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Atmospherically abundant, volatile water-soluble organic compounds formed through gas-phase chemistry (e.g., glyoxal (C2), methylglyoxal (C3), and acetic acid) have great potential to form secondary organic aerosol (SOA) via aqueous chemistry in clouds, fogs, and wet aerosols. This paper (1) provides chemical insights into aqueous-phase OH-radical-initiated reactions leading to SOA formation from methylglyoxal and (2) uses this and a previously published glyoxal mechanism (Lim et al., 2010) to provide SOA yields for use in chemical transport models. Detailed reaction mechanisms including peroxy radical chemistry and a full kinetic model for aqueous photochemistry of acetic acid and methylglyoxal are developed and validated by comparing simulations with the experimental results from previous studies (Tan et al., 2010, 2012). This new methylglyoxal model is then combined with the previous glyoxal model (Lim et al., 2010), and is used to simulate the profiles of products and to estimate SOA yields. At cloud-relevant concentrations (∼ 10−6 − ∼ 10-3 M; Munger et al., 1995) of glyoxal and methylglyoxal, the major photooxidation products are oxalic acid and pyruvic acid, and simulated SOA yields (by mass) are ∼ 120% for glyoxal and ∼ 80% for methylglyoxal. During droplet evaporation oligomerization of unreacted methylglyoxal/glyoxal that did not undergo aqueous photooxidation could enhance yields. In wet aerosols, where total dissolved organics are present at much higher concentrations (∼ 10 M), the major oxidation products are oligomers formed via organic radical-radical reactions, and simulated SOA yields (by mass) are ∼ 90% for both glyoxal and methylglyoxal. Non-radical reactions (e.g., with ammonium) could enhance yields.

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Atmos. Chem. Phys., 13, 8651–8667, 2013

www.atmos-chem-phys.net/13/8651/2013/

doi:10.5194/acp-13-8651-2013

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Chemical insights, explicit chemistry, and yields of secondary

organic aerosol from OH radical oxidation of methylglyoxal and

glyoxal in the aqueous phase

Y. B. Lim

, Y. Tan

, and B. J. Turpin

Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA

Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA

Correspondence to: Y. B. Lim (ylim@envsci.rutgers.edu)

Received: 19 January 2013 – Published in Atmos. Chem. Phys. Discuss.: 19 February 2013

Revised: 26 July 2013 – Accepted: 26 July 2013 – Published: 3 September 2013

Abstract. Atmospherically abundant, volatile water-soluble

organic compounds formed through gas-phase chemistry

(e.g., glyoxal (C

), methylglyoxal (C

), and acetic acid) have

great potential to form secondary organic aerosol (SOA)

via aqueous chemistry in clouds, fogs, and wet aerosols.

This paper (1) provides chemical insights into aqueous-

phase OH-radical-initiated reactions leading to SOA forma-

tion from methylglyoxal and (2) uses this and a previously

published glyoxal mechanism (Lim et al., 2010) to provide

SOA yields for use in chemical transport models. Detailed

reaction mechanisms including peroxy radical chemistry and

a full kinetic model for aqueous photochemistry of acetic

acid and methylglyoxal are developed and validated by com-

paring simulations with the experimental results from pre-

vious studies (Tan et al., 2010, 2012). This new methylgly-

oxal modelis then combined with the previous glyoxal model

(Lim et al., 2010), and is used to simulatethe proﬁles of prod-

ucts and to estimate SOA yields.

At cloud-relevant concentrations (∼ 10

−6

− ∼ 10

−3

Munger et al., 1995) of glyoxal and methylglyoxal, the ma-

jor photooxidation products are oxalic acid and pyruvic acid,

and simulated SOA yields (by mass) are ∼ 120% for glyoxal

and ∼ 80% for methylglyoxal. During droplet evaporation

oligomerization of unreacted methylglyoxal/glyoxal that did

not undergo aqueous photooxidation could enhance yields.

In wet aerosols, where total dissolved organics are present

at much higher concentrations (∼ 10M), the major oxidation

products are oligomers formed via organic radical–radical re-

actions, and simulated SOA yields (by mass) are ∼ 90 % for

both glyoxal and methylglyoxal. Non-radical reactions (e.g.,

with ammonium) could enhance yields.

1 Introduction

Water is predicted to be the largest component of ﬁne par-

ticles (PM

2.5

) globally (Liao and Seinfeld, 2005) and in re-

gions with high relative humidity and hygroscopic aerosol

species. Water inclouds, fogs,and aerosols provides an abun-

dant and important medium for chemistry, including chem-

istry 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). Recent laboratory, ﬁeld and modeling studies

suggest that several water-soluble organic compounds dis-

solve in atmospheric waters (e.g., cloud/fog droplets and wet

aerosols) andundergo aqueous radical and non-radical chem-

istry to form SOA (e.g., Blando and Turpin, 2000; Ervens et

al., 2011; Gong et al., 2011; Myriokefalitakis et al., 2011;

Lee et al., 2011, 2012; Zhou et al., 2011; Tan et al., 2012;

Ortiz-Montalvo et al., 2012; J. Liu et al., 2012; Y. Liu et al.,

2012; Lin et al., 2012; McNeill et al., 2012). Hereafter, this

type of SOA is denoted as aqSOA.

In a previous publication (Lim et al., 2010), we devel-

oped a full kinetic model including detailed radical chem-

istry to describe aqSOA formation via OH radical oxidation

of glyoxal, an abundant and highly water-soluble compound

Published by Copernicus Publications on behalf of the European Geosciences Union.

8652 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

formed through photooxidation of alkenes and aromatics.

The current work is focused on methylglyoxal. Methylgly-

oxal 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 photoox-

idation of both anthropogenic VOCs like aromatic hydro-

carbons (Nishino et al., 2010) and biogenic VOCs includ-

ing 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 reac-

tions in clouds and wet aerosols, due to its high water solu-

bility (H

eff

= 3.71 × 10

Matm

−1

; Betterton and Hoffmann,

1988), ability to form oligomers via acid catalysis, and reac-

tivity with OH radicals (Blando and Turpin, 2000; De Haan

et al., 2009; Sareen et al., 2010; Tan et al., 2010, 2012).

Acetic acid (H

eff

= 5.50 × 10

Matm

−1

; Herrmann et al.,

2005) is highly water soluble, atmospherically abundant both

in the gas phase (∼ 300ppt; Nolte et al., 1999) and in the

aqueous phase (Khare et al., 1999), and also a major inter-

mediate product of methylglyoxal + OH (Tan et al., 2012). It

should be noted that methylglyoxal and acetic acid are much

more reactive with OH radicals in the aqueous phase than in

the gas phase (lifetimes in the aqueous phase are ∼ 26min

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 rad-

ical oxidation of methylglyoxal is proposed. Detailed radi-

cal chemistry includes peroxy radical (RO

) chemistry initi-

ated by bimolecular reactions (RO

–RO

reactions). We val-

idate, in part, the methylglyoxal model by comparing results

from aqueous photooxidation experiments developed by Tan

et al. (2010, 2012) with model simulations of these exper-

iments. Note that in aqueous photooxidation experiments,

OH radicals are formed through UV photolysis of H

whereas in the atmosphere, uptake from the gas phase is the

dominant known source (Ervens et al., 2003a), with addi-

tional contributions from aqueous (e.g., Fenton, nitrate) re-

actions (Arakaki and Faust, 1998; Zepp et al., 1987). In this

work, experimental results are better captured after taking

into account the absorptionof UV by H

and organic com-

pounds. Finally, the combined glyoxal and methylglyoxal

model is used to simulate aqSOA formation via OH radical

oxidation under a range of atmospheric conditions, including

cloud-relevant conditions (10µM) and higher concentrations.

Runs at 10M are intended to provide insights into OH radical

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

∼ 2–3µgCm

−3

and estimated aerosol water concentrations

of ∼ 10µgm

−3

at RH>70%; Hennigan et al., 2009; Volka-

mer et al., 2009). Note that non-radical chemistry is not in-

cluded in these aerosol SOA yields. Methylglyoxal and gly-

oxal aqSOA yields are reported for conditions encountered

by clouds and by wet aerosols based on two types of simu-

lations: a "batch reactor" approach, in which the precursor

(methylglyoxal or glyoxal) is depleted as OH radical reac-

tions proceed, and a steady-state "continuously stirred tank

reactor" (CSTR) approach, in which the precursor is replen-

ished (held constant) in the aqueous phase.

