = 0.02). This agrees with a previous study where only 1% of PhOH interactions with a triplet resulted in destruction of the parent PhOH.23 While we are describing this PhOH−3C* 1053

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start by comparing the kinetics for phenol oxidation by 3C* and OH in fog and cloud drops impacted by wood combustion. For hydroxyl radical, we use a range of aqueous concentrations ((0.5−10) × 10−15 M) based on measurements in midlatitude cloud and fog waters with estimated contributions from gas-todrop partitioning of •OH.44 We use literature values45 for the second-order rate constants of ArOH with •OH at pH 5. Values at pH 2 are 6.5, 3.5, and 1.6 times lower than the pH 5 values for PhOH, GUA, and SYR, respectively, based on a sigmoidal fit of literature and experimental rate constants (SI Section S5). For the kinetics of 3C* with phenols, we use winter-solsticenormalized rate constants from eq 6 with ArOH concentrations at 10 μM and DMB concentrations of 1, 5, and 10 μM. At pH 5, and with intermediate levels of oxidants, the rates of phenol oxidation by 3C* and •OH are similar (Figure 6A): triplets are least important for PhOH (with a ratio k′ArOH,3C*/ k′ArOH,•OH of 0.1), but for GUA and SYR reactions with 3C* are 2.2 and 2.4 times faster than with •OH (SI Table S7). At pH 2, the triplet state of DMB is much more reactive (Figure 2) and triplets dominate phenol oxidation: the ratio (k′ArOH,3C*/ k′ArOH,•OH) is 15, 6.7, and 6.2 for PhOH, GUA, and SYR, respectively (SI Table S7). These results suggest that in areas of significant biomass combustion, the destruction of atmospheric aqueous phenols by organic triplet excited states is comparable to •OH at high pH (>5) and dominates at lower pH. While values of k′ArOH,•OH depend on the atmospheric concentration of •OH, the ratio k′ArOH,3C*/k′ArOH,•OH is greater than 1 for all three pairs of •OH and 3C* concentrations, with the exception of PhOH (SI Table S7).44 Furthermore, Figure 6A reiterates that syringol is the most reactive of the three phenols, while PhOH is the least reactive. Considering both 3C* and •OH at their intermediate concentrations, the aqueous lifetimes of SYR, GUA, and PhOH are 0.8, 1.0, and 3.4 h, respectively, at pH 5 and 0.6, 1.1, and 1.8 h at pH 2. To better estimate the importance of aqueous 3C* and •OH for SOA formation from the oxidation of phenols, we also examined the product of the normalized first-order rate constant for ArOH loss and the corresponding SOA mass yield for a given oxidant. This product, k′ArOH × YSOA, is essentially a “rate constant” for SOA formation in the aqueous phase. As shown in Figure 6B, the “rate constant” for SOA formation from triplet excited states is similar or larger than that for •OH at all pH values for GUA and SYR, while •OH is the dominant pathway for aqueous SOA from PhOH at pH values above roughly 3. Values for the ratio (k′ArOH, 3C* × YSOA,3C*/k′ArOH,•OH × YSOA,•OH) for PhOH range from 0.1 at pH above 6 to near 11 at pH below 2; SYR and GUA values are less pH dependent, with ratios ranging from approximately 1 above pH 5 to 4−6 above pH 3. We also compare the relative importance of aqueous- and gas-phase formation of SOA from phenols. For these calculations we use typical Davis, CA winter fog conditions (T = 5 °C, liquid water content =1.0 × 10−7 L-aq L-g−1) and literature Henry’s Law constants (1.5 × 104, 5.0 × 103, and 2.5 × 104 M atm−1 for PhOH, GUA and SYR, respectively46) to determine the gas-aqueous partitioning of the phenols. Using these parameters we calculated the ratio of aqueous-phase and gas-phase formation of phenol SOA: faq × ((k′ArOH, 3C* × YSOA,aq) + (k′ArOH, •OH × YSOA,aq))/( fg × (k′ArOH, OH,g × YSOA,g)) where faq and fg are the fractions of each phenol in the aqueous and gas phase, respectively. As shown in Figure 6C, aqueousphase oxidation is more important for the formation of SOA from syringol, while gas-phase oxidation is more important as a •

