secondary organic aerosol production from aqueous reactions of atmospheric phenols with an organic...

Secondary Organic Aerosol Production from Aqueous Reactions ofAtmospheric Phenols with an Organic Triplet Excited StateJeremy 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 ofEnvironmental Toxicology, University of CaliforniaDavis, 1 Shields Avenue, Davis, California 95616, United States

*S Supporting Information

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

■ INTRODUCTION

While much of atmospheric organic aerosol (OA) is secondary,traditional models of secondary organic aerosol (SOA) oftenfail to account for observed mass loadings.1,2 In part, this mightbe because the models only include gas-phase pathways to formlow-volatility products, while more recent work has shown thataqueous reactions can also be an important source of SOA.3−8

In the latter pathway, organic gases partition into the aqueousphase and are then oxidized to lower-volatility products thatremain in the particle phase even after the condensed-phasewater evaporates.The aqueous formation of SOA can occur through both

thermal (dark) and photochemical reactions. For photo-chemical reactions, the hydroxyl radical (•OH) is typicallyconsidered the dominant oxidant for aqueous organics and thusthe dominant source of SOA from the aqueous phase.9

However, fog and cloud drops (and, by extension, aqueousaerosol particles) contain a number of other oxidants, includingsinglet molecular oxygen, peroxyl radicals, peroxides, and tripletexcited states of organic compounds (3C*).10−13 These speciesmight play a significant role in the aqueous formation of SOA,but they have received little attention. We are particularlyinterested in the possible roles of 3C*, which are formed byillumination of light-absorbing organic carbon such as aromaticcarbonyls.12−14 There is evidence that triplet excited states areimportant in the formation of HOOH and HONO, and the lossof phenols in atmospheric drops and particles,12,13,15 but theirpotential role in SOA formation has not been examined.

As for the volatile organic precursors of aqueous SOA, recentstudies have mainly focused on small aliphatic compounds suchas 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 aswell studied. Our interest here is on phenols, which arearomatic alcohols emitted from biomass combustion. Biomassburning is a large atmospheric source of phenols and, morebroadly, organic carbon, including nonphenolic aromaticcarbonyls.19−22 When exposed to sunlight, aromatic carbonylsare excited to their triplet states, which can abstract a hydrogenatom from phenol, resulting in phenoxy radicals that couple toform oligomers.12,23 Similar reactions initiated by hydroxylradical oxidation of phenols also produce low volatility,oligomeric SOA,24−27 but aqueous SOA formation by the3C*-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) withthree model phenolsphenol, guaiacol (2-methoxyphenol),and syringol (2,6-dimethoxyphenol)that represent the majorclasses of phenols emitted from biomass combustion.19,20 Wereport the kinetics of these reactions under atmosphericallyrelevant conditions and the mass yields of the low-volatilityproducts formed.

Received: October 11, 2013Revised: December 18, 2013Accepted: December 23, 2013Published: December 23, 2013

Article

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© 2013 American Chemical Society 1049 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 asreceived: phenol (PhOH; 99%), 3,4-dimethoxybenzaldehyde(DMB; 99%), syringol (SYR; 99%), and acetonitrile (HPLCgrade) from Aldrich; guaiacol (GUA; 98%) from TCI America;and sodium borate (ACS grade) and sulfuric acid (Trace Metalgrade) from Fisher. All solutions were made using purifiedwater (Milli-Q) from a Milli-Q Plus system (Millipore; ≥18.2MΩ cm) with an upstream Barnstead activated carboncartridge. Solution pH was adjusted using sulfuric acid for pH≤ 5 and sodium borate for pH > 5. Aluminum cups were madeby 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 andweighed 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 orfog drops in areas with significant wood combustion.12,19,21 Inthis work, we use the abbreviation “PhOH” to refer specificallyto phenol (i.e., the compound C6H5OH) and the terms“phenol” and “ArOH” to refer generically to any of the threephenolic 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 WXe arc lamp with downstream optical filters.11 Online, real-timemeasurements of SOA yields were performed using air-saturated solutions in stirred, 118 mL Pyrex tubes (thusfiltering light below 285 nm) illuminated in a New EnglandUltraviolet Company RPR-200 photoreactor equipped with300, 350, and 419 nm bulbs (2, 7, and 7 bulbs, respectively).For each experiment, we also ran a dark control samplewrapped in aluminum foil, but otherwise treated identically tothe illuminated sample.Chemical Analysis. Periodically during illumination small

aliquots of solution were removed from the illuminated anddark cells to measure the concentrations of ArOH and DMBusing an HPLC consisting of the following: Shimadzu LC-10AT pump, ThermoScientific BetaBasic-18 C18 column,Shimadzu-10AT UV−vis detector (detection wavelengths of270, 276, 268, and 307 nm for PhOH, GUA, SYR, and DMB,respectively), degassed eluent of 20:80 acetonitrile/water, and aflow rate of 0.70 mL min−1. We measured the photon flux oneach experimental day by determining the photolysis rateconstant (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 pHmeter.Kinetic Analysis. The measured apparent first-order rate

constant for phenol loss (k′LIGHT) was determined as thenegative of the slope of a plot of ln([ArOH]t/[ArOH]0) versusillumination time, where [ArOH] is the concentration ofphenol (at times t and zero). We used an analogous procedureto determine DMB loss. There was no significant loss (p <0.05) for phenol or DMB in any dark control. Values of k′LIGHTwere normalized to sunlight conditions at midday on the wintersolstice at Davis, CA11 (j2NB,win = 0.0070 s−1) and werecorrected for the small amount of internal light screening dueto DMB:

′ =′

××

λ

⎡⎣⎢⎢

⎤⎦⎥⎥k

kS j

jArOHLIGHT

2NB,exp2NB,win

(1)

In this equation, k′ArOH is the normalized first-order decayconstant for phenol loss and Sλ is the internal light screeningfactor,29,30 determined for wavelengths at the peak in the lightabsorption action spectrum for DMB (310 − 335 nm). For ourtypical DMB concentration of 5 μM, Sλ ranged from 0.85 for a5-cm cell to 0.97 for a 1-cm cell, showing that light screeningwas minor.We fit the pH-dependence of the phenol decay rate constant

in illuminated DMB solutions using a sigmoidal regression:12

′ = ′ +′ − ′

++k k

k k

1 HK

ArOH,Calcd HTT HT

[ ]

a (2)

where k′ArOH,Calcd is the calculated value of the normalized first-order decay constant from the regression, k′HT and k′T arefitted values for the pseudo-first order rate constant for phenolreaction with the protonated (HT) and neutral (T) tripletstates of DMB, respectively (e.g., k′HT = kArOH+HT[HT]), and Kais the fitted acid dissociation constant for the triplet excitedstate of DMB. Equation 2 is derived from the mole fractionequations (αHT = 1 + [H+]/Ka and αT = 1− αHT) and eq 5.12

The regressions for eq 2 (sigmoidal 3-variable fit), and for eq 6below (2-variable rational fit), were performed using SigmaPlotVersion 11 (Systat Inc.). The initial rate of phenol (RArOH,0)loss under Davis winter sunlight was calculated as follows:

= ′ ×R k [ArOH]ArOH,0 ArOH 0 (3)

Measurement of Secondary Organic Aerosol Yields.SOA mass yields were determined for pH 5 solutionscontaining 100 μM phenol and 5 μM DMB. Yields areexpressed relative to the amount of phenol reacted in theaqueous phase. We used the upper end of expectedatmospheric phenol concentrations in order to form enoughSOA to measure gravimetrically. SOA yields were measuredusing two different methods: (1) solution blow-down followedby gravimetric mass measurement, and (2) real-time measure-ments using an online High-Resolution Time-of-Flight AerosolMass Spectrometer (HR-ToF-AMS). In the first method,solutions were illuminated with simulated sunlight (with acorresponding dark control) until approximately half of theinitial phenol had reacted. The mass of low volatility productswere determined by transferring 10 or 12 mL of the light anddark solutions to separate clean, preweighed aluminum cups.The light and dark solutions were then evaporated to drynessusing Specialty grade (99.997%) nitrogen gas (Praxair), and thecups were weighed again. The difference in mass before andafter evaporation represents the low volatility components ineach solution; the difference between the illuminated and darksamples represents the SOA mass. The SOA mass yield, i.e., themass of SOA formed per mass of phenol reacted duringillumination, was calculated using the following:

=−

Y(mass of illuminated sample mass of dark sample)

mass of phenol reacted

SOA

(4)

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

Environmental Science & Technology Article

dx.doi.org/10.1021/es4045715 | Environ. Sci. Technol. 2014, 48, 1049−10571050

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

■ RESULTS AND DISCUSSIONPhotochemical Behavior. We start by describing the

general photochemical behavior of phenols in the presence ofthe triplet excited state of 3,4-dimethoxybenzaldehyde (DMB).Figure 1 illustrates the first-order decay kinetics of PhOH in

illuminated solution and the lack of reactivity for PhOH (andDMB) in the dark. In approximately 75% of our experiments,there was no loss of DMB in the illuminated solution, but in theother experiments DMB was photodegraded, with a rateconstant that was generally less than 20% of the correspondingphenol value. These general observationsa rapid loss ofphenol in solutions illuminated with simulated sunlight but noreactivity in the darkare consistent with a past studyperformed with 313 nm radiation.12 Our finding of occasionalloss of DMB is in contrast to this past work, possibly becausewe 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 inour experiments, while contributions from singlet molecularoxygen (1O2*) and hydroxyl radical (•OH) are insignificant.We also examined the direct photodegradation of the threephenols: PhOH and GUA have no direct photodegradationunder our conditions, whereas SYR does when the initial

concentration is 100 μM. However, because this directphotodegradation is much slower than loss due to the tripletexcited 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 phenolconcentration on phenol loss kinetics in illuminated solutionscontaining DMB. As shown in Figure 2, the apparent rate

constant for loss of PhOH, k′ArOH, is strongly dependent uponpH, increasing by a factor of nearly 20 between pH 6 and pH 2for 10 μM phenol solutions. To examine whether increasingionic strength contributed to the increase in k′ArOH withdecreasing pH in Figure 2, we performed a control experimentwhere we adjusted a pH 6, 100 μM PhOH solution to the ionicstrength of a pH 2 sample using sodium sulfate. As shown bythe gray square in Figure 2, this increase in ionic strength doesnot affect k′ArOH. Since some triplet states can react withchloride,31 we also tested whether the addition of 100 μMNH4Cl slows the kinetics of phenol loss in a 10 μM PhOHsolution. As shown by the gray triangle in Figure 2, k′ArOH inthe NH4Cl solution is very similar to the predicted value fromsolutions without chloride, indicating no significant effect.Finally, we also examined whether temperature affects thekinetics of PhOH loss by 3C*: as seen by the gray circle at pH1.8 in Figure 2, k′ArOH at 5 °C is within the range of valuesmeasured at the default temperature of 20 °C, indicating nosignificant effect of temperature.As described previously,12 the sigmoidal behavior of k′ArOH

with pH suggests that the DMB triplet state is protonated to amore reactive form in acidic solutions. Fitting eq 2 to thisFigure 2 data gives an average pKa of 3.3 ± 0.2 for the DMBtriplet (SI Table S2), which is similar to the previous value (3.6± 0.1) determined by measuring HOOH production during313 nm illumination.12 Previous work has investigated theprotonation of triplet excited states and found that generally thepKA 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 bythe triplet excited state of DMB; results are shown for pH 2 solutionscontaining 100 μM PhOH and 5 μM DMB. Open symbols showconcentrations of PhOH (squares) and DMB (circles) in illuminatedsolutions, while filled symbols show concentrations for dark controls.

Figure 2. Dependence of PhOH destruction rate constant on pH forilluminated solutions containing 5 μM DMB and either 10 or 100 μMPhOH at 20 °C. Black lines are regression fits to eq 2; fittedparameters are tabulated in SI Table S2. The orange line representsαHT, the fraction of triplet excited DMB that is protonated. There arethree control experiments: the gray circle represents a 100 μM PhOHexperiment at 5 °C; the gray square represents a 100 μM PhOHexperiment at high ionic strength (I = 0.2 M), and the gray trianglerepresents a 10 μM PhOH experiment with 100 μM NH4Cl added.Error bars represent ±1 SE, propagated from the standard errors ofk′LIGHT and j2NB; most error bars are smaller than the symbols.



ground state.12 Figure 2 illustrates that the measured apparentfirst-order rate constant for ArOH loss (k′ArOH) is a mole-fraction-weighted sum of the rate constants for phenol with theprotonated (HT) and neutral (T) forms of the triplet excitedstate:

α α′ = ′ + ′k k kArOH HT HT T T (5)

where αHT and αT are the mole fractions of the two forms ofthe triplet excited state, and k′HT and k′T are the pseudo first-order rate constants for ArOH decay. To more simplydetermine k′HT and k′T, we performed subsequent experimentsat pH 2 and 5, since under these conditions the protonated andneutral states of the triplet, respectively, are dominant: αHT is95% at pH 2 and αT is 98% at pH 5. Since k′HT is much largerthan k′T, at pH 2 k′HT ≈ k′ArOH, while at pH 5 we subtract thesmall contribution of the protonated triplet from the measuredvalue 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 pH2 and 5 over a range of phenol concentrations of 1 to 100 μM,approximately the range expected in areas with significant woodcombustion.12 As shown in Figure 3, the protonated tripletexcited state of DMB is also more reactive than the neutraltriplet for guaiacol and syringol: values of k′HT (Figure 3A) arelarger than values of k′T (Figure 3B) for all three phenols,although PhOH is most affected. The reactivity order shown in

Figure 3A,B (SYR > GUA > PhOH) is consistent with the factthat adding electron-donating methoxy groups to phenolsdecreases the electron oxidation potential, thus increasing thereaction rate.32,33

Figure 3 also demonstrates that the apparent rate constantfor phenol loss by triplet excited states decreases withincreasing phenol concentration, consistent with the behaviorof the acidic 10 and 100 μM PhOH solutions in Figure 2. Asimilar pattern has been reported in a previous surface waterstudy of methoxy phenols and humic acids.33 We hypothesizethat the phenol concentration dependence of k′ArOH in Figure 3is due to a competition between three fates of the triplet excitedstate: reaction with phenol, quenching by oxygen to formsinglet molecular oxygen (1O2*), and unimolecular decay backto the DMB ground state (Figure 4 and SI Section S3). In

natural water systems, the O2 pathway is the dominant sink for3C* at low concentrations of phenol.23,34−36 However, asphenol concentrations approach 100 μM, phenol becomes animportant sink, thus suppressing the triplet steady-stateconcentration, [3C*], which decreases k′ArOH since k′ArOH =kArOH+3C*[

3C*] (where kArOH+3C* is the second-order rateconstant for reaction of 3C* with phenol). This effect isillustrated for all three phenols, at both pH values, in Figure 3.Viewed in terms of the rate of phenol loss, RArOH, the rateapproaches a plateau at higher phenol concentrations as phenolbecomes a major sink (SI Figure S2). At a concentration of 100μM, reaction with phenols accounts for approximately 40−60%of the loss of the DMB triplet, with the exception of PhOH atpH 5, which is an insignificant reactive sink for the DMB triplet(SI Figure S5), although it can physically quench the triplet (SITable S3). Using the model of 3C* formation and loss shownin Figure 4, we can derive an expression for the observedconcentration dependence of k′ArOH (SI Section S3; eq S10):

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

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



′ =

+φ φ

+ ′×

+×

+ * *

+ *

+ *

+ *⎜ ⎟⎛⎝

⎞⎠

k1

[ArOH]k kj C k

k k

j C k

ArOH

[O ][ ] [ ]

O2 3C 2 3C

hv,abs ISC ArOH 3C

ArOH 3C Q

hv,abs ISC ArOH 3C

(6)

Here, kO2+3C* is the second-order rate constant for reaction of3C* with molecular oxygen, k′3C* is the first-order rate constantfor unimolecular decay of 3C* to the ground state C, jhv,abs isthe rate constant for light absorption by the chromophore C,φISC is the intersystem crossing quantum yield of the excitedsinglet (1C*) to triplet (3C*), [C] is the chromophoreconcentration (DMB in our experiments), and kQ is thesecond-order rate constant for nontransformative interactionbetween the 3C* and a phenol. The apparent second-order rateconstant kArOH+3C* is a mole-fraction-weighted combination ofthe second-order phenol rate constants with the protonatedand neutral triplet states (kArOH+HT and kArOH+T, respectively):

α α= ++ * + +k k kArOH 3C HT ArOH HT T ArOH T (7)

We fit eq 6 to our data using a two-parameter equation:

′ =+

ka b

1[ArOH]ArOH

(8)

The results of these regressions are listed in Table 1 for all threephenols 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 tripletexcited state (3DMB*) with phenol (i.e., kArOH+3C*) and thesecond-order rate constant for nonreactive quenching of thetriplet state by phenol (kQ). On the basis of our Figure 3regression fits, values of kArOH+3C* for the triplet state of DMBrange 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). Weobtain similar values from regressions of phenol destructionrate versus concentration (Figure S2 and Table S4); however,we recommend the values reported in Table 1. The fast rateconstants for phenols with the neutral DMB triplet are similarto values previously reported for three other triplets with a suiteof phenols at near neutral pH.23 We find that the rate constantsfor phenols with the protonated triplet state of DMB are evenfaster, by factors of 1.3, 1.9, and 31 for GUA, SYR, and PhOH,respectively (Table 1). Overall, the slowest rate constant forchemical reaction is for PhOH with the neutral DMB tripletstate. However, under these conditions, PhOH rapidlyphysically 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-tripletinteractions result in reaction (i.e., kArOH+3C*/(kQ + kArOH+3C*)= 0.02). This agrees with a previous study where only 1% ofPhOH interactions with a triplet resulted in destruction of theparent PhOH.23 While we are describing this PhOH−3C*

interaction as nonreactive physical quenching, it has beensuggested that the interaction initially forms a phenoxy radical,which is then reduced by superoxide to regenerate the parentPhOH, leading to no apparent reaction.23 In contrast to thedominant role of quenching for PhOH at pH 5, for all of theother conditions in our experiments the ArOH-DMBinteraction is dominated by chemical reaction, with littleevidence of quenching (SI Table S3).On the basis of our mechanistic understanding of the

competition between dissolved oxygen and phenols for thetriplet excited states of DMB (eq 6) we can predict the first-order rate constant for phenol loss (k′ArOH) in our illuminatedsolutions for any pH or phenol concentration As shown in SIFigure S7, the loss of all three of the phenols depends onphenol concentration and pH, with PhOH being especiallysensitive to pH. Thus the environmental lifetimes of aqueousphenols due to reactions with 3C* depend both upon theconcentration of triplet precursors as well as the concentrationsof triplet sinks (e.g., phenols) and acidity. Extending thisframework to quantify rates of phenol loss by triplet excitedstates in atmospheric waters requires information about the rateof triplet formation and the pseudo-first order rate constant forloss of triplets by phenols and other reactants. Unfortunately,we cannot currently constrain either of these parametersbecause of a lack of data.

Mass Yields of Secondary Organic Aerosol. Recentwork has shown that the oxidation of phenols by aqueous •OHproduces significant amounts of low-volatility products.27 Tounderstand whether oxidation by aqueous triplet excited statesalso efficiently produces SOA, we measured SOA mass yields inilluminated pH 5 solutions containing 5 μM DMB and 100 μMphenol.Our first method of determining SOA yields uses gravimetric

measurements and so we must first understand our “back-ground” mass. To do this, we measured the mass of low-volatility material remaining in a series of dark control solutionsblown 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 solutionsafter approximately half of the initial ArOH has reacted, theaverage blown-down masses are 7.0, 6.9, and 12.8 μg mL−1 forPhOH, 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 thatSOA mass yields in our solutions are near 100% for all threephenols, 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 valuesfrom aqueous PhOH, GUA, and SYR solutions with •OH asthe oxidant (Anastasio and Sun; in preparation). SOA massyields greater than 100% indicate incorporation of other atomsinto the products, most likely oxygen, probably via hydrox-

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

k′HTa k′Ta kArOH+3C*b(109 M−1 s−1)

a (min−1) b (min μM−1) a (min−1) b (min μM−1) kArOH+HT kArOH+T

PhOH 123 (±14) 0.8 (±0.3) 3190 (±420) 30 (±10) 3.4 (±1.2) 0.13 (±0.09)GUA 79 (±5) 0.6 (±0.2) 99 (±9) 0.6 (±0.2) 5.3 (±1.9) 4.2 (±3.0)SYR 37 (±2) 0.6 (±0.1) 59 (±20) 0.5 (±0.2) 11 (±3) 5.8 (±4.1)

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



ylation of the aromatic rings.27 Figure 5 also shows the real-time SOA mass yields determined by aerosolizing theilluminated solutions, drying the drops, and analyzing withonline HR-ToF-AMS. The real-time yield determined after halfof PhOH has reacted is similar to the gravimetric determinationat the same time point, but the real-time YSOA values for GUAand SYR are, on average, 65 ± 35% lower than those measuredgravimetrically. This suggests that the N2 blowdown can causesemivolatile components to react and form low-volatilityproducts, possibly because of the sulfuric acid; evidence forsuch reactions in solutions with different organics wasreportedly recently.37 However, the real-time YSOA valuesdetermined at the end of illumination are not statisticallydifferent (p > 0.05) from the gravimetric determinations for anyof the phenols (Figure 5). Understanding the reasons for thesemethodological differences in YSOA, and the possible influenceof N2 blowdown and illumination time on SOA mass willrequire further study. Despite this uncertainty, our aqueousyields of phenolic SOA are clearly higher than values reportedfor phenol oxidation by •OH in the gas phase.38−42 Forexample, Lauraguais et al.41 found SOA yields of 10−36% forsyringol reacting with gaseous hydroxyl radical under high NOxconcentrations. Under low NOx conditions, Yee et al.42

measured higher SOA mass yields of 25−44%, 44−50%, and25−37% from the •OH oxidation of gas-phase phenol, guaiacoland syringol, respectively. The more efficient production of lowvolatility products from phenol oxidation in solution reflects theefficient formation and coupling of phenoxyl radicals in theaqueous phase.12,23,43 In contrast, gas-phase oxidation ofphenols produces a larger amount of volatile products throughfragmentation of the aromatic rings,38,42 in addition tofunctionalization by addition of •OH and NO2 to thephenols.39

Comparison of Aqueous SOA Formation fromPhenols via Triplet Excited States and •OH. In thissection, we compare the relative importance of 3C* and •OHfor making SOA from the aqueous oxidation of phenols. We

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 midlatitudecloud and fog waters with estimated contributions from gas-to-drop partitioning of •OH.44 We use literature values45 for thesecond-order rate constants of ArOH with •OH at pH 5. Valuesat pH 2 are 6.5, 3.5, and 1.6 times lower than the pH 5 valuesfor PhOH, GUA, and SYR, respectively, based on a sigmoidalfit of literature and experimental rate constants (SI Section S5).For the kinetics of 3C* with phenols, we use winter-solstice-normalized rate constants from eq 6 with ArOH concentrationsat 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* are2.2 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) andtriplets 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 ofsignificant biomass combustion, the destruction of atmosphericaqueous phenols by organic triplet excited states is comparableto •OH at high pH (>5) and dominates at lower pH. Whilevalues of k′ArOH,•OH depend on the atmospheric concentrationof •OH, the ratio k′ArOH,3C*/k′ArOH,•OH is greater than 1 for allthree pairs of •OH and 3C* concentrations, with the exceptionof PhOH (SI Table S7).44 Furthermore, Figure 6A reiteratesthat syringol is the most reactive of the three phenols, whilePhOH is the least reactive. Considering both 3C* and •OH attheir intermediate concentrations, the aqueous lifetimes of SYR,GUA, and PhOH are 0.8, 1.0, and 3.4 h, respectively, at pH 5and 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 alsoexamined the product of the normalized first-order rateconstant for ArOH loss and the corresponding SOA massyield for a given oxidant. This product, k′ArOH × YSOA, isessentially a “rate constant” for SOA formation in the aqueousphase. As shown in Figure 6B, the “rate constant” for SOAformation from triplet excited states is similar or larger thanthat for •OH at all pH values for GUA and SYR, while •OH isthe dominant pathway for aqueous SOA from PhOH at pHvalues above roughly 3. Values for the ratio (k′ArOH, 3C* ×YSOA,3C*/k′ArOH,•OH × YSOA,•OH) for PhOH range from 0.1 atpH above 6 to near 11 at pH below 2; SYR and GUA values areless pH dependent, with ratios ranging from approximately 1above 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 thesecalculations we use typical Davis, CA winter fog conditions(T = 5 °C, liquid water content =1.0 × 10−7 L-aq L-g−1) andliterature Henry’s Law constants (1.5 × 104, 5.0 × 103, and 2.5× 104 M atm−1 for PhOH, GUA and SYR, respectively46) todetermine the gas-aqueous partitioning of the phenols. Usingthese parameters we calculated the ratio of aqueous-phase andgas-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 aqueousand gas phase, respectively. As shown in Figure 6C, aqueous-phase oxidation is more important for the formation of SOAfrom syringol, while gas-phase oxidation is more important as a

Figure 5. Average (±1 σ) mass yields of SOA (YSOA) formed from the3DMB*-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 ofsolutions 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 forPhOH, GUA, SYR, respectively. The middle and right bars for eachArOH are YSOA measured in real-time by online HR-ToF-AMS, at one-half-life (middle bar, light shading) and at the end of each onlineexperiment (right bar, white), at 85, 95, and 100% loss for PhOH,GUA, and SYR, respectively.



source of SOA from PhOH and (especially) GUA. Thesignificant aqueous contributions for SYR and PhOH show theimportance of aqueous kinetics and SOA yields: while only 5and 3% of these phenols, respectively, are in the aqueous phaseunder these conditions, this weak partitioning is offset by rapidphenol oxidation (Figure 3) and high SOA mass yields (Figure5) in the aqueous phase.

Atmospheric Implications. Our results suggest thataqueous transformations of phenols by excited triplet statesof light-absorbing organic compounds can be a significantsource of SOA: triplet excited states can be a rapid sink foraqueous phenols, and they form SOA with very high massyields. In areas impacted by biomass combustion, the 3C*-mediated oxidation of aqueous phenols appears to be generallyfaster than reaction with •OH, especially under acidicconditions. In addition, while our experiments were conductedunder conditions similar to cloud and fog drops, similarchemistry likely occurs in aqueous aerosol particles. However,due to the low fraction of phenols in the aqueous phase, gas-phase oxidation of phenols will still be a significant sink formost phenols. While our results reveal a previouslyunappreciated role for excited triplet states in atmosphericchemistry, more quantitatively assessing this role requiresunderstanding the concentrations and reactivities of excitedtriplet states in atmospheric drops and particles.

■ ASSOCIATED CONTENT*S Supporting InformationA detailed description of our kinetic model derivation, furtherkinetic data, and control experiments. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 530-754-6095; fax: 530-752-1552; e-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFunding for this research was provided by the National ScienceFoundation (Grant No. AGS-1036675), the University ofCalifornia Toxic Substances Research and Teaching Program(TSR&TP) through the Atmospheric Aerosols and HealthLead Campus Program, and the California AgriculturalExperiment Station (Project CA-D*-LAW-6403-RR).

■ REFERENCES(1) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.;Simpson, D.; et al. The formation, properties and impact of secondaryorganic 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 organicaerosols in anthropogenically-influenced Northern Hemispheremidlatitudes. Geophys. Res. Lett. 2007, 34 (13), L13801.(3) Blando, J. D.; Turpin, B. J. Secondary organic aerosol formationin 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 organicaerosol formation from acetylene (C2H2): Seed effect on SOA yieldsdue to organic photochemistry in the aerosol aqueous phase. Atmos.Chem. Phys. 2009, 9, 1907−1928.

Figure 6. Panel A compares the calculated pseudo-first-order rateconstants for the aqueous oxidation of phenols by •OH and 3C* at pH2 (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 rangeof aqueous oxidant concentrations for fog waters in Davis, CA atmidday 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 aproxy, with initial concentrations of 1, 5, and 10 μM for the bottom,middle, and top lines, respectively. Values of k′ArOH for 3C* werecalculated 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, andkSYR+OH = 2.6 × 1010 M−1s−1) and experimentally determined values atpH 2 (SI Section S5). Panel B shows the pH dependence of the ratioof aqueous SOA formation from phenol oxidation by 3C* compared to•OH. The black horizontal line represents a ratio of 1, where the twooxidants are equally important. Conditions: middle values for oxidantsfrom Panel A; 10 μM phenol; SOA yields from blown-down samples(Figure 5 for 3C* reactions and unpublished data for •OH reactionyields (1.06, 1.09, and 1.14 for PhOH, GUA, and SYR, respectively);yields were assumed independent of pH; k′ArOH+•OH values wereestimated (SI Section S5). Panel C compares the ratio of SOAproduction in the aqueous phase by •OH and 3C* (using middleconcentrations 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 thefractions of each phenol in the aqueous and gas phase, respectively.For this calculation, we have assumed that the actinic flux in anaqueous drop is 1.5 times that of the surrounding gas phase.47 Gas-phase rate constants and SOA mass yields are from Yee et al.42



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