Interactive comment on “ Chemical insights , explicit chemistry and yields of secondary organic aerosol from methylglyoxal and glyoxal ”

This paper describes a detailed model of the aqueous-phase oxidation of glyoxal and methylglyoxal. Main improvements over a previous glyoxal oxidation model have to do with simulating laboratory experiment conditions better, namely the attenuation of the photolysis beam by HOOH absorption. In addition, tetraperoxy compound decomposition rates were allowed to vary by an order of magnitude (staying within published limits) in order to better fit the data. However, in order to make the simulations match the data, two additional adjustments were made. First, glyoxylic acid photolysis rates and product ratios were adjusted away from published measurements. Second, the


Introduction
Water is predicted to be the largest component of fine particles (PM 2.5 ) globally (Liao and Seinfeld, 2005)  medium for chemistry, including chemistry that forms secondary organic aerosol (SOA).The vast majority of organics are emitted in the gas phase.Gas-phase photochemistry fragments and oxidizes these emissions, making water soluble organics (e.g., acetic acid, glyoxal) ubiquitous and abundant in the atmosphere (Millet et al., 2005).
In a previous publication (Lim et al., 2010), we developed a full kinetic model including detailed radical chemistry to describe aqSOA formation via OH radical oxidation of glyoxal, an abundant and highly water soluble compound formed through photooxidation of alkenes and aromatics.The current work is focused on methylglyoxal.Methylglyoxal is a common α-carbonyl in the atmosphere (Munger et al., 1995), with a globally estimated source of 140 Tg annually (Fu et al., 2008).It is formed from the photooxidation of both anthropogenic VOCs like aromatic hydrocarbons (Nishino et al., 2010) and biogenic VOCs including isoprene (Atkinson et al., 2006).The major sinks are gas-phase UV photolysis and photooxidation (Tadic et al., 2006;Fu et al., 2008).Like glyoxal, methylglyoxal also has great potential to form SOA through aqueous-phase reactions in clouds and wet aerosols, due to its high water solubility (H eff = 3.71 × 10 3 M atm −1 ; Betterton and Hoffmann, 1988), ability to form oligomers via acid catalysis, and reactivity with OH radicals (Blando and Turpin, 2000;De Haan et al., 2009;Sareen et al., 2010;Tan et al., 2010Tan et al., , 2012)).Acetic acid (H eff = 5.50×10 3 M atm −1 ; Herrmann et al., 2005) is highly water soluble, atmospherically abundant both in the gas phase (∼ 300 ppt;Nolte et al., 1999) and in the aqueous phase (Khare et al., 1999), and also one of major intermediate products of methylglyoxal + OH reactions Figures  (Tan et al., 2012).It should be noted that methylglyoxal and acetic acid are much more reactive with OH radical in the aqueous phase than in the gas phase (lifetimes in the aqueous phase are ∼ 26 min for methylglyoxal and ∼ 17 h for acetic acid; however, in the gas phase, lifetimes are ∼ 0.9 day for methylglyoxal and 17 days for acetic acid).
In this paper, a full kinetic model for the aqueous OH radical oxidation of methylglyoxal is proposed.Detailed radical chemistry includes peroxy radical (RO 2 ) chemistry initiated by bimolecular reactions (RO 2 -RO 2 reactions).We validate, in part, the methylglyoxal model by comparing results from aqueous photooxidation experiments developed by Tan et al. (2010Tan et al. ( , 2012) ) with model simulations of these experiments.
Note that in aqueous photooxidation experiments, OH radicals are formed through UV photolysis of H 2 O 2 , whereas in the atmosphere uptake from the gas phase is the dominant known source (Ervens et al., 2003a), with additional contributions from aqueous (e.g., Fenton, nitrate) reactions (Arakaki and Faust, 1998;Zepp et al., 1987).In this work, experimental results are better captured after taking into account the absorption of UV by H 2 O 2 and organic compounds.Finally, the combined glyoxal and methylglyoxal model is used to simulate aqSOA formation under a range of atmospheric conditions, including cloud-relevant conditions (10 µM) and higher concentrations.Runs at 10 M are intended to provide insights into the chemistry in wet aerosols using glyoxal or methylglyoxal as a surrogate for the mix of dissolved water-soluble organics (i.e., Based on water soluble organic carbon compounds of ∼ 2-3 µg C m −3 and estimated aerosol water concentrations of ∼ 10 µg m −3 at RH > 70 %; Hennigan et al., 2009;Volkamer et al., 2009).Methylglyoxal and glyoxal aqSOA yields are reported for conditions encountered by clouds and by wet aerosols based on two types of simulations: a "batch reactor" approach, in which the precursors (methylglyoxal or glyoxal) is depleted as OH radical reactions proceed, and a steady-state "continuous stirred tank reactor" (CSTR) approach, in which the precursors is replenished (held constant) in the aqueous phase.Introduction

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Experiments used to evaluate chemical modeling
Aqueous methylglyoxal chemistry and yields are developed herein, making use of chemical theory and previously published aqueous photooxidation experiments conducted with OH radicals and methylglyoxal or acetic acid (an intermediate product).
Experiments were conducted at cloud relevant and higher concentrations, but concentrations were still several orders of magnitude lower than the concentrations of water soluble organic compounds in wet aerosol.Experimental details are provided elsewhere (Tan et al., 2010(Tan et al., , 2012)).Briefly,methylglyoxal (30,300,and 3000 µM) or acetic acid (20, 100 and 1000 µM) was dissolved in 18 M Ω milli-Q water.OH radicals (10 −14 -10

Peroxy radical chemistry
Peroxy radical chemistry plays an important role in the aqueous chemistry of methylglyoxal, which is described in detail in Sect.3. As in the gas phase, OH radical reactions in the aqueous phase produce peroxy radicals due to the presence of dissolved O 2 in atmospheric waters (Herrmann, 2003).Peroxy radicals subsequently undergo two Figures

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Full possible reaction pathways: (1) self decomposition giving off HO 2 and forming acids, and (2) bimolecular RO 2 -RO 2 reaction.In the glyoxal-OH reaction, glyoxylic acid and oxalic acid are formed by the decomposition pathway, which is also the dominant pathway (Lim et al., 2010).In the methylglyoxal-OH reaction, pyruvic acid is formed by decomposition.However, further OH reactions of pyruvic acid and acetic acid, two major products of the methylglyoxal + OH, involve RO 2 -RO 2 reactions.Figure 1a illustrates peroxy radical chemistry initiated by RO 2 -RO 2 reactions.For acetic acid/pyruvic acid + OH reactions, peroxy radicals form by the addition of O 2 to the primary carbon, followed by RO 2 -RO 2 reactions forming tetroxides.This pathway is preferred over decomposition because of the absence of a hydroxy group nearby.Two well-known decomposition pathways from teteroxides are alkoxy radical/O 2 formation (A in Fig. 1a) suggested by Benson (1965) and alcohol/aldehyde/O 2 formation (B in Fig. 1a) suggested by Russell et al. (1957).The Benson pathway (A) and the Russell pathway (B) are not related and independent because the Russell pathway (B) is a concerted reaction, so none of the products are formed via alkoxy radical chemistry.
Alkoxy radicals formed in the Benson pathway (A) undergo either decomposition (I) or a 1,2-hydride shift (J) (Figs.1-3).In Fig. 1a, resulting products through decomposition of alkoxy radicals (I) are organic radicals (R q ) and aldehydes (= O).Gas-phase chamber studies suggest that decomposition of alkoxy radicals is likely to occur if a radical position in an organic radical product (R q ) is at secondary or tertiary carbons, and is enhanced when functional groups (e.g., hydroxy or carboxylic groups) are adjacent to these carbons (Atkinson et al., 2007) due to radical stabilization (Lim and Ziemann, 2009).Alkoxy radicals formed in the aqueous phase contain hydroxyl/carboxylic functional groups since the parent organic precursors are water soluble.Decomposition in the aqueous phase (I) is, therefore, more favorable than in the gas phase.For acetic/pyruvic acid in the aqueous phase (Fig. 1b), alkoxy radicals decompose to organic radicals and formaldehydes.Organic radicals are stabilized by a carboxylic group for acetic acid or a diol (since a carbonyl group will undergo hydration) for pyruvic acid (Fig. 1c), While significant 1,2-H shift (followed by O 2 reactions to form carbonyls) is Introduction

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Full not observed in the gas phase (Atkinson, 2007), alkoxy radicals in the aqueous phase do undergo 1,2-H shift.Although the detailed reaction mechanisms are not well understood, the 1,2-H shift is very likely to be assisted by water molecules (Von Sonntag et al., 1997).

Kinetic model
As done previously for glyoxal (Lim et al., 2010), we developed a full kinetic model for aqueous chemistry of methylglyoxal with OH radical at cloud-and aerosol-relevant concentrations including: (1) the formation of organic acids such as acetic, glyoxylic, glycolic, pyruvic, oxalic, and mesoxalic acid (Lim et al., 2005;Tan et al., 2009Tan et al., , 2010Tan et al., , 2012)); (2) organic radical-radical reactions to form higher carbon number acids and oligomers; and (3) peroxy radical chemistry, including self decomposition and bimolecular RO 2 -RO 2 reactions.The model was first validated by comparison against acetic acid + OH radical experiments (Tan et al., 2012), since acetic acid is an important intermediate product.Then, using the same rate constants, model predictions were compared with concentration dynamics from methylglyoxal + OH radical experiments (Tan et al., 2010(Tan et al., , 2012)).Most of the kinetic rate constants were obtained from literature documented in Tan et al. (2009Tan et al. ( , 2010Tan et al. ( , 2012)), or determined using an estimation method based on structure-activity relationships (Monod et al., 2005(Monod et al., , 2008)).Values from Ervens et al. (2003b) were also used for OH radical initiated reactions.For the radical-O 2 (peroxy radical formation) and organic radical-radical reactions, the rate constants of 1×10 6 M −1 s −1 and 1.3×10 9 M −1 s −1 , respectively were used as suggested by Guzman et al. (2006).The following were used for peroxy radical chemistry: a rate constant of 3 × 10 8 M −1 s −1 for bimolecular RO 2 -RO 2 reactions (Lim et al., 2010), a rate constant of 1 × 10 7 s −1 for the 1,2-H shift (Gilbert et al., 1976), and a rate constant on the order of Introduction

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However, in the previous work light absorption by H 2 O 2 was not taken into account.
The H 2 O 2 photolysis rate constant (k photo ) can be corrected using Beer's law, where b ext = extinction coefficient (M −1 cm −1 ) and L = path length (cm).An extinction coefficient for H 2 O 2 of 18.4 M −1 cm −1 was used (Stefan et al., 1996).
A path length of 0.80 cm provides the best fit for all three H 2 O 2 concentrations Introduction

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Full (Fig. 4d-f).This value is reasonable since it is close to the actual path length of the reaction vessel (1.04 cm).Accounting for UV absorption by H 2 O 2 provides substantial improvement in the R 2 values at the highest concentration from R 2 = 0.80 (Fig. 4a) to 0.96 with correction (Fig. 4d).Thus, in the revised model, Methylglyoxal and pyruvic acid are also light absorbing compounds and their photolysis reactions are included in the model -methylglyoxal photolysis is even corrected by using extinction coefficient of 12.7 M −1 cm −1 (Tan et al., 2010).However, these photolysis reactions turn out to be negligible during OH radical reactions because photolysis rates are much slower than OH radical reaction rates (Tan et al., 2010).

Both simulated OH concentrations (∼ 10
−12 M) and simulated and measured pH (3 to 5) reasonably reflect cloud conditions (Faust, 1994;Hermann, 2003).Simulated dissolved O 2 remains saturated during the entire reaction (Fig. S1 in Supplement), in agreement with measured O 2 at the beginning and end of experiments.Note that dissolved O 2 in atmospheric waters is expected to be saturated due to high the surfaceto-volume ratio of cloud droplets and wet aerosols.

Atmospheric simulations
Unlike laboratory experiments, the major source of OH radicals in the atmospheric aqueous phase is believed to be uptake from the gas phase, although aqueous sources also contribute (e.g., through Fenton and nitrate reactions; Arakaki and Faust, 1998;Lim et al., 2005;Lim et al., 2010;Zepp et al., 1987).In atmospheric simulations, the OH radical concentration in the aqueous phase was set to be constant at 2.44 × 10 −12 M, a value maintained by Henry's law equilibrium with the gas-phase OH radical concentration of 2 × 10 6 molecule cm phase) concentration from photooxidation of 30 µM of initial glyoxal is ∼ 20 µM, which is reasonable in the atmospheric waters according to Henry's law equilibrium with an atmospheric concentration of H 2 O 2 (∼ 0.2 ppb) in the gas phase (Warneck, 1999).
The following atmospheric processes are required to form aqSOA: (1) glyoxal and methylglyoxal production via gas-phase photooxidation, (2) glyoxal and methylglyoxal uptake by atmospheric waters (i.e., Henry's law equilibrium between gas-and aqueousphase glyoxal and methylglyoxal), and (3) aqueous-phase reactions in the atmospheric waters forming low or semivolatile products.This atmospheric process can be approximated in a batch or CSTR framework.In the batch reactor approximation, aqueousphase OH radical reactions are limited by photochemical production of glyoxal and methylglyoxal in the gas phase.Slow glyoxal/methylglyoxal production results in its depletion in the atmospheric waters by aqueous-phase OH radical reactions.In the CSTR approximation, however, aqueous-phase OH radical reactions are not limited by gasphase photochemical production of glyoxal and methylglyoxal.In contrast to the batch reactor, glyoxal and methylglyoxal are continuously taken up by atmospheric waters and never depleted out in that medium.This is a better assumption when gas-phase production is faster than aqueous reactions.Which of these approximations is more appropriate depends on what the major precursors are in the region of study.

SOA yield from atmospheric simulations
Given: where A = glyoxal or methylglyoxal and P i = product i , the product yield for P i (Y P i ) is given by: Tables Figures

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Full Then overall SOA yield (Y SOA ) is defined as: where F i = particle fraction of P i and [A] reacted = concentration of unhydrated A reacted with OH radical in the aqueous phase.For glyoxal-OH reactions, the SOA-forming products are oxalate (OXLAC) and oligomers (OLIG).For methylglyoxal-OH reactions, the SOA-forming products are pyruvate (PYRAC), oxalate (OXLAC) and oligomers (OLIG).
To use these yields in a chemical transport model, the model must simulate the gas phase concentration of A, the uptake of A into the aqueous phase, and the change in the aqueous concentration of A as a result of reactions with OH ([A] reacted ) over the course of a time step.[A] reacted is then multiplied by Y SOA to produce SOA.

Product yield
In the previous glyoxal-OH model, the maximum yields of oxalic acid and oligomers were simulated (Lim et al., 2010), but in this work average yields are estimated.For example, the simulated molar yield of oxalic acid that is formed from the OH radical initiated reaction of 10 µM initial [glyoxal] is plotted with the reaction time (x-axis) in Fig. 5a.In the previous work (Lim et al., 2010), the maximum yield of 0.91 was estimated.But in this work, Fig. 5a is replotted to 5b where the x axis is [glyoxal] reacted and the y axis is [oxalic acid]; therefore, the slope represents the yield of oxalic acid.In Fig. 5b, oxalic acid increases as glyoxal reacts; then the curve drops sharply when glyoxal is depleted.The slope of ∼ 0.80, obtained by the linear regression on the product formation curve from the starting point of aqueous-phase photochemistry (t = 0) to the peak (t max = 38 min) giving a reasonably low error (R 2 ∼ 0.9), represents the average (molar) yield of oxalic acid.In CSTR simulation plots, oxalic acid continuously increases and never drops as glyoxal reacts (Fig. 5c).A similar oxalic acid yield (the slope = 0.84 with R 2 ∼ 1) was obtained by the linear regression over 60 min aqueousphase photochemistry.Introduction

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Full Plots of batch simulations conducted from 10 µM to 10 M of initial glyoxal concentrations are similar to Fig. 5b (R 2 ≥∼ 0.9, 0 ≤ t ≤ t max ∼ 20-∼ 40 min), with oxalic acid being the main product below ∼ 10 mM) and oligomers above ∼ 10 mM.Oligomers were calculated as the sum of products with higher carbon number than the precursor (Lim et al., 2010).Plots of CSTR simulations conducted from 0.1 to 100 µM of initial glyoxal concentrations are similar to Fig. 5c (R 2 ∼ 1, 0 ≤ t ≤ 60 min), with oxalic acid being the main product.

Particle fraction
SOA yields also depend on what fraction of each aqueous chemistry remains in the particle phase (Seinfeld and Pankow, 2003).We expect that oligomers stay entirely in the particle phase.In this work, we assume that dicarboxylic acid products of C 3 or higher, such as malonate (C 3 ) or tartarate (C 4 ), remain entirely in the particle phase.
The gas-particle partitioning of the smaller organic acids (e.g., oxalate, pyruvate) depends on whether they are present in the atmosphere as acids or salts, since their salts have much lower vapor pressures (Limbeck, et al., 2001;Martinelango et al., 2007;Smith et al., 2009;Ortiz-Montalvo et al., 2012).For example, the vapor pressure of oxalic acid (at 25 • C) is 8.26×10 −5 Torr (Saxena and Hildemann, 1996), whereas the vapor pressure of ammonium oxalate is 5.18 × 10 −8 Torr (EPA, 2011).In this work, we assume that 90 % of oxalate and 70 % of pyruvate remain in the particle phase, based on atmospheric measurements (Lim et al., 2005;Ervens et al., 2007).However, these particle fractions could vary based on availability of organic/inorganic constituents (e.g., NH 3 , amines, sodium).Note that the yields calculated in this work neglect the formation of glyoxal and methylglyoxal oligomers through droplet evaporation (Loeffler et al., 2006;De Haan et al., 2009).In summary, SOA yields were estimated using the simulation results from glyoxal/methylglyoxal precursor concentrations from 10 −5 to 10 M, and the literature particle fraction values from atmospheric measurements (e.g., 90 % for oxalate, 70 % for pyruvate and 100 % for oligomers) (Table 1).Introduction

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Full model radical-radical reactions are excluded when the radical is on the primary carbon (e.g., acetic acid/pyruvic-OH radical reactions).

Aqueous-phase reactions of methylglyoxal with OH radical
Figure 3 illustrates the reaction mechanisms for the aqueous-phase OH radical reactions of methylglyoxal.Major products are pyruvic, acetic, and oxalic acid.Bold arrows indicate the major pathways.Pyruvic acid is the major first-generation product from OH radical reaction of methylglyoxal, and acetic acid is formed substantially from OH radical reactions of pyruvic acid and partially from bimolecular peroxy radical reactions and H 2 O 2 -pyruvic acid reactions.Oxalic acid is formed directly from glyoxylic and mesoxalic acids, which are products of every pathway shown in Fig. 3.
The first step of the OH radical reactions is H-atom abstraction from the primary carbon (minor) or the carbon in between the diol (major), then peroxy radical formation by O 2 addition.In the minor pathway, peroxy radicals undergo RO 2 -RO 2 reactions, and form alkoxy radicals (the Benson pathway A) or C 3 organic compounds (the Russell pathway B).The alkoxy radical decomposes to formaldehyde and an organic radical compound, and this organic radical later becomes glyoxylic acid.The C 3 organic compounds from the Russell pathway B react with OH radical and eventually form mesoxalic acid.In the major pathway, peroxy radicals either decompose to pyruvic acid while losing HO 2 (major) or undergo RO 2 -RO 2 reactions (minor), which eventually lead to the formation of carbon dioxide and acetic acid.It should be noted that pyruvic acid reacts with H 2 O 2 and forms acetic acid, carbon dioxide, and water; however, this is minor and OH radical oxidation is the major pathway.The OH radical oxidation of pyruvic acid occurs by H-atom abstraction from the primary carbon or the carboxylic group.The peroxy radical from the radical on the primary carbon forms via the O 2 addition, and undergoes RO 2 -RO 2 reactions.In this RO 2 -RO 2 reaction, oxalic acid and mesoxalic acid are eventually formed via Benson/Russell pathways and alkoxy radical chemistry.The organic radical product from the H-atom abstraction from the carboxylic group Introduction

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Full decomposes to carbon dioxide and a C 2 aldehyde radical, which eventually becomes acetic acid.

Kinetic model
Reactions and rate/equilibrium constants used in the full kinetic model of glyoxal/methylglyoxal + OH are provided in Table S1.Detailed reaction mechanisms for the decomposition of tetroxides are still not understood, and therefore, calculation of the branching ratio for two pathways A and B from theory is not possible (Dibble, 2007).
In this work, the same branching ratio of 95 % (A) to 5 % (B) was used for acetic, pyruvic acids and methylglyoxal, and this branching ratio was determined based on the ESI-MS intensities of the acetic acid oxidation products, glyoxylic and glycolic acid with an assumption that glycolic acid is only produced in the Russell pathway (B), whereas glyoxylic acid is produced in the both A and B pathways (Figs. 2 and 3).Note that this branching ratio is expected to be independent of initial precursor concentrations because decomposition of tetroxides is the unimolecular decay.
Although there is a literature rate constant (6×10 7 M −1 s −1 ; Stefan and Bolton, 1999) for the OH radical reaction of pyruvic acid, in our knowledge, there are no detailed literature rate constants for H-atom abstractions from the primary carbon and from the carboxylic group.In this work the branching ratio of 85 % to 15 % (H-atom abstraction from the primary carbon vs. from the carboxylic group) was used based on the estimation method suggested Monod et al. (2005).Introduction

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Full the model was expanded by including comprehensive methylglyoxal-OH radical chemistry: OH radical reactions of acetic acid, pyruvic acid, and methylglyoxal, and light absorption by H 2 O 2 and light absorbing organic compounds (e.g., methylglyoxal, pyruvic acid).The model, then, was validated by simulating the laboratory experiments of Tan et al. (2009Tan et al. ( , 2010Tan et al. ( , 2012)).

Glyoxal-OH radical model
The light absorption correction (Sect.2.4) was validated by simulating glyoxal + OH experiments (Fig. 6).For low concentration experiments (initial [glyoxal] = 30 µM), oxalic acid predicted by the previous glyoxal model (Lim et al., 2010) and this new model are identical and agree well with the experimental results (Fig. 6a).Simulations are identical because the H 2 O 2 concentration (decreasing from 150 µM H 2 O 2 ) was too low to affect photochemistry.H 2 O 2 absorbed less than 1 % of the transmitted light, and therefore including light absorption in the model had a negligible effect on OH production from H 2 O 2 photolysis.However, the light absorption correction by H 2 O 2 substantially improves the glyoxal-OH radical model simulation (Fig. 6b) at the higher radical chemistry: RO 2 -RO 2 reactions, the Benson/Russell pathways and the alkoxy radical chemistry (Fig. 7).In the model, the rate constant for the 1,2-H shift from the alkoxy radical is set to be 1 × 10 7 s −1 (Gilbert et al., 1976), while decomposition rates vary: 5 × 10 6 , 8 × 10 6 , 2 × 10 7 s −1 for initial [acetic acid] = 20, 100, 1000 µM, respectively.Those values were determined by fitting to the experimental results while their range is within literature values (∼ 10 6 -10 7 s −1 ) from Gilbert et al. (1981).Ideally, the decomposition rate from the alkoxy radical is constant regardless of the initial acetic acid concentration due to the first order, but pH or cage effects by water molecules could affect the rate.

Methylglyoxal-OH radical model
Simulations of the methylglyoxal + OH experiments are shown in Fig. 8.The methylglyoxal-OH model contains the same parameters used in the acetic acid-OH radical model except the decomposition rate at the initial [methylglyoxal] = 3000 µM: 3.2 × 10 7 s −1 instead of 2 × 10 7 s −1 .Using these values, the performance of the model simulations was substantially improved (Fig. 8a, b).It should be pointed out that in order to obtain the best fit to the experimental results we adjusted the literature rate constant and product (molar) yields of pyruvic acid photolysis (i.e., pyruvic acid → 0.45 acetic acid +0.55CO 2 , rate constant = 5 × 10 −4 s −1 ; Carlton et al., 2006).The rate constant for the photolysis of pyruvic acid must be adjusted to 1 × 10 −4 s −1 , which is 5 times slower than the literature rate.Moreover, it is assumed in the model that pyruvic acid photolyzes to only acetic acid, no CO 2 (Reaction 213 in Table S1).This need for such adjustments particularly by using a slower rate that the literature value could be evidence that in our reaction system, photolysis is not as important as OH-radical initiated reactions.Note: the purpose of the 254 nm UV lamp in these experiments is to provide an atmospherically-relevant OH radical concentration in the aqueous phase, and not to study photolysis.methylglyoxal, at 3000 µM it still does not capture the timing and the magnitude of oxalic acid formation until ∼ 200 min.Accounting for light absorption by H 2 O 2 is not sufficient to explain the oxalic acid profile in the 3000 µM methylglyoxal experiments.We hypothesize that this is because of the formation of light absorbing organic products that have the same time profile as pyruvic acid but a higher extinction coefficient (1500 cm −1 M −1 ) at 254 nm.Incorporating these "pyruvic acid surrogates" into the model significantly improves the model performance, resulting in an excellent agreement to the experimental values.While we did not identify these light absorbing products in our reaction vessel, light absorbing (brown carbon) products of other methylglyoxal reactions have been observed by others.For example, Sareen et al. (2010) observed the presence of UV light absorbing products with an estimated extinction coefficient of ∼ 5000 cm −1 M −1 from non-radical reactions of methylglyoxal in highly concentrated aqueous ammonium sulfate solutions.Our methylglyoxal-OH experiments did not contain ammonium sulfate or any other source of nitrogen.In these experiments light absorption could be due to a π-conjugate system formed possibly via aldol condensation (Sareen et al., 2010;Lim et al., 2010).Further work is needed to investigated to this hypothesis.

aqSOA yields under atmospheric conditions
According to the field study by Munger et al. (1995), glyoxal and methylglyoxal concentrations in the cloudwater are similar, ranging ∼ 0.1-∼ 300 µM.For the CSTR runs, 10 −7 -10 −4 M of glyoxal/methylglyoxal concentrations in the aqueous phase are considered.The equivalent gas-phase concentrations due to Henry's law are ∼ 0.3 ppt-∼ 0.3 ppb for glyoxal and ∼ 30 ppt-∼ 30 ppb for methylglyoxal, and those ranges reasonably agree with literature (Fu et al., 2008).For the runs, 10

Batch reactor approximation
Mass-based SOA yields for glyoxal and methylglyoxal are obtained by performing the batch reactor approximation are illustrated in Fig. 9 and summarized Again, this material is predicted to be entirely oligomeric.These particle phase product yields were estimated by fitting to the simulations (Fig. 9a, b) and making use of Eqs. ( 3) and (4).The total particle-phase yield as a function of concentration is given by: Similarly, for methylglyoxal, the total particle-phase yield as a function of concentration is given by: where

CSTR model
In order to estimate the yield, the reacted glyoxal or methylglyoxal concentrations should be known.Since it is not possible to directly obtain the reacted precursor Figures

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Full formaldehyde and CO 2 ), and all of the stoichiometric coefficients are assumed to be 1 except for CO 2 , which is 2 because two CO 2 molecules are produced from one oxalic acid molecule.A linear regression method is used to get the product yield during ∼ 1 h of glyoxal-OH reactions and ∼ 20 min of methylglyoxal-OH reactions (Fig. 5C), resulting R-square values close to 1.At cloud-relevant concentration, Y SOA CSTR(glyoxal) is 1.19 and solely contributed by oxalate (Fig. S2a).Y SOA CSTR(methylglyoxal) is 0.803, which is the sum of 0.759 from pyruvate and 0.0439 from oxalate (Fig. S2b).These CSTR yields are quite similar to batch yields at cloud conditions (Fig. 10a, b) and also summarized in Table 1.

Conclusions
Volatile but highly water soluble glyoxal (≤ 276 µM = the concentration measured in atmospheric waters), methylglyoxal (0.02-128 µM), and acetic acid/acetate (0.4-245 µM) are common organic compounds found in the atmosphere (Tan et al., 2012).They are mostly gas-phase photochemical fragments of anthropogenic/biogenic VOCs.Due to their small carbon number (C 2 -C 3 ), these compounds form no semivolatile products and, therefore, no SOA by gas-phase oxidation alone.However, since they are water soluble, they form SOA via aqueous-phase photochemistry in atmospheric waters.In clouds, the major products are oxalate and pyruvate, which remain in the particle phase by forming organic salts with inorganic or ammonium ions.In wet aerosols, the major products are oligomers, which stay entirely in the particle phase.The aqueous photochemistry of these compounds is initiated predominantly by OH radical, the current understanding is that the main source of aqueous OH radicals is uptake from the gas phase.Peroxy radical chemistry is key to understanding aqueous-phase OH radical reactions.At the cloud-relevant conditions, H-atom abstracted organic products react with dissolved O 2 forming peroxy radicals, which decompose to carboxylic acids (e.g., glyoxylic acid, oxalic acid, or pyruvic acid) and HO 2 .At the aerosol-relevant conditions, oligomers are formed via radical-radical reactions.The bimolecular-diffused RO 2 -RO 2 reactions constitute important peroxy radical chemistry for OH radical reactions of acetic, pyruvic acid and methylglyoxal.Alcohols and carbonyls are produced in the Russell pathway, whereas alkoxy radicals are produced in the Benson pathway.Alkoxy radicals undergo subsequent decomposition or a 1,2-H shift.
In this work, a full kinetic model was developed based on detailed reaction mechanisms and validated against laboratory experiments.The batch and CSTR simulations predict similar and substantial aqSOA yields.These simulation results are consistent with the expectation that aqueous chemistry is a substantial source of SOA globally.Certainly, other water soluble organic precursor will undergo similar aqueous chemistry.The processes examined in this work depend on the availability of OH radicals in clouds, fogs, and wet aerosols.Uptake from the gas phase is generally thought to be the major source of OH radicals to atmospheric waters, and steady-state aqueous concentrations of OH radicals from the gas phase may be influenced by droplet surfaceto-volume ratios and aqueous concentrations of reactants.The photo-Fenton reaction (i.e., the photooxidation of Fe ions in the presence of H 2 O 2 ) is considered the major OH radical formation inside the wet aerosol (Lim et al., 2005(Lim et al., , 2010)).Photolysis of nitrite/nitrate (Hullar and Anastasio, 2011) and organic matter (Dong and Rosario-Ortiz, 2012) can also produce OH radicals in aqueous particles.However, the degree of OH Introduction

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Full radical formation or "recycling" in atmospheric waters is not well understood and could represent an important, yet unrecognized oxidant source.Another issue that could impact SOA yields pertains to the gas-particle partitioning of products.The gas-particle partitioning of the carboxylic acid products depends on the availability of these products to form carboxylate salts, which have lower vapor pressures than the corresponding acids (e.g., oxalic acid vs. ammonium oxalate; Ortiz-Montalvo et al., 2012).Available measurements put oxalate predominantly in the particle phase, presumably as a salt.However, oxalate measurements are limited, and this might not be uniformly true.(Tan et al., 2012), and previous/current model simulations (using the model of Lim et al., 2005, this work).In (C), simulations were performed with and without the absorption (abs) correction by hypothetical light absorbing organic products, with the same time profile as pyruvic acid, but a much higher extinction coefficient (1500 cm −1 M −1 ).Introduction

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Full and in regions with high relative humidity and hygroscopic aerosol species.Water in clouds, fogs and aerosols provides an abundant and important Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | −3 (Finlayson-Pitts and Pitts, 2000).The initial concentration of H 2 O 2 in the aqueous phase was set to be zero.The maximum simulated H 2 O 2 concentration (largely formed via bimolecular HO 2 -HO 2 reactions in the aqueous Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | concentration (initial [glyoxal] = 3000 µM), where the initial H 2 O 2 concentration was 15 mM.(Note, higher H 2 O 2 concentrations were used in experiments with higher glyoxal/methylglyoxal concentrations in order to maintain similar OH concentrations in all experiments.)Note both the Lim et al. (2010) and the current model include organic radical-radical reactions, resulting in improved prediction of oxalic acid in the 3000 µM experiments compared to the Lim et al. (2005) dilute chemistry model.By correcting for light absorption by H 2 O 2 in the current work, the model now captures the timing of the peak (Fig. 6b).4.1.2Acetic acid-OH radical model Next, the performance of the expanded model was evaluated by simulating acetic acid + OH experiments.Model performance was improved by including detailed peroxy Discussion Paper | Discussion Paper | Discussion Paper | Figure 8c (initial [methylglyoxal] = 3000 µM) is interesting.Although the new model successfully fit oxalate measurements from the OH reactions of 30, 300 µM of Discussion Paper | Discussion Paper | Discussion Paper | −7 -10 M of glyoxal/methylglyoxal concentrations are considered with the highest concentrations comparable to concentrations of water soluble organics in wet aerosols.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | online at: http://www.atmos-chem-phys-discuss.net/13/4687Discussion Paper | Discussion Paper | Discussion Paper | Arakaki, T. and Faust, B. C.: Sources, sinks, and mechanisms of hydroxyl radical (OH) photoproduction and consumption in authentic acidic continental cloud waters from Whiteface Mountain, New York: the role of Fe(r) (r = II, III) photochemical cycle, J. Geophys.Res., 103, 3487-3504, 1998.Atkinson, R.: Rate constants for the atmospheric reactions of alkoxy radicals: an updated esti-Discussion Paper | Discussion Paper | Discussion Paper | Nishino, N., Arey, J., and Atkinson, R.: Formation yields of glyoxal and methylglyoxal from the gas-phase OH radical-initiated reactions of toluene, xylenes, and trimethylbenzenes as a function of NO 2 concentration, J. Phys.Chem.A, 114, 10140-10147, 2010.Nolte, C. G., Fraser, M. P., and Cass, G. R.: Gas phase C 2 -C 10 organic acids concentrations in the Los Angeles atmosphere, Environ.Sci.Technol., 33, 540-545, 1999Discussion Paper | Discussion Paper | Discussion Paper | Tan, Y., Carlton, A. G., Seitzinger, S. P., and Turpin, B. J.: SOA from methylglyoxal in clouds and wet aerosols: measurements and prediction of key products, Atmos.Environ., 44, 5218-5226, 2010.Tan, Y., Lim, Y. B., Altieri, K. E., Seitzinger, S. P., and Turpin, B. J.: Mechanisms leading to oligomers and SOA through aqueous photooxidation: insights from OH radical oxidation of Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .
Fig. 1.Mechanism for peroxy radical reactions (a).The pathway A is suggested by Benson (1965) forming alkoxry radicals, followed by decomposition (I) or 1,2-hydride shift (J).The pathway B forming no organic radical product (i.e., B is a concerted reaction) is suggested by Russell et al. (1957).Parent precursors can be acetic acid or pyruvic acid (b).Fragmented organic radicals (R*) are expected to be stabilized by a carboxylic group for acetic acid (c left), a diol, which results from hydration of a carbonyl group for pyruvic acid (c right).

Fig. 1 .Fig. 2 .
Fig. 1.Mechanism for peroxy radical reactions (a).The pathway A is suggested byBenson (1965) forming alkoxry radicals, followed by decomposition (I) or 1,2-hydride shift (J).The pathway B forming no organic radical product (i.e., B is a concerted reaction) is suggested byRussell et al. (1957).Parent precursors can be acetic acid or pyruvic acid (b).Fragmented organic radicals (R * ) are expected to be stabilized by a carboxylic group for acetic acid (c left), and a diol, which results from hydration of a carbonyl group for pyruvic acid (c right).

Fig. 10 .
Fig. 10.(A) The CSTR simulation and the batch simulation for particle-phase mass based yields of oxalate (Y OXLAC ) with increasing initial concentrations of glyoxal for glyoxal + OH. (B) The CSTR simulation and the batch simulation for particle-phase mass based yields of pyruvate (Y PYRAC ) and oxalate (Y OXLAC ) with increasing initial concentrations of methylglyoxal for methylglyoxal + OH.
in Table 1.At cloud-relevant concentrations, Y SOA Batch(glyoxal) is 1.20 and is solely contributed by oxalate.Y SOA Batch(methylglyoxal) is 0.772, which is the sum of 0.659 from pyruvate and 0.113 from oxalate.Using glyoxal as a surrogate for dissolved water-soluble organics, the aerosol-relevant Y SOA Batch(glyoxal) is 0.937.If instead methylglyoxal is used as a surrogate for total dissolved water-soluble organics, the aerosol-relevant

Table 1 .
Product yields, particle-phase product yields and SOA yields.