Figure 5. Average (±1 σ) mass yields of SOA (YSOA) formed from the 3 DMB*-mediated aqueous oxidation of phenols (PhOH), guaiacol (GUA), and syringol (SYR) at pH 5. Dark bars (left bar in each trio) represent YSOA determined gravimetrically from N2 blowdown of solutions illuminated until the half-life of PhOH (n = 3), GUA (n = 5), or SYR (n = 9). The average half-lives were 1308, 153, and 59 min for PhOH, GUA, SYR, respectively. The middle and right bars for each ArOH are YSOA measured in real-time by online HR-ToF-AMS, at onehalf-life (middle bar, light shading) and at the end of each online experiment (right bar, white), at 85, 95, and 100% loss for PhOH, GUA, and SYR, respectively.

ylation of the aromatic rings.27 Figure 5 also shows the realtime SOA mass yields determined by aerosolizing the illuminated solutions, drying the drops, and analyzing with online HR-ToF-AMS. The real-time yield determined after half of PhOH has reacted is similar to the gravimetric determination at the same time point, but the real-time YSOA values for GUA and SYR are, on average, 65 ± 35% lower than those measured gravimetrically. This suggests that the N2 blowdown can cause semivolatile components to react and form low-volatility products, possibly because of the sulfuric acid; evidence for such reactions in solutions with different organics was reportedly recently.37 However, the real-time YSOA values determined at the end of illumination are not statistically different (p > 0.05) from the gravimetric determinations for any of the phenols (Figure 5). Understanding the reasons for these methodological differences in YSOA, and the possible influence of N2 blowdown and illumination time on SOA mass will require further study. Despite this uncertainty, our aqueous yields of phenolic SOA are clearly higher than values reported for phenol oxidation by •OH in the gas phase.38−42 For example, Lauraguais et al.41 found SOA yields of 10−36% for syringol reacting with gaseous hydroxyl radical under high NOx concentrations. Under low NOx conditions, Yee et al.42 measured higher SOA mass yields of 25−44%, 44−50%, and 25−37% from the •OH oxidation of gas-phase phenol, guaiacol and syringol, respectively. The more efficient production of low volatility products from phenol oxidation in solution reflects the efficient formation and coupling of phenoxyl radicals in the aqueous phase.12,23,43 In contrast, gas-phase oxidation of phenols produces a larger amount of volatile products through fragmentation of the aromatic rings,38,42 in addition to functionalization by addition of •OH and NO2 to the phenols.39 Comparison of Aqueous SOA Formation from Phenols via Triplet Excited States and •OH. In this section, we compare the relative importance of 3C* and •OH for making SOA from the aqueous oxidation of phenols. We 1054

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source of SOA from PhOH and (especially) GUA. The significant aqueous contributions for SYR and PhOH show the importance of aqueous kinetics and SOA yields: while only 5 and 3% of these phenols, respectively, are in the aqueous phase under these conditions, this weak partitioning is offset by rapid phenol oxidation (Figure 3) and high SOA mass yields (Figure 5) in the aqueous phase. Atmospheric Implications. Our results suggest that aqueous transformations of phenols by excited triplet states of light-absorbing organic compounds can be a significant source of SOA: triplet excited states can be a rapid sink for aqueous phenols, and they form SOA with very high mass yields. In areas impacted by biomass combustion, the 3C*mediated oxidation of aqueous phenols appears to be generally faster than reaction with •OH, especially under acidic conditions. In addition, while our experiments were conducted under conditions similar to cloud and fog drops, similar chemistry likely occurs in aqueous aerosol particles. However, due to the low fraction of phenols in the aqueous phase, gasphase oxidation of phenols will still be a significant sink for most phenols. While our results reveal a previously unappreciated role for excited triplet states in atmospheric chemistry, more quantitatively assessing this role requires understanding the concentrations and reactivities of excited triplet states in atmospheric drops and particles.

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ASSOCIATED CONTENT

S Supporting Information *

A detailed description of our kinetic model derivation, further kinetic data, and control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 6. Panel A compares the calculated pseudo-first-order rate constants for the aqueous oxidation of phenols by •OH and 3C* at pH 2 (lighter colored, left bar in each pair) and pH 5 (darker bar on right). Boxes show values of k′ArOH at 10 μM phenol based on a likely range of aqueous oxidant concentrations for fog waters in Davis, CA at midday on the winter solstice: •OH concentrations range from 5 × 10−15 (bottom line in •OH boxes) to 7.5 × 10−15 (middle line) to 10 × 10−15 M (top line),44 while for excited triplet states, we use DMB as a proxy, with initial concentrations of 1, 5, and 10 μM for the bottom, middle, and top lines, respectively. Values of k′ArOH for 3C* were calculated using eqs 5 and 7, with kArOH+3C* values from Table 1. Values of k′ArOH for •OH reactions at pH 5 were calculated using the • OH concentrations above and literature rate constants45 for pH 5 (kPhOH+OH = 1.4 × 1010 M−1 s−1, kGUA+OH = 2 × 1010 M−1 s−1, and kSYR+OH = 2.6 × 1010 M−1s−1) and experimentally determined values at pH 2 (SI Section S5). Panel B shows the pH dependence of the ratio of aqueous SOA formation from phenol oxidation by 3C* compared to • OH. The black horizontal line represents a ratio of 1, where the two oxidants are equally important. Conditions: middle values for oxidants from Panel A; 10 μM phenol; SOA yields from blown-down samples (Figure 5 for 3C* reactions and unpublished data for •OH reaction yields (1.06, 1.09, and 1.14 for PhOH, GUA, and SYR, respectively); yields were assumed independent of pH; k′ArOH+•OH values were estimated (SI Section S5). Panel C compares the ratio of SOA production in the aqueous phase by •OH and 3C* (using middle concentrations from Panel A) to that in the gas-phase by •OH (1× 106 mlc cm−3). The y-axis is faq × (k′ArOH, 3C* × YSOA,aq + k′ArOH, •OH × YSOA,aq)/( fgas × (k′ArOH+OH,gas × YSOA,gas)), where faq and fgas are the fractions of each phenol in the aqueous and gas phase, respectively. For this calculation, we have assumed that the actinic flux in an aqueous drop is 1.5 times that of the surrounding gas phase.47 Gasphase rate constants and SOA mass yields are from Yee et al.42

*Phone: 530-754-6095; fax: 530-752-1552; e-mail: canastasio@ ucdavis.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the National Science Foundation (Grant No. AGS-1036675), the University of California Toxic Substances Research and Teaching Program (TSR&TP) through the Atmospheric Aerosols and Health Lead Campus Program, and the California Agricultural Experiment Station (Project CA-D*-LAW-6403-RR).

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REFERENCES

(1) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; et al. The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 2009, 9 (14), 5155−5236. (2) Zhang, Q.; Jimenez, J. L.; Canagaratna, M. R.; Allan, J. D.; Coe, H.; et al. Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes. Geophys. Res. Lett. 2007, 34 (13), L13801. (3) Blando, J. D.; Turpin, B. J. Secondary organic aerosol formation in cloud and fog droplets: A literature evaluation of plausibility. Atmos. Environ. 2000, 34 (10), 1623−1632. (4) Volkamer, R.; Ziemann, P. J.; Molina, M. Secondary organic aerosol formation from acetylene (C2H2): Seed effect on SOA yields due to organic photochemistry in the aerosol aqueous phase. Atmos. Chem. Phys. 2009, 9, 1907−1928. 1055

dx.doi.org/10.1021/es4045715 | Environ. Sci. Technol. 2014, 48, 1049−1057

Environmental Science & Technology

Article

(5) Lim, Y. B.; Tan, Y.; Perri, M. J.; Seitzinger, S. P.; Turpin, B. J. Aqueous chemistry and its role in secondary organic aerosol (SOA) formation. Atmos. Chem. Phys. 2010, 10, 10521−10539. (6) Huang, X. H.; Ip, H. S. S.; Yu, J. Z. Secondary organic aerosol formation from ethylene in the urban atmosphere of Hong Kong: A multiphase chemical modeling study. J. Geophys. Res. 2011, 116, D03206. (7) Lee, A. K. Y.; Herckes, P.; Leaitch, W. R.; Macdonald, A. M.; Abbatt, J. P. D. Aqueous OH oxidation of ambient organic aerosol and cloud water organics: Formation of highly oxidized products. Geophys. Res. Lett. 2011, 38, L11805. (8) Ervens, B.; Turpin, B. J.; Weber, R. J. Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): A review of laboratory, field and model studies. Atmos. Chem. Phys. 2011, 11 (21), 11069−11102. (9) Herrmann, H.; Hoffmann, D.; Schaefer, T.; Brauer, P.; Tilgner, A. Tropospheric aqueous-phase free-radical chemistry: Radical sources, spectra, reaction kinetics and prediction tools. ChemPhysChem 2010, 11, 3796−3822. (10) Faust, B. C.; Allen, J. M. Sunlight-initiated partial inhibition of the dissolved iron(III)-catalyzed oxidation of S(IV) species by molecular oxygen in aqueous solution. Atmos. Environ. 1994, 28, 745−748. (11) Anastasio, C.; McGregor, K. G. Chemistry of fog waters in California’s Central Valley: 1. In situ photoformation of hydroxyl radical and singlet molecular oxygen. Atmos. Environ. 2001, 35 (6), 1079−1089. (12) Anastasio, C.; Faust, B. C.; Rao, C. J. Aromatic carbonyl compounds as aqueous-phase photochemical sources of hydrogen peroxide in acidic sulfate aerosols, fogs, and clouds 0.1. Non-phenolic methoxybenzaldehydes and methoxyacetophenones with reductants (phenols). Environ. Sci. Technol. 1997, 31 (1), 218−232. (13) Vione, D.; Maurino, V.; Minero, C.; Pelizzetti, E.; Harrison, M. A. J.; Olariu, R.; Arsene, C. Photochemical reactions in the tropospheric aqueous phase and on particulate matter. Chem. Soc. Rev. 2006, 35, 441−453. (14) Canonica, S.; Jans, U.; Stemmler, K.; Hoigne, J. Transformation kinetics of phenols in waterphotosensitization by dissolved natural organic material and aromatic ketones. Environ. Sci. Technol. 1995, 29 (7), 1822−1831. (15) Perri, M. J.; Seitzinger, S. P.; Turpin, B. J. Secondary organic aerosol production from aqueous photooxidation of glycolaldehyde: Laboratory experiments. Atmos. Environ. 2009, 43, 1487−1497. (16) Lee, A.; Goldstein, A. H.; Kroll, J. H.; Ng, N. L.; Varutbangkul, V.; Flagan, R. C.; Seinfeld, J. H. Gas-phase products and secondary aerosol yields from the photooxidation of 16 different terpenes. J. Geophys. Res. 2006, 111 (D17), D17305. (17) Tan, Y.; Perri, M. J.; Seitzinger, S. P.; Turpin, B. J. Effects of precursor concentration and acidic sulfate in aqueous glyoxal −OH radical oxidation and implications for secondary organic aerosol. Environ. Sci. Technol. 2009, 43, 8105−8112. (18) Lim, H.; Carlton, A. G.; Turpin, B. J. Isoprene forms secondary organic aerosol through cloud processing: Model simulations. Environ. Sci. Technol. 2005, 39, 4441−4446. (19) Nolte, C. G.; Schauer, J. J.; Cass, G. R.; Simoneit, B. R. T. Highly polar organic compounds present in wood smoke and in the ambient atmosphere. Environ. Sci. Technol. 2001, 35, 1912−1919. (20) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of emissions from air pollution sources. 3. C-1−C-29 organic compounds from fireplace combustion of wood. Environ. Sci. Technol. 2001, 35 (9), 1716−1728. (21) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Sources of fine organic aerosol. 9. Pine, oak and synthetic log combustion in residential fireplaces. Environ. Sci. Technol. 1998, 32 (1), 13−22. (22) Bond, T. C.; Streets, D. G.; Yarber, K. F.; Nelson, S. M.; Woo, J.; Klimont, Z. A technology-based global inventory of black and organic carbon emissions from combustion. J. Geophys. Res. 2004, 109, D14203.

(23) Canonica, S.; Hellrung, B.; Wirz, J. Oxidation of phenols by triplet aromatic ketones in aqueous solution. J. Phys. Chem. A 2000, 104 (6), 1226−1232. (24) Gelencser, A.; Hoffer, A.; Kiss, G.; Tombacz, E.; Kurdi, R.; Bencze, L. In-Situ formation of light-absorbing organic matter in cloud water. J. Atmos. Chem. 2003, 45, 25−33. (25) Hoffer, A.; Kiss, G.; Blazso, M.; Gelencser, A. Chemical characterization of humic-like substances (HULIS) formed from a lignin-type precursor in model cloud water. Geophys. Res. Lett. 2004, 31 (6), L06115. (26) Chang, J. L.; Thompson, J. E. Characterization of colored products formed during irradiation of aqueous solutions containing H2O2 and phenolic compounds. Atmos. Environ. 2010, 44 (4), 541− 551. (27) Sun, Y. L.; Zhang, Q.; Anastasio, C.; Sun, J. Insights into secondary organic aerosol formed via aqueous-phase reactions of phenolic compounds based on high resolution mass spectrometry. Atmos. Chem. Phys. 2010, 10 (10), 4809−4822. (28) Galbavy, E. S.; Ram, K.; Anastasio, C. 2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry. J. Photochem. Photobiol., A 2010, 209, 186−192. (29) Zhang, Q.; Anastasio, C. Conversion of fogwater and aerosol organic nitrogen to ammonium nitrate, and NOx during exposure to simulated sunlight and ozone. Environ. Sci. Technol. 2003, 37, 3522− 3530. (30) Leifer, A. The Kinetics of Environmental Aquatic Photochemistry: Theory and Practice; American Chemical Society: Washington, DC, 1988; p 100. (31) Jammoul, A.; Dumas, S.; D’Anna, B.; George, C. Photoinduced oxidation of sea salt halides by aromatic ketones: A source of halogenated radicals. Atmospheric Chemistry and Physics 2009, 9, 4229−4237. (32) Leigh, W. J.; Lanthioor, E. C.; Pierre, M. J. S. Photoinduced hydrogen abstraction from phenols by aromatic ketones. A new mechanism for hydrogen abstraction by carbonyl n,π* and π,π* triplets. J. Am. Chem. Soc. 1996, 118, 12339−12348. (33) Canonica, S.; Hoigne, J. Enhanced oxidation of methoxy phenols at micromolar concentration photosensitized by dissolved natural organic material. Chemosphere 1995, 30 (12), 2365−2374. (34) Sharpless, C. M. Lifetimes of triplet dissolved organic matter (DOM) and the effect of NaBH4 reduction on singlet oxygen quantum yields: Implications for DOM photophysics. Environ. Sci. Technol. 2012, 46 (7), 3912−3920. (35) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Photosensitized transformation involving electronic energy transfer in natural waters: Role of humic substances. Environ. Sci. Technol. 1985, 19, 74−81. (36) Halladja, S.; Ter Halle, A.; Aguer, J.-P.; Boulkamh, A.; Richard, C. Inhibition of humic subtances mediated photooxygenation of furfuryl alcohol by 2,4,6-trimethylphenol. Evidence for reactivity of the phenol with triplet excited states. Environ. Sci. Technol. 2007, 41, 6066−6073. (37) Nguyen, T. B.; Lee, P. B.; Updyke, K. M.; Bones, D. L.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. Formation of nitrogen- and sulfurcontaining light-absorbing compounds accelerated by evaporation of water from secondary organic aerosols. J. Geophys. Res. 2012, 117, D01207. (38) Ofner, J.; Kruger, H. U.; Grothe, H.; Schmitt-Kopplin, P.; Whitmore, K.; Zetzsch, C. Physico-chemical characterization of SOA derived from catechol and guaiacola model substance for the aromatic fraction of atmospheric HULIS. Atmos. Chem. Phys. 2011, 11 (1), 1−15. (39) Chhabra, P. S.; Ng, N. L.; Canagaratna, M. R.; Corrigan, A. L.; Russell, L. M.; Worsnop, D. R.; Flagan, R. C.; Seinfeld, J. H. Elemental composition and oxidation of chamber organic aerosol. Atmos. Chem. Phys. 2011, 11, 8827−8845. (40) Coeur-Tourneur, C.; Tomas, A.; Guilloteau, A.; Henry, F.; Ledoux, F.; Visez, N.; Riffault, V.; Wenger, J. C.; Bedjanian, Y. Aerosol formation yields from the reaction of catechol with ozone. Atmos. Environ. 2009, 43 (14), 2360−2365. 1056

dx.doi.org/10.1021/es4045715 | Environ. Sci. Technol. 2014, 48, 1049−1057

Environmental Science & Technology

Article

(41) Lauraguais, A.; Coeur-Tourneur, C.; Cassez, A.; Seydi, A. Rate constant and secondary organic aerosol yields for the gas-phase reaction of hydroxyl radicals with syringol (2,6-dimethoxyphenol). Atmos. Environ. 2012, 55, 43−48. (42) Yee, L. D.; Kautzman, K. E.; Loza, C. L.; Schilling, K. A.; Coggon, M. M.; et al. Secondary organic aerosol formation from biomass burning intermediates: Phenol and methoxyphenols. Atmos. Chem. Phys. 2013, 13, 3485−3532. (43) Das, T. N. Monomer and dimer radical cations of benzene, toluene, and naphthalene. J. Phys. Chem. A 2009, 113, 6489−6493. (44) Arakaki, T.; Anastasio, C.; Kuroki, Y.; Nakajima, H.; Okada, K.; et al. A general scavenging rate constant for reaction of hydroxyl radical with organic carbon in atmospheric waters. Environ. Sci. Technol. 2013, 47 (15), 8196−8203. (45) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O−) in aqueous solutions. J. Phys. Chem. Ref. Data 1988, 17 (2), 513−886. (46) Sander, R. Compilation of Henry’s Law Constants for inorganic and organic species of potential importance in environmental chemistry (Version 3). http://www.henrys-law.org (47) Mayer, B.; Madronich, S. Actinic flux and photolysis in water droplets: Mie calculations and geometrical optics limit. Atmos. Chem. Phys. 2004, 4, 2241−2250.

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