2 Methods

2.1 Experiments used to evaluate chemical modeling

Aqueous methylglyoxal chemistry and yields are developed

herein, making use of chemical theory and previously pub-

lished aqueous photooxidation experiments conducted with

OH radicals and methylglyoxal or acetic acid (an intermedi-

ate product). Experiments were conducted at cloud-relevant

and higher concentrations, but concentrations were still sev-

eral orders of magnitude lower than the concentrations of

water-soluble organic compounds in wet aerosol. Experi-

mental details are provided elsewhere (Tan et al., 2010,

2012). Brieﬂy, methylglyoxal (30, 300, and 3000µM) or

acetic acid (20, 100, and 1000 µM) was dissolved in 18 M

Milli-Q water. OH radicals (10

−14

–10

−12

M) were generated

by photolysis (254nm with Hg UV lamp) of hydrogen per-

oxide. Reaction temperature was maintained at ∼ 25

◦

C. Dis-

solved O

measured at the beginning and the end of each ex-

periment was saturated. pH decreased from 6.6 to 3.3 over

the course of the 360min experiments. Samples were ana-

lyzed by ion chromatography (IC), unit mass resolution elec-

trospray ionization mass spectrometry (ESI-MS), and ESI-

MS after preseparation in the IC (IC-ESI-MS). Control ex-

periments were conducted for both organics (methylglyoxal

and acetic acid) as follows: organic + UV, organic + H

+ UV, mixed standard + H

, and mixed standard +

UV. Mixed standards contained pyruvic, acetic, formic, ox-

alic, glyoxylic, glycolic, succinic,and malonic acids (250µM

each). Note that H

was measured by the triiodide method

(Banerjee et al., 1964) using a UV-visible spectrometer in

+ UV experiments.

2.2 Peroxy radical chemistry

Peroxy radical chemistry plays an important role in the aque-

ous chemistry of methylglyoxal, which is described in de-

tail 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

in atmospheric waters (Herrmann,

2003). Peroxy radicals subsequently undergo two possible

reaction pathways: (1) self decomposition giving off HO

and forming acids, and (2) bimolecular RO

–RO

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. How-

ever, further OH reactions of pyruvic acid and acetic acid,

Atmos. Chem. Phys., 13, 8651–8667, 2013 www.atmos-chem-phys.net/13/8651/2013/

Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8653

Fig. 1. Mechanism for peroxy radical reactions (a). The pathway A is suggested by Benson (1965), forming alkoxy 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) and a diol, which results from hydration of a carbonyl group for pyruvic acid ((c) right).

two major products of the methylglyoxal + OH, involve

–RO

reactions.

Figure 1a illustrates peroxy radical chemistry initiated by

–RO

reactions. For acetic acid/pyruvic acid + OH re-

actions, peroxy radicals form by the addition of O

to the

primary carbon, followed by RO

–RO

reactions forming

tetroxides. This pathway is preferred over decomposition be-

cause of the absence of a hydroxyl group nearby. Two well-

known decomposition pathways from tetroxide are alkoxy

radical/O

formation (A in Fig. 1a) suggested by Benson

(1965), and alcohol/aldehyde/O

formation (B in Fig. 1a)

suggested by Russell et al. (1957). The Benson pathway (A)

and the Russell pathway (B) are not related and are indepen-

dent because the Russell pathway (B) is a concerted reaction,

so none of the products are formed via alkoxy radical chem-

istry.

Alkoxy radicals formed in the Benson pathway (A) un-

dergo either decomposition (I) or a 1,2-hydride shift (J)

(Figs. 1, 2, and 3). In Fig. 1a, resulting products through de-

composition of alkoxy radicals (I) are organic radicals (R

)

and aldehydes (=O). Gas-phase chamber studies suggest that

decomposition of alkoxy radicals is likely to occur if a radi-

cal position in an organic radical product (R

) is at secondary

or tertiary carbons, and is enhanced when functional groups

(e.g, hydroxylor carboxylic groups) are adjacent to these car-

bons (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. Decompo-

sition 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 radi-

cals 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-hydride shift (followed by O

reactions

to form carbonyls) is not observed in the gas phase (Atkin-

son, 2007), alkoxy radicals in the aqueous phase do undergo

1,2-hydride shift. Although the detailed reaction mechanisms

are not well understood, the 1,2-hydride shift is very likely to

be assisted by water molecules (Von Sonntag et al., 1997).

2.3 Kinetic model

As done previously for glyoxal (Lim et al., 2010), we devel-

oped a full kinetic model for aqueous chemistry of methyl-

glyoxal with OH radicals at cloud- and aerosol-relevant con-

centrations including the following: (1) the formation of or-

ganic acids such as acetic, glyoxylic, glycolic, pyruvic, ox-

alic, and mesoxalic acid (Lim et al., 2005; Tan et al., 2009,

2010, 2012); (2) organic radical–radical reactions to form

higher carbon number acids and oligomers; and (3) peroxy

radical chemistry, includingself decomposition and bimolec-

ular RO

–RO

reactions. The model was ﬁrst validated by

comparison against acetic acid + OH radical experiments

(Tan et al., 2012), since acetic acid is an important intermedi-

ate product. Then, using the same rate constants, model pre-

dictions were compared with concentration dynamics from

www.atmos-chem-phys.net/13/8651/2013/ Atmos. Chem. Phys., 13, 8651–8667, 2013

8654 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

Fig. 2. Reaction mechanisms for the reactions of acetic acid with OH radicals in the aqueous phase. RO

represents bimolecular RO

–RO

reactions.

methylglyoxal + OH radical experiments (Tan et al., 2010,

2012).

Most of the kinetic rate constants were obtained from the

literature documented in Tan et al. (2009, 2010, 2012), or

determined using an estimation method based on structure–

activity relationships (Monod et al., 2005, 2008). Values

from Ervens et al. (2003b) were also used for OH-radical-

initiated reactions. For the radical–O

(peroxy radical for-

mation) and organic radical–radical reactions, the rate con-

stants of 1 × 10

−1

and 1.3 × 10

−1

, respec-

tively, were used as suggested by Guzman et al. (2006). The

following were used for peroxy radical chemistry: a rate con-

stant of 3 × 10

−1

for bimolecular RO

–RO

reactions

(Lim et al. 2010), a rate constant of 1 × 10

−1

for the 1,2-

hydride shift (Gilbert et al., 1976), and a rate constant on the

order of 10

to 10

−1

for decomposition from the alkoxy

radical (Gilbert et al., 1981).

This model, which we describe in detail in Sect. 3, was

validated in part by comparison with laboratory experiments

and then used to simulate the atmosphere.

2.4 Determining the rate constants for H

photolysis

The performance of a glyoxal model (Lim et al., 2010),

which includes detailed radical reactions (e.g., H-atom ab-

straction by OH radicals, peroxy/alkoxy radical reactions,

decompositions, and organic radical–radical reactions) was

substantially improved at high concentrations (∼ mM) by in-

cluding organic radical–radical reactions in the model. Be-

low we describe a further improvement to the glyoxal model

that we then apply also to methylglyoxal. Speciﬁcally, since

OH radicals in experiments are produced in situ from H

photolysis and H

is light absorbing, we correct for light

absorption by H

Previously (Tan et al., 2010), the rate constant (1.1 × 10

−1

) for 254nm UV photolysis of H

at 0.15, 1.5, and

20mM in experiments was determined by ﬁtting modeled

concentrations (R1–R7) to measurements in H

UV control experiments (Fig. 4a–c) as documented by Tan et

al. (2009, 2010, 2012).

→ 2OH (k

= 1.1 × 10

−1

) (R1)

OH + H

→ HO

+ H

O (k

= 2.7 × 10

−1

) (R2)

+ H

→ OH + H

O + O

= 3.7M

−1

) (R3)

2HO

→ H

+ O

= 8.3 × 10

−1

) (R4)

OH + HO

→ H

O + O

= 7.1 × 10

−1

) (R5)

−

→H

=1.0 × 10

−1

) (R6)

Atmos. Chem. Phys., 13, 8651–8667, 2013 www.atmos-chem-phys.net/13/8651/2013/

Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8655

Fig. 3. Reaction mechanisms for the reactions of methylglyoxal with OH radicals in the aqueous phase. The bold arrows represent dominant

pathways.

2OH → H

= 1.0 × 10

−1

) (R7)

However, in the previous work, light absorption by H

was

not taken into account. The H

photolysis rate constant

photo

) can be corrected using Beer's law,

photo

) = k

−b

ext

L [H

]

−1

), (1)

where b

ext

is extinction coefﬁcient (M

−1

) and L is path

length (cm).

An extinction coefﬁcient for H

of 18.4M

−1

was

used (Stefan et al., 1996). A path length of 0.80 cm provides

the best ﬁt for all three H

concentrations (Fig. 4d–f). This

valueis reasonable since it is close to the actualpath length of

the reaction vessel (1.04cm). Accounting for UV absorption

by H

provides substantial improvement in the R

values

at the highest concentration from R

= 0.80 (Fig. 4a) to 0.96

with correction (Fig. 4d). Thus, in the revised model,

photo

) = (1.1 × 10

)10

−18.4 × 0.80 ×

[

]

−1

). (2)

www.atmos-chem-phys.net/13/8651/2013/ Atmos. Chem. Phys., 13, 8651–8667, 2013

8656 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

Fig. 4. Real-time proﬁles of 254 nm UV photolysis decay of H

(where initial concentrations are 0.15, 1.5, and 20 mM) without light-

absorption correction (A, B, and C) and with the correction (D, E, and F). The correction was made by multiplying the transmittance by the

photolysis decay rate constant, where –log (transmittance) = 18.4 × 0.80 × [H

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 12.7M

−1

(Tan et al., 2010).

However, these photolysis reactions turn out to be negligi-

ble during OH radical reactions because photolysis rates are

much slower than OH radical reaction rates (Stefan et al.,

1996; Tan et al., 2010).

Both simulated OH concentrations ( ∼ 10

−12

M) and simu-

lated and measured pH (3 to 5) reasonably reﬂect cloud con-

ditions (Faust, 1994; Hermann, 2003). Simulated dissolved

remains saturated during the entire reaction (Fig. S1 in

Supplement), in agreement with measured O

at the begin-

ning and end of experiments. Note that dissolved O

in at-

mospheric waters is expected to be saturated due to high the

surface-to-volume ratio of cloud droplets and wet aerosols.

2.5 Atmospheric simulations

Unlike laboratory experiments, the major source of OH rad-

icals in the atmospheric aqueous phase is believed to be up-

take from the gas phase, although aqueous sources also con-

tribute (e.g., through Fenton and nitrate reactions; Arakaki

and Faust, 1998; Lim et al., 2005, 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

a value maintained by Henry's law equilibrium with the gas-

phase OH radical concentration of 2 × 10

moleculecm

−3

(Finlayson-Pitts and Pitts, 2000). This is likely an upper

bound, as discussed in Sect. 5. The initial concentration of

in the aqueous phase was set to be zero. The maxi-

mum simulated H

concentration (largely formed via bi-

molecular HO

–HO

reactions in the aqueous phase) from

photooxidation of 30µM of initial glyoxal is ∼ 20µM, which

is reasonable in atmospheric waters according to Henry's

law equilibrium with an atmospheric concentration of H

(∼ 0.2 ppb) in the gas phase (Warneck, 1999).

The following atmospheric processes are needed to model

aqSOA: (1) glyoxal and methylglyoxal production via gas-

phase photooxidation, (2) glyoxal and methylglyoxal uptake

by atmospheric waters (i.e., Henry's law equilibrium be-

tween gas- and aqueous-phase glyoxal and methylglyoxal),

(3) uptake or condensed-phase production of oxidants (e.g.,

OH radicals), (4) aqueous-phase reactions in the atmospheric

waters forming low or semivolatile products, and (5) gas–

particle partitioning of products. Ideally, all these processes

Atmos. Chem. Phys., 13, 8651–8667, 2013 www.atmos-chem-phys.net/13/8651/2013/

Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8657

including detailed aqueous chemistry would be included in

the model. When this is not practical, SOA yields can be use-

ful, enabling prediction of SOA by multiplying the amount

of "aqueous glyoxal reacted" in a time step by the estimated

SOA yield. The production of particle-phase products via

aqueous OH radical oxidation (step 4–5) can be approxi-

mated ina batch or CSTR framework.In the batch reactor ap-

proximation, aqueous-phase OH radical reactions are limited

by photochemical production of glyoxal and methylglyoxal

in the gas phase. Slow glyoxal/methylglyoxal production re-

sults 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 lim-

ited by gas-phase photochemical production of glyoxal and

methylglyoxal. In contrast to the batch reactor, glyoxal and

methylglyoxal are continuously taken up into atmospheric

waters and never depleted in that medium. This is a better as-

sumption when gas-phase production is faster than aqueous

reaction. Which of these approximations is more appropriate

depends on what the major precursors are in the particular

study area. For example, a batch approximation would be

appropriate if the dominant methylglyoxal precursors were

toluene. The gas-phase lifetime of toluene due to OH (when

[OH] in the gas phase is 2 × 10

moleculecm

−3

(12 h day-

time average); Finlayson-Pitts and Pitts, 2000) is ∼ 1 day

(k = 5.63 × 10

−12

molecule

−1

; Atkinson and Arey,

2003) and it produces ∼ 20% methylglyoxal (Nishino et al.,

2010). This methylglyoxal can dissolve in cloud/fog waters

and react with OH radicals with the lifetime of 4.3 h (when

[OH] in the aqueous phase is assumed to be 1 × 10

−13

M).

A batch approximation is appropriate since in this case the

gas-phase production of methylglyoxal is slower than the

aqueous-phase OH oxidation of methylglyoxal. In contrast, a

CSTR approximation would be more appropriate if the pro-

duction of methylglyoxal was dominated by compounds like

1,3,5-trimethylbenzene (which produces ∼ 50 % methylgly-

oxal; Nishino et al., 2010). This is because the gas-phase life-

time of 1,3,5-trimethylbenzene due to OH radicals is only 2.5

h. Thus, in this case the gas-phase production of the methyl-

glyoxal precursor is now faster than its aqueous oxidation.

While we expect OH oxidation to be the dominant daytime

aqSOA formation pathway in clouds and an important con-

tributor to wet aerosol chemistry (Lim et al., 2010), it should

be recognized that the chemistry in wet aerosols is highly

complexand poorlyunderstood. Both radical and non-radical

reactions are likely to occur in wet aerosols and the rela-

tive importance of these pathways may depend on the degree

to which OH radicals are produced or recycled in the con-

densed phase, which is not well understood currently. Un-

doubtedly, organic–inorganic interactions play an important

role in wet aerosol chemistry (e.g., formation of organosul-

fates and organic nitrogen compounds); the yields reported

here are not intended to represent all processes, only aqSOA

formed through OH radical reaction.

2.6 SOA yields from atmospheric photochemical

simulations

Given

A + OH →

, (R8)

where A is glyoxal or methylglyoxal and P

is product i , the

product yield for P

) is given by

]

[A]

reacted

. (3)

Then the overall SOA yield (Y

SOA

) is deﬁned as

SOA

]

[A]

reacted

, (4)

where F

is the particle fraction of P

and [A]

reacted

the concentration of unhydrated A reacted with OH rad-

ical 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.

2.6.1 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 10µM initial [glyoxal] is

plotted with the reaction time (x axis) in Fig. 5a. In the previ-

ous 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 photochem-

istry ( t = 0) to the peak (t

max

= 38min) giving a reasonably

low error (R

∼ 0.9), represents the average (molar) yield of

oxalic acid. In CSTR simulation plots, oxalic acid continu-

ously increases and never drops as glyoxal reacts (Fig. 5c).

A similar oxalic acid yield (slope = 0.84 with R

∼ 1) was

obtained by linear regression over 60min of aqueous-phase

photochemistry.

Plots of batch simulations conducted from 10µM to 10M

of initial glyoxal are similar to Fig. 5b (R

≥ 0.9, 0 ≤ t ≤

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8658 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

Fig. 5. Simulated results for the OH radical oxidation of glyoxal,

where initial [glyoxal] is 10

−5

M. (A) Time proﬁle of oxalic acid

yield from the OH radical reactions of glyoxal, batch reactor ap-

proximation. The maximum molar yieldof ∼ 0.9 was estimated pre-

viously (Lim et al., 2010). (B) Oxalic acid (M) vs. glyoxal reacted

(M), batch reactor approximation. In this plot, the linear regres-

sion was performed for 0–38 min (y = 0.797x , R

= 0.909), and

the slope (0.797) represents the average molar yield of oxalic acid.

for 0–38min. The average molar yield of oxalic acid (the slope) is

0.841 with higher precision (R

= 0.998).

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

are similar to Fig. 5c (R

∼ 1, 0 ≤ t ≤ 60min), with oxalic

acid being the main product.

2.6.2 Particle fraction

SOA yields also depend on what fraction of each aqueous

product 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 prod-

ucts of C

or higher, such as malonate (C

) or tartrate (C

remain entirely in the particle phase. The gas–particle parti-

tioning 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 pres-

sures (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 pyru-

vate remain in the particle phase (Lim et al., 2005; Ervens

et al., 2007). However, these particle fractions could vary

based on availability of organic/inorganic constituents (e.g.,

, 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 simula-

tion results from glyoxal/methylglyoxal precursorconcentra-

tions from 10

−5

to 10M, and literature particle fraction val-

ues from atmospheric measurements (e.g., 90% for oxalate,

70% for pyruvate, and 100% for oligomers) (Table 1).

3 Aqueous photochemistry of methylglyoxal:

mechanisms and kinetic model

3.1 Aqueous-phase reactions of acetic acid with OH

radical

Reaction mechanisms for the aqueous-phase OH radical ox-

idation of acetic acid are proposed in Fig. 2. Acetic acid

is oxidized by H-atom abstraction either from OH in the

carboxylic group (*O(O)CCH

; * represents radical) or the

methyl group (HO(O)CC*H

). [*O(O)CCH

] decomposes

to carbon dioxide and a methyl radical (*CH

). This [*CH

]

forms [*OOCH

] by O

addition and eventually becomes

methanol and formaldehyde via RO

–RO

reactions includ-

ing the Benson/Russell pathways and the alkoxy radical

chemistry of decomposition and a 1,2-hydride shift.

The dominant H-atom abstraction from acetic acid occurs

from the methyl group with a kinetic rate ∼ 5 times faster

than abstraction from the carboxylic group (Tan et al., 2012).

Through O

addition to [HO(O)CC*H

], the peroxy radical

[HO(O)CH

OO*] forms, followed by RO

–RO

reactions.

In the Benson pathway (A), the alkoxy radical decomposes

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Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8659

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

[Precursor]

(M) Y

OXLAC

OXLAC-P

PYRAC

PYRAC-P

OLIG

(= Y

OLIG-P

)

SOA

[G]

[MG]

Batch CSTR Batch CSTR Batch CSTR Batch CSTR Batch CSTR Batch CSTR

−7

N/A

1.34 1.32 1.20 1.19

N/A N/A N/A N/A

0 0 1.20 1.19

−6

1.33 1.30 1.20 1.17 0 0 1.20 1.17

−5

1.34 1.31 1.21 1.17 0 0 1.21 1.17

−4

1.28 1.15 1.15 1.03 0.01 0.01 1.16 1.04

−3

0.87

N/A

0.78

N/A

0.08

N/A

0.86

N/A

−2

0.28 0.25 0.32 0.56

−1

0.03 0.03 0.68 0.70

0 0 0.91 0.91

10 0 0 0.97 0.97

N/A

−7

0.05 0.05 0.05 0.04 1.02 1.09 0.71 0.76 0 0 0.76 0.80

−6

0.05 0.05 0.05 0.04 1.02 1.08 0.71 0.76 0 0 0.76 0.80

−5

0.12 0.05 0.11 0.04 0.98 1.08 0.68 0.76 0 0 0.79 0.80

−4

0.12 0.05 0.11 0.04 0.96 1.03 0.67 0.72 0 0 0.78 0.76

−3

0.11

N/A

0.10

N/A

0.81

N/A

0.57

N/A

0.03

N/A

0.70

N/A

−2

0.06 0.05 0.56 0.39 0.22 0.66

−1

0.01 0.01 0.13 0.09 0.60 0.70

0 0 0 0 0.90 0.90

10 0 0 0 0 0.95 0.95

[G]

= initial [glyoxal] (M), [MG]

= initial [methylglyoxal] (M)

All yields are mass based.

OXLAC = oxalate; PYRAC = pyruvate; OLIG = oligomer

OXLAC

= OXLAC yield; Y

PYRAC

= PYRAC yield; Y

OLIG

= OLIG yield

OXLAC-P

= particle-phase OXLAC yield; Y

PYRAC-P

= particle-phase PYRAC yield; Y

OLIG-P

= particle-phase OLIG yield

OXLAC-P

= Y

OXLAC

× 0.90; Y

PYRAC-P

= Y

PYRAC

× 0.70; Y

OLIG-P

= Y

OLIG

× 1

SOA

= Y

OXLAC-P

+ Y

PYRAC-P

+ Y

OLIG-P

to formaldehyde and carbon dioxide, or, via the 1,2-hydride

shift, forms [HO(O)CC*H(OH)], which then becomes gly-

oxylic acid via O

reactions and is oxidized further to oxalic

acid.

In the Russell pathway (B), the organic products are gly-

colic acid and glyoxylic acid. OH/O

oxidation of glycolic

acid forms [HO(O)CH

OO*], which again becomes gly-

oxylic acid, and is later oxidized to oxalic acid.

Note that Tan et al. (2012) did not observe oligomer for-

mation from the OH radical (∼ 10

−12

M) oxidation of 1mM

acetic acid, whereas oligomers do form from the OH radi-

cal oxidation of glyoxal and methylglyoxal at identical con-

centrations (Tan et al., 2009, 2010). Oligomers form from

glyoxal and methylglyoxal oxidation because of radical sta-

bilization (Guzman et al., 2006; Lim et al., 2010). In the ﬁrst

H-atom-abstracted product of acetic acid (HO(O)C*H

), the

radical is rather unstable because it is at the primary carbon.

In contrast, for glyoxal/methylglyoxal reactions, the radi-

cal is at the triply substituted carbon (i.e., carbon bound to

three non-H atoms), and these stabilized radicals allow for

oligomers' formation via radical–radical reaction (Lim et al.,

2010; Tan et al., 2012). Based on these results, in this kinetic

model radical–radical reactions are excluded when the radi-

cal is on the primary carbon (e.g., acetic acid/pyruvic acid–

OH radical reactions). Note that organic radical–radical re-

actions for all organic species except acetic and pyruvic acid

are always "turned on" in our model. At low concentrations,

organic radical–O

reactions are dominant, whereas at high

concentrations organic radical–radical reactions are impor-

tant.

3.2 Aqueous-phase reactions of methylglyoxal with OH

radical

Figure 3 illustrates the reaction mechanisms for the aqueous-

phase OH radical reaction of methylglyoxal. Major prod-

ucts are pyruvic, acetic, and oxalic acid. Bold arrows in-

dicate the major pathways. Pyruvic acid is the major ﬁrst-

generation product from OH radical reaction of methylgly-

oxal, and acetic acid is formed substantially from OH radi-

cal reactions of pyruvic acid and partially from bimolecular

peroxy radical reactions and H

–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 ab-

straction from the primary carbon (minor) or the carbon in

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8660 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

between the diol (major), then peroxy radical formation by

addition. In theminor pathway, peroxy radicals undergo

–RO

reaction, and form alkoxy radicals (the Benson

pathway A) or C

organic compounds (the Russell pathway

B). The alkoxy radical decomposes to formaldehyde and an

organic radical compound, and this organic radical later be-

comes glyoxylic acid. The C

organic compounds from the

Russell pathwayB react with OH radical and eventuallyform

mesoxalic acid. In the major pathway, peroxy radicals either

decompose to pyruvic acid while losing HO

(major) or un-

dergo RO

–RO

reaction (minor), which eventually leads to

the formation of carbon dioxide and acetic acid. It should be

noted that pyruvic acid reacts with H

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 radi-

cal from the radical on the primary carbon forms via O

addi-

tion, and undergoes RO

–RO

reaction. In this RO

–RO

re-

action, oxalic acid and mesoxalic acid are eventually formed

via Benson/Russell pathways and alkoxy radical chemistry.

The organic radical product from H-atom abstraction from

the carboxylic group decomposes to carbon dioxide and a C

aldehyde radical, which eventually becomes acetic acid.

3.3 Kinetic model

Reactions and rate/equilibrium constants used in the full ki-

netic model of glyoxal/methylglyoxal + OH are provided in

Table S1. Detailed reaction mechanisms for the decomposi-

tion of tetroxides are still not understood, and therefore, cal-

culation of the branching ratio for the 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 and pyruvic acids and methylglyoxal, and this branch-

ing ratio was determined based on the ESI-MS intensities of

the acetic acid oxidation products and glyoxylic and glycolic

acid with an assumption that glycolic acid isonly producedin

the Russell pathway (B), whereas glyoxylic acid is produced

in both the A and B pathways (Figs. 2 and 3). Note that this

branching ratio is expected to be independent of initial pre-

cursor concentration because decomposition of tetroxides is

unimolecular decay.

Although there is a literature rate constant (6 × 10

−1

; Stefan and Bolton, 1999) for the OH radical reaction of

pyruvic acid, to our knowledge, there are no detailed litera-

ture rate constants for H-atom abstraction from the primary

carbon and from the carboxylic group. In this work a branch-

ing ratio of 85 % to 15% (H-atom abstraction from the pri-

mary carbon vs. from the carboxylic group) was used based

on the estimation method of Monod et al. (2005).

4 Simulation results and discussion

4.1 Model validation: simulating laboratory

experiments

Previously, a full kinetic model for the aqueous chemistry of

glyoxal with OH radical was developed by including detailed

radical chemistry (Lim et al., 2010). In this work, 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

and light-absorbing organic compounds (e.g., methyl-

glyoxal, pyruvic acid). The model, then, was validated by

simulating the laboratory experiments of Tan et al. (2009,

2010, 2012).

4.1.1 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] = 30µM), ox-

alic 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

concentration (decreasing from 150µM

) was too low to affect photochemistry. H

absorbed

less than 1 % of the transmitted light, and therefore includ-

ing light absorption in the model had a negligible effect on

OH production from H

photolysis. However, the light-

absorption correction by H

substantially improves the

glyoxal–OH radical model simulation (Fig. 6b) at the higher

concentration (initial [glyoxal] = 3000µM), where the ini-

tial H

concentration was 15mM (note that higher H

concentrations were used in experiments with higher gly-

oxal/methylglyoxal concentrations in order to maintain sim-

ilar OH concentrations in all experiments). Note that both

the Lim et al. (2010) and the current model include organic

radical–radical reactions, resulting in improved prediction of

oxalic acid in the 3000µM experiments compared to the Lim

et al. (2005) dilute chemistry model. By correcting for light

absorption by H

in the current work, the model now cap-

tures the timing of the peak (Fig. 6b).

4.1.2 Acetic acid–OH radical model

Next, the performance of the expanded model was evaluated

by simulating acetic acid + OH experiments. Model per-

formance was improved by including detailed peroxy radi-

cal chemistry: RO

–RO

reactions, the Benson/Russell path-

ways, and the alkoxy radical chemistry (Fig. 7). In the model,

the rate constant for the 1,2-hydride shift from the alkoxy

radical is set to be 1 × 10

−1

(Gilbert et al., 1976), while

decomposition rates vary: 5 × 10

, 8 × 10

, and 2 × 10

−1

for initial [acetic acid] = 20, 100, and 1000µM, respectively.

Those values were determined by ﬁtting to the experimental

results, while their range is within literature values (∼ 10

–

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Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8661

Fig. 6. Real-time proﬁles of oxalic acid produced from OH radical

oxidation of glyoxal at initial concentrations of 30µM (A) and 3000

µM (B); experimental results (Tan et al., 2009) and previous/current

model simulations (using the model of Lim et al., 2005, 2010; this

work).

−1

) from Gilbert et al. (1981). Ideally, the decomposi-

tion rate from the alkoxy radical is constant regardless of the

initial acetic acid concentration due to being ﬁrst order, but

pH or cage effects by water molecules could affect the rate.

4.1.3 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 is 3.2 × 10

−1

instead of 2 × 10

−1

. Using

these values, the performance of the model simulations was

substantially improved (Fig. 8a, b). The best ﬁt to the exper-

imental results was obtained by neglecting pyruvic acid pho-

tolysis (see comparison, Fig. 8a) rather than including the

literature rate constant and product (molar) yields for pyru-

vic acid photolysis (i.e., pyruvic acid → 0.45 acetic acid

+ 0.55 CO

, rate constant = 5 × 10

−4

−1

; Carlton et al.,

2006). This has been observed previously for similar exper-

iments conducted with acetone (Stefan et al., 1996; Stefan

and Bolton, 1999). This is expected because the molar ab-

sorptivity of H

(Bolton and Carter, 1994) is much larger

than the molar absorptivity of pyruvic acid. Thus, photolysis

in this experimental system is negligible in the presence of

Fig. 7. Real-time proﬁles of oxalic acid produced from OH radical

reactions of acetic acid at the initial acetic acid concentrations of

20µM (A) and 100µM (B) and 1000µM (C); experimental results

(Tan et al., 2012) and previous/current model simulations (using the

model of Lim et al., 2005; this work).

OH radicals. Note that the purpose of the 254nm UV lamp

in these experiments is to provide an atmospherically rele-

vant OH radical concentration in the aqueous phase and not

to study photolysis.

Figure 8c (initial [methylglyoxal] = 3000 µM) is interest-

ing. Although the new model successfully ﬁt oxalate mea-

surements from the OH reactions of 30 and 300µM of

methylglyoxal, at 3000µM it still does not capture the timing

and the magnitude of oxalic acid formation until ∼ 200min.

Accounting for light absorption by H

is not sufﬁcient

to explain the oxalic acid proﬁle in the 3000µM methyl-

glyoxal experiments. We hypothesize that this is because of

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8662 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

Fig. 8. Real-time proﬁles of oxalic acid produced from OH radical

reactions of methylglyoxal at the initial methylglyoxal concentra-

tions of 30 µM (A) and 300µM (B) and 3000µM (C); experimental

results (Tan et al., 2012) and previous/current model simulations

(using the model of Lim et al., 2005; this work). In (C), simula-

tions were performed with and without the absorption (abs) correc-

tion by hypothetical light-absorbing organic products, withthe same

time proﬁle as pyruvic acid, but a muchhigher extinction coefﬁcient

(1500cm

−1

the formation of light-absorbing organic products that have

the same time proﬁle as pyruvic acid but a higher extinc-

tion coefﬁcient (1500 cm

−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 iden-

tify these light-absorbing products in our reaction vessel,

light-absorbing (brown carbon) products of other methyl-

glyoxal reactions have been observed by others. For exam-

ple, Sareen et al. (2010) observed the presence of UV-light-

absorbing products with an estimated extinction coefﬁcient

of ∼ 5000 cm

−1

from non-radical reactions of methyl-

glyoxal in highly concentrated aqueous ammonium sulfate

solutions. Our methylglyoxal–OH experiments did not con-

tain 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; Noziere et al., 2010).

Further work is needed to investigate this hypothesis.

4.2 aqSOA yields under atmospheric conditions

According to the ﬁeld study by Munger et al. (1995), gly-

oxal and methylglyoxal concentrations in the cloud water are

similar, ranging from ∼ 0.1 to 300µM. For the CSTR runs,

−7

–10

−4

M glyoxal/methylglyoxal in the aqueous phase

were considered. The equivalent gas-phase concentrations

due to Henry's law are ∼ 0.3ppt–0.3ppb for glyoxal and

∼ 30 ppt–30 ppb for methylglyoxal, and those ranges reason-

ably agree with literature (Fu et al., 2008). For the batch re-

actor runs, 10

−7

–10M of glyoxal/methylglyoxal concentra-

tions were considered with the highest concentrations com-

parable to concentrations of water-soluble organics in wet

aerosols.

4.2.1 Batch reactor approximation

Mass-based SOA yields for glyoxal and methylglyoxal are

obtained using the batch reactor approximation are illus-

trated in Fig. 9 and summarized 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 0.77,

which is the sum of 0.66 from pyruvate and 0.11 from

oxalate. Using glyoxal as a surrogate for dissolved water-

soluble organics, the aerosol-relevant Y

SOA

Batch(glyoxal) is

0.94. If instead methylglyoxal is used as a surrogate for

total dissolved water-soluble organics, the aerosol-relevant

SOA

Batch(methylglyoxal) is 0.94. Again, this material

is predicted to be entirely oligomeric (note that detailed

oligomer distributions for 1M glyoxal and 1M methylgly-

oxal are provided in Fig. S3). It should be recognized that the

chemistry in wet aerosols is complex and poorly understood.

It is unlikely that all dissolved, water-soluble organics in an

aerosol particle will have the same ability to form oligomers.

However, glyoxal or methylglyoxal are reasonable surrogates

for dissolved water-soluble organics because (1) aldehydes

and alcohols are the major known water-soluble organic pre-

cursors (Blando and Turpin, 2001), (2) they are remarkably

reactive to OH radicals in the aqueous phase, (3) and they are

expected to have similar oxidation mechanisms.

The total particle-phase yield as a function of concentra-

tion is given by

SOA

(

glyoxal

)

1.20

1 + 491[G]

0.931

1 +

0.0243

[G]

. (5)

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Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8663

Fig. 9. Atmospheric batch simulations (A) for particle-phase mass-

based yields of oxalate (Y

OXLAC

), oligomers (Y

OLIG

), and SOA

(= Y

OXLAC

+ Y

OLIG

) with increasing initial concentrations of gly-

oxal (x axis) for aqueous-phase OH radical reactions, and (B) for

particle-phase mass-based yields of oxalic acid (Y

OXLAC

), pyruvate

PYRAC

), oligomers (Y

OLIG

), and SOA (= Y

OXLAC

+ Y

PYRAC

OLIG

) with increasing initial concentrations of methylglyoxal (x

axis) for aqueous-phase OH radical reactions. Similarly, O/C (on

the right-hand y axis) vs. initial concentrations of glyoxal (A) and

O/C vs. initial concentrations of methylglyoxal (B).

Similarly, for methylglyoxal, the total particle-phase yield as

a function of concentration is given by

SOA

(

methylglyoxal

)

0.659

1 + 71.5[MG]

0.113

1 + 107[MG]

0.940

1 +

0.0459

[MG]

, (6)

where [G]

is initial [glyoxal] and [MG]

is initial [methyl-

glyoxal]. These particle-phase product yields were estimated

by ﬁtting to the simulations (Fig. 9a and b) and making use of

Eqs. (3) and (4). Note that at 1 × 10

−2

M of initial glyoxal in

Fig. 9a, the SOA yield is aminimum (56%) for the following

reason. For this initial glyoxal concentration substantial (up

to 4.6mM) H

forms by HO

+ HO

and HO

+ O

−

reac-

tions. The reaction of glyoxylic acid + H

forming formic

acid competes with the reaction of glyoxylic acid + OH radi-

cals to form oxalate, and also competes with organic radical–

radical reactions to form oligomers (Lim et al., 2010). This

ﬁnding from atmospheric simulation is consistent with previ-

ous experimental results at similar concentrations suggesting

a role for H

(Lee et al., 2012).

4.2.2 CSTR model

In order to estimate the yield, the reacted glyoxal or methyl-

glyoxal concentrations should be known. Since it is not

possible to directly obtain the reacted precursor concentra-

tions in the steady-state CSTR model, they were calculated

by summing the major products (e.g., oxalic acid, pyru-

vic acid, glyoxylic acid, formic acid, acetic acid, formalde-

hyde, and CO

), and all of the stoichiometric coefﬁcients

are assumed to be 1 except for CO

, which is 2 because

two CO

molecules are produced from one oxalic acid

molecule. A linear regression method was used to get the

product yield during ∼ 1 h of glyoxal–OH reaction and

∼ 20 min of methylglyoxal–OH reaction (Fig. 5c), resulting

in R

values close to 1. At cloud-relevant concentrations,

SOA

CSTR(glyoxal) is 1.19 and solely contributed by ox-

alate (Fig. S2A). Y

SOA

CSTR(methylglyoxal) is 0.80, which

is the sum of 0.76 from pyruvate and 0.044 from oxalate

(Fig. S2B). These CSTR yields are quite similar to the batch

yields at the same conditions (Fig. 10a and b) and also sum-

marized in Table 1.

4.3 O/C ratio

O/ C ratios of aqSOA for glyoxal and methylglyoxal un-

der atmospheric conditions are estimated based on yield

fractions of oxalate (O / C = 2), pyruvate (O/C = 1), and

oligomers (Fig. 9a and b). According to FTICR-MS analy-

sis, the O/ C ratio of glyoxal oligomers (m/z

−

200–500) is

1.2 (Lim et al., 2010) and the O/ C ratio of methylglyoxal

oligomers (m/z

−

245–800) is 0.69 (Altieri et al., 2008). O/ C

ratios decrease from 2 to 1 for glyoxal (Fig. 9a) and from 1

to 0.7 for methylglyoxal (Fig. 9b), as initial concentrations

of glyoxal or methylglyoxal increase from cloud conditions

to wet aerosol conditions.

5 Conclusions

Volatile but highly water-soluble glyoxal (≤ 276µM – the

concentration measured in atmospheric waters), methylgly-

oxal (0.02–128µM), 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 photochemi-

cal fragments of anthropogenic/biogenic VOCs. Due to their

small carbon number (C

-C

), 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 of their reaction with

OH radicals 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

www.atmos-chem-phys.net/13/8651/2013/ Atmos. Chem. Phys., 13, 8651–8667, 2013

8664 Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA

Fig. 10. The CSTR simulation and the batch simulation for particle-

phase mass-based yields of oxalate (Y

OXLAC

) with increasing ini-

tial 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.

oligomers, which stay entirely in the particle phase. The

aqueous radical chemistry of these compounds is initiated

predominantly by OH radicals, and the current understand-

ing is that the main source of aqueous OH radicals is up-

take from the gas phase. Peroxy radical chemistry is a key

to understanding aqueous-phase OH radical reactions. At

cloud-relevant conditions, H-atom-abstracted organic prod-

ucts react with dissolved O

forming peroxy radicals, which

decompose to carboxylic acids (e.g., glyoxylic acid, oxalic

acid, or pyruvic acid) and HO

. At aerosol-relevant con-

ditions, oligomers are formed via radical–radical reactions.

The bimolecular-diffused RO

–RO

reactions constitute im-

portant peroxy radical chemistry for OH radical reactions of

acetic and pyruvic acid and methylglyoxal. Alcohols and car-

bonyls are produced in the Russell pathway, whereas alkoxy

radicals are produced in the Benson pathway. Alkoxy rad-

icals undergo subsequent decomposition or a 1,2-hydride

shift.

In this work, a full kinetic model was developed based on

detailed reaction mechanisms and validated against labora-

tory experiments. The batch and CSTR simulations predict

similar and substantial aqSOA yields. These simulation re-

sults are consistent with the expectation that aqueous chem-

istry is a substantial source of SOA globally. Although uncer-

tainties are large, recent model studies (Carlton et al., 2008;

Fu et al., 2008; Liu et al., 2012) suggest that the magnitude of

aqSOA is comparable to SOA formed via gas–particle parti-

tioning of semivolatile products from gas-phase oxidation of

VOCs. Certainly, other water-soluble organic precursor will

also undergo aqueous chemistry leading to SOA formation.

While glyoxal and methylglyoxal are likely to be present

at concentrations up to ∼ 10mM (in sulfate-containing wet

aerosols H

eff

for glyoxal has been enhanced by a factor of

∼ 500 according to Kampf et al., 2013), our work includes

model runs at concentrations as high as molar. In these model

runs, glyoxal and methylglyoxal are being treated as surro-

gates for the complex mixture of water-soluble organics in

wet aerosols, recognizing that there are many water-soluble

compounds that will exist in atmospheric waters and will

form radicals via reaction with OH. In wet aerosols we ex-

pect that these radicals will react with each other. Thus, we

expect that a complex array of oligomeric products will form

in wet aerosols, not just tartaric and malonic acids.

The processes examined in this work depend on the avail-

ability of OH radicals in clouds, fogs, and wet aerosols (Wax-

man et al., 2013). Uptake from the gas phase is generally

thought to be the major source of OH radicals to atmo-

spheric waters, and steady-state aqueous concentrations of

OH radicals from the gas phase may be inﬂuenced by droplet

surface-to-volume ratios and aqueous concentrations of re-

actants. The photo-Fenton reaction (i.e., the photooxidation

of Fe ions in the presence of H

) is considered the major

source of OH radicals inside wet aerosols (Arakaki and Faust

1998; Lim et al., 2005, 2010). Photolysis of nitrite/nitrate

(Hullar and Anastasio, 2011) and organic matter (Dong and

Rosario-Ortiz, 2012) can also produce OH radicals in aque-

ous particles. However, the degree of OH radical formation

or "recycling" in atmospheric waters is not well understood

and could represent an important yet unrecognized oxidant

source. The sources and fate of oxidants in wet aerosols con-

stitute an important area of current and future research, and

may determine to what degree condensed-phase oxidation

reactions occur near the surface versus in the bulk of wet

aerosols, and the relative importance of radical versus non-

radical reactions in aerosols.

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

extent to which these products are present as carboxylate

salts, which have lower vapor pressures than the correspond-

ing 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, gas-phase particle partitioning measurements of

Atmos. Chem. Phys., 13, 8651–8667, 2013 www.atmos-chem-phys.net/13/8651/2013/

Y. B. Lim et al.: Chemical insights, explicit chemistry, and yields of SOA 8665

carboxylic acids are limited, and this might not be uniformly

true.

Supplementary material related to this article is

available online at http://www.atmos-chem-phys.net/13/

8651/2013/acp-13-8651-2013-supplement.pdf.

Acknowledgements. This research has been supported by a grant

from the US Environmental Protection Agency's Science to

Achieve Results (STAR) program (grant R833751), the National

Science Foundation (NSF; 0630298; 1052611), National Oceanic

and Atmospheric Administration (NOAA; NA07OAR4310279),

US Department of Agriculture (USDA-NIFA), and the New

Jersey Agricultural Experiment Station. Although the research

described in this paper has been funded wholly or in part by the US

Environmental Protection Agency's STAR program, it has not been

subjected to any EPA review and therefore does not necessarily

reﬂect the views of the EPA, and no ofﬁcial endorsement should

be inferred. The authors acknowledge helpful discussions with

Annmarie Carlton, Kostas Tsigaridis, and Barbara Ervens.

Edited by: R. Volkamer

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www.atmos-chem-phys.net/13/8651/2013/ Atmos. Chem. Phys., 13, 8651–8667, 2013

... We also estimate the upper limit of oxalate mass yield from CHOCHO over SMA as ~ 10 % based on the oxalate fraction (~2 %) in OA (Fig. 6c). Our analysis shows lower oxalate yield than previously reports from the OH oxidation in cloud process (Tan et al., 2009, Lim et al., 2013Ortiz-Montalvo et al., 2014). We presume our lower yield has likely originated from the existence of many fates in uptaken CHOCHO, such as oligomerization (Lim et al., 2013), reaction with sulfate (Lim et al., 2016) and ammonium (Yu et al., 2011;Ortiz-Montalvo et al., 2014), etc., since our analysis is over SMA 370 area. ...

... Our analysis shows lower oxalate yield than previously reports from the OH oxidation in cloud process (Tan et al., 2009, Lim et al., 2013Ortiz-Montalvo et al., 2014). We presume our lower yield has likely originated from the existence of many fates in uptaken CHOCHO, such as oligomerization (Lim et al., 2013), reaction with sulfate (Lim et al., 2016) and ammonium (Yu et al., 2011;Ortiz-Montalvo et al., 2014), etc., since our analysis is over SMA 370 area. From our dataset, in cloud process case, analysis was impossible due to low abundance in gas-phase CHOCHO concentration (below detection limit). ...

Dongwook Kim
Changmin Cho
Seokhan Jeong
Kyung-Eun Min

Glyoxal (CHOCHO), the simplest dicarbonyl in the troposphere, is an important precursor for secondary organic aerosol (SOA) and brown carbon (BrC) affecting air-quality and climate. The airborne measurement of CHOCHO concentrations during the KORUS-AQ (KORea-US Air Quality study) campaign in 2016 enables detailed quantification of loss mechanisms, pertaining to SOA formation in the real atmosphere. The production of this molecule was mainly from oxidation of aromatics (59 %) initiated by hydroxyl radical (OH), of which glyoxal forming mechanisms are relatively well constrained. CHOCHO loss to aerosol was found to be the most important removal path (69 %) and contributed to roughly ~20 % (3.7 μg sm−3 ppmv−1 hr−1, normalized with excess CO) of SOA growth in the first 6 hours in Seoul Metropolitan Area. To our knowledge, we show the first field observation of aerosol surface-area (Asurf)-dependent CHOCHO uptake, which diverges from the simple surface uptake assumption as Asurf increases in ambient condition. Specifically, under the low (high) aerosol loading, the CHOCHO effective uptake rate coefficient, keff,uptake, linearly increases (levels off) with Asurf, thus, the irreversible surface uptake is a reasonable (unreasonable) approximation for simulating CHOCHO loss to aerosol. Dependency of photochemical impact, as well as aerosol viscosity, are discussed as other possible factors influencing CHOCHO uptake rate. Our inferred Henry's law coefficient of CHOCHO, 7.0 × 108 M atm−1, is ~2 orders of magnitude higher than those estimated from salting-in effects constrained by inorganic salts only, which urges more understanding on CHOCHO solubility under real atmospheric conditions.

... Precursor emission sources for ac-etate and formate include plants, soil, vehicles, and biomass burning, with key production routes including the oxidation of isoprene, the ozonolysis of olefins, and peroxy radical reactions (Khare et al., 1999, and references therein). Pyruvate is considered the most abundant aqueous reaction product of methylglyoxal, generated by the oxidation of gas-phase anthropogenic volatile organic compounds (Boris et al., 2014;Carlton et al., 2006;Lim et al., 2013;Stefan et al., 1996;Tan et al., 2010). Glycolate has been linked to aqueous processing of acetate and is a precursor of glyoxylate (Boris et al., 2014), which is formed via the oxidation of glycolaldehyde by hydroxide radicals (Thomas et al., 2016). ...

Connor Stahl
Ewan Crosbie
Paola Angela Bañaga
Armin Sorooshian

This work focuses on total organic carbon (TOC) and contributing species in cloud water over Southeast Asia using a rare airborne dataset collected during NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), in which a wide variety of maritime clouds were studied, including cumulus congestus, altocumulus, altostratus, and cumulus. Knowledge of TOC masses and their contributing species is needed for improved modeling of cloud processing of organics and to understand how aerosols and gases impact and are impacted by clouds. This work relies on 159 samples collected with an axial cyclone cloud-water collector at altitudes of 0.2–6.8 km that had sufficient volume for both TOC and speciated organic composition analysis. Species included monocarboxylic acids (glycolate, acetate, formate, and pyruvate), dicarboxylic acids (glutarate, adipate, succinate, maleate, and oxalate), methanesulfonic acid (MSA), and dimethylamine (DMA). TOC values range between 0.018 and 13.66 ppm C with a mean of 0.902 ppm C. The highest TOC values are observed below 2 km with a general reduction aloft. An exception is samples impacted by biomass burning for which TOC remains enhanced at altitudes as high as 6.5 km (7.048 ppm C). Estimated total organic matter derived from TOC contributes a mean of 30.7 % to total measured mass (inorganics + organics). Speciated organics contribute (on a carbon mass basis) an average of 30.0 % to TOC in the study region and account for an average of 10.3 % to total measured mass. The order of the average contribution of species to TOC, in decreasing contribution of carbon mass, is as follows (±1 standard deviation): acetate (14.7 ± 20.5 %), formate (5.4 ± 9.3 %), oxalate (2.8 ± 4.3 %), DMA (1.7 ± 6.3 %), succinate (1.6 ± 2.4 %), pyruvate (1.3 ± 4.5 %), glycolate (1.3 ± 3.7 %), adipate (1.0 ± 3.6 %), MSA (0.1 ± 0.1 %), glutarate (0.1 ± 0.2 %), and maleate (

... system updates and evaluation aqueous chemistry for several bromine species. KMT2 builds upon CMAQ's existing in-cloud sulfur (S) oxidation chemistry, replacing the yield parameterization of in-cloud SOA formation from GLY/MGLY + OH with a mechanistic representation of small dicarboxylic acid formation from the reactions of OH with glyoxal, methylglyoxal, glycolaldehyde, and acetic acid (Lim et al., 2005(Lim et al., , 2010(Lim et al., , 2013Sareen et al., 2013). It also includes additional aqueous chemistry for S, nitrogen (N), carbon (C), and hydroxide (OH − ) species largely based on the aqueous mechanism of ReLACS-AQ (Leriche et al., 2013), a compact cloud chemistry mechanism built from and tested against more comprehensive models. ...

The Community Multiscale Air Quality (CMAQ) model version 5.3 (CMAQ53), released to the public in August 2019 and followed by version 5.3.1 (CMAQ531) in December 2019, contains numerous science updates, enhanced functionality, and improved computation efficiency relative to the previous version of the model, 5.2.1 (CMAQ521). Major science advances in the new model include a new aerosol module (AERO7) with significant updates to secondary organic aerosol (SOA) chemistry, updated chlorine chemistry, updated detailed bromine and iodine chemistry, updated simple halogen chemistry, the addition of dimethyl sulfide (DMS) chemistry in the CB6r3 chemical mechanism, updated M3Dry bidirectional deposition model, and the new Surface Tiled Aerosol and Gaseous Exchange (STAGE) bidirectional deposition model. In addition, support for the Weather Research and Forecasting (WRF) model's hybrid vertical coordinate (HVC) was added to CMAQ53 and the Meteorology-Chemistry Interface Processor (MCIP) version 5.0 (MCIP50). Enhanced functionality in CMAQ53 includes the new Detailed Emissions Scaling, Isolation and Diagnostic (DESID) system for scaling incoming emissions to CMAQ and reading multiple gridded input emission files. Evaluation of CMAQ531 was performed by comparing monthly and seasonal mean daily 8 h average (MDA8) O3 and daily PM2.5 values from several CMAQ531 simulations to a similarly configured CMAQ521 simulation encompassing 2016. For MDA8 O3, CMAQ531 has higher O3 in the winter versus CMAQ521, due primarily to reduced dry deposition to snow, which strongly reduces wintertime O3 bias (2–4 ppbv monthly average). MDA8 O3 is lower with CMAQ531 throughout the rest of the year, particularly in spring, due in part to reduced O3 from the lateral boundary conditions (BCs), which generally increases MDA8 O3 bias in spring and fall (∼0.5 µg m−3). For daily 24 h average PM2.5, CMAQ531 has lower concentrations on average in spring and fall, higher concentrations in summer, and similar concentrations in winter to CMAQ521, which slightly increases bias in spring and fall and reduces bias in summer. Comparisons were also performed to isolate updates to several specific aspects of the modeling system, namely the lateral BCs, meteorology model version, and the deposition model used. Transitioning from a hemispheric CMAQ (HCMAQ) version 5.2.1 simulation to a HCMAQ version 5.3 simulation to provide lateral BCs contributes to higher O3 mixing ratios in the regional CMAQ simulation in higher latitudes during winter (due to the decreased O3 dry deposition to snow in CMAQ53) and lower O3 mixing ratios in middle and lower latitudes year-round (due to reduced O3 over the ocean with CMAQ53). Transitioning from WRF version 3.8 to WRF version 4.1.1 with the HVC resulted in consistently higher (1.0–1.5 ppbv) MDA8 O3 mixing ratios and higher PM2.5 concentrations (0.1–0.25 µg m−3) throughout the year. Finally, comparisons of the M3Dry and STAGE deposition models showed that MDA8 O3 is generally higher with M3Dry outside of summer, while PM2.5 is consistently higher with STAGE due to differences in the assumptions of particle deposition velocities to non-vegetated surfaces and land use with short vegetation (e.g., grasslands) between the two models. For ambient NH3, STAGE has slightly higher concentrations and smaller bias in the winter, spring, and fall, while M3Dry has higher concentrations and smaller bias but larger error and lower correlation in the summer.

Quynh Tran
Yi-Hsueh Chuang
Steve Tan
Hsin-Hsin Tung

Isopropyl alcohol (IPA) is a significant pollutant in the wastewater of semiconductor manufacturing industry. This study investigated the degradation of IPA in the microwave (MW)-assisted oxidation process using hydrogen peroxide (H2O2) as the oxidant. Complete elimination of IPA was noted in the MW/H2O2 system within 90 min of irradiation. In comparison, only 4.8, 6.1, and 68.2% of IPA, respectively, was removed in MW irradiation alone, H2O2 oxidation, and the system using the combination of thermal (TH) and H2O2. The degradation kinetics of IPA followed the pseudo-first-order in MW/H2O2 and TH/H2O2 systems, whereas the pseudo-zero-order reaction kinetics was observed in others. The degradation rates increased on increasing the hydrogen peroxide dose to a certain level. An excess H2O2 would trap the hydroxyl radicals (•OH) to form weaker radicals that inhibit IPA oxidation. A series of degradation intermediates were identified and quantified corresponding to acetone and short-chain organic acids. Finally, the degradation pathways of IPA were proposed and validated by the total organic carbon mass balance.

The Jülich Aqueous-phase Mechanism of Organic Chemistry (JAMOC) is developed and implemented in the Module Efficiently Calculating the Chemistry of the Atmosphere (MECCA; version 4.5.0). JAMOC is an explicit in-cloud oxidation scheme for oxygenated volatile organic compounds (OVOCs), suitable for global model applications. It is based on a subset of the comprehensive Cloud Explicit Physico-chemical Scheme (CLEPS; version 1.0). The phase transfer of species containing up to 10 carbon atoms is included, and a selection of species containing up to 4 carbon atoms reacts in the aqueous phase. In addition, the following main advances are implemented: (1) simulating hydration and dehydration explicitly; (2) taking oligomerisation of formaldehyde, glyoxal, and methylglyoxal into account; (3) adding further photolysis reactions; and (4) considering gas-phase oxidation of new outgassed species. The implementation of JAMOC in MECCA makes a detailed in-cloud OVOC oxidation model readily available for box as well as for regional and global simulations that are affordable with modern supercomputing facilities. The new mechanism is tested inside the box model Chemistry As A Boxmodel Application (CAABA), yielding reduced gas-phase concentrations of most oxidants and OVOCs except for the nitrogen oxides.

Air-water interfaces are ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atmospheric aqueous aerosols. The aerosol droplets interface, in particular, plays a critical role in numerous atmospheric chemistry processes. Methyl vinyl ketone (MVK) and methacrolein (MACR), two abundant volatile organic compounds, are the significant precursors of Criegee intermediates and secondary organic aerosol. In this work, the physicochemical properties of MVK and MACR at the air-water interface are studied from a theoretical perspective. The free energy wells of MVK and MACR occur at the air-water interface, and the absorption probabilities of them are 71% and 67%, respectively. Repulsion dominates the interactions between MVK/MACR and water molecules in the bulk region, while attraction is dominant at the interface. The two molecules tend to tilt at the interface, with the CC bond exposed at the outer interface. The most likely reaction scenario of O3-initiated MVK/MACR reaction in the troposphere is also determined for the first time. Based on the molecular dynamics simulation results, the activity sequence of MVK + O3 is given at four different environments by the density functional theory method: air-water interface, mineral clusters interface, bulk solution, and homogeneous gas. The interfacial water molecule can catalyze the reaction of MVK with O3, and the rate constant at the air-water interface is ∼6 times larger than that on the mineral surface model. Compared with mineral particles, aqueous particles play a more significant role in modifying the reaction properties of atmospheric organic species.

Connor Stahl
Ewan Crosbie
Paola Angela Bañaga
Armin Sorooshian

This work focuses on total organic carbon (TOC) and contributing species in cloud water over Southeast Asia using a rare airborne dataset collected during NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), in which a wide variety of maritime clouds were studied, including cumulus congestus, altocumulus, altostratus, and cumulus. Knowledge of TOC levels and their contributing species is needed for improved modeling of cloud processing of organics and to understand how aerosols and gases impact and are impacted by clouds. This work relies on 159 samples collected with an Axial Cyclone Cloud water Collector at altitudes of 0.2–6.8 km that had sufficient volume for both TOC and speciated organic composition analysis. Species included monocarboxylic acids (glycolate, acetate, formate, and pyruvate), dicarboxylic acids (glutarate, adipate, succinate, maleate, and oxalate), methanesulfonate (MSA), and dimethylamine (DMA). TOC values range between 0.018–13.660 ppm C with a mean of 0.902 ppm C. The highest TOC values are observed below 2 km with a general reduction aloft. An exception is samples impacted by biomass burning for which TOC remains enhanced as high as 6.5 km (7.048 ppm C). Estimated total organic matter derived from TOC contributes a mean of 30.7 % to total measured mass (inorganics + organics). Speciated organics contribute (on carbon mass basis) an average of 30.0 % to TOC in the study region, and account for an average of 10.3 % to total measured mass. The order of the average contribution of species to TOC, in decreasing contribution of carbon mass, is as follows: acetate (14.7 ± 20.5 %), formate (5.4 ± 9.3 %), oxalate (2.8 ± 4.3 %), DMA (1.7 ± 6.3 %), succinate (1.6 ± 2.4 %), pyruvate (1.3 ± 4.5 %), glycolate (1.3 ± 3.7 %), adipate (1.0 ± 3.6 %), MSA (0.1 ± 0.1 %), glutarate (0.1 ± 0.2 %), maleate (

With the increasing concerns on summertime atmospheric photochemical pollution, the diagnosis and prevention of ozone pollution have been paid close attention. Both formaldehyde (HCHO) and glyoxal (CHOCHO) are ubiquitous oxidation intermediates of volatile organic compounds (VOCs). The ratio of glyoxal to formaldehyde (RGF) is used as a metric for VOCs emission sources. In this study, the mixing ratios of HCHO and CHOCHO have been measured by the active differential optical absorption spectroscopy (DOAS) method in the urban area of Shanghai during summertime in 2018, as well as other trace gases. The average levels of HCHO and CHOCHO are 3.31 ± 1.43 ppbv and 0.164 ± 0.073 ppbv, respectively. The similar diurnal patterns and high correlation between HCHO, CHOCHO and ozone levels implied that daytime photochemical processes are the dominant formation pathway for these trace gases. We find that with increased NOx levels, HCHO shows higher ozone formation potential relative to glyoxal. The RGF ratio increases with temperature and decreases with NO2 levels. By investigating the coupling of typical VOCs species such as acetylene, toluene and isoprene with HCHO and CHOCHO, RGF is found to be strongly impacted by the ambient VOCs profiles, suggesting that RGF should be used with caution when linking it to a given VOC precursor source. Finally, the RGF variations with ozone pollution episodes and weather processes are also discussed.

The atmospheric chemistry of isoprene has broad implications for regional air quality and the global climate. Allylic radicals, taking 13-17% yield in the isoprene oxidation by •Cl, can contribute as much as 3.6-4.9% to all possible formed intermediates in local regions at daytime. Considering the large quantity of isoprene emission, the chemistry of the allylic radicals is therefore highly desirable. Here, we investigated the atmospheric oxidation mechanism of the allylic radicals using quantum chemical calculations and kinetics modeling. The results indicate that the allylic radicals can barrierlessly combine with O2 to form peroxy radicals (RO2•). Under ≤100 ppt NO and ≤50 ppt HO2• conditions, the formed RO2• mainly undergo two times "successive cyclization and O2 addition" to finally form the product fragments 2-alkoxy-acetaldehyde (C2H3O2•) and 3-hydroperoxy-2-oxopropanal (C3H4O4). The presented reaction illustrates a novel successive cyclization-driven autoxidation mechanism. The formed 3-hydroperoxy-2-oxopropanal product is a new isomer of the atmospheric C3H4O4 family and a potential aqueous-phase secondary organic aerosol precursor. Under >100 ppt NO condition, NO can mediate the cyclization-driven autoxidation process to form C5H7NO3, C5H7NO7, and alkoxy radical-related products. The proposed novel autoxidation mechanism advances our current understanding of the atmospheric chemistry of both isoprene and RO2•.

Large amounts of small α-dicarbonyls (glyoxal and methylglyoxal) are produced in the atmosphere from photochemical oxidation of biogenic isoprene and anthropogenic aromatics, but the fundamental mechanisms leading to secondary organic aerosol (SOA) and brown carbon (BrC) formation remain elusive. Methylglyoxal is commonly believed to be less reactive than glyoxal because of unreactive methyl substitution, and available laboratory measurements showed negligible aerosol growth from methylglyoxal. Herein, we present experimental results to demonstrate striking oligomerization of small α-dicarbonyls leading to SOA and BrC formation on sub-micrometer aerosols. Significantly more efficient growth and browning of aerosols occur upon exposure to methylglyoxal than glyoxal under atmospherically relevant concentrations and in the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile oligomers and light-absorbing nitrogen-heterocycles are identified as the dominant particle-phase products. The distinct aerosol growth and light absorption are attributed to carbenium ion-mediated nucleophilic addition, interfacial electric field-induced attraction, and synergetic oligomerization involving organic/inorganic species, leading to surface- or volume-limited reactions that are dependent on the reactivity and gaseous concentrations. Our findings resolve an outstanding discrepancy concerning the multiphase chemistry of small α-dicarbonyls and unravel a new avenue for SOA and BrC formation from atmospherically abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.

B. Ervens
Barbara J. Turpin
R. J. Weber

Progress has been made over the past decade in predicting secondary organic aerosol (SOA) mass in the atmosphere using vapor pressure-driven partitioning, which implies that SOA compounds are formed in the gas phase and then partition to an organic phase (gasSOA). However, discrepancies in predicting organic aerosol oxidation state, size and product (molecular mass) distribution, relative humidity (RH) dependence, color, and vertical profile suggest that additional SOA sources and aging processes may be important. The formation of SOA in cloud and aerosol water (aqSOA) is not considered in these models even though water is an abundant medium for atmospheric chemistry and such chemistry can form dicarboxylic acids and "humic-like substances" (oligomers, high-molecular-weight compounds), i.e. compounds that do not have any gas phase sources but comprise a significant fraction of the total SOA mass. There is direct evidence from field observations and laboratory studies that organic aerosol is formed in cloud and aerosol water, contributing substantial mass to the droplet mode. This review summarizes the current knowledge on aqueous phase organic reactions and combines evidence that points to a significant role of aqSOA formation in the atmosphere. Model studies are discussed that explore the importance of aqSOA formation and suggestions for model improvements are made based on the comprehensive set of laboratory data presented here. A first comparison is made between aqSOA and gasSOA yields and mass predictions for selected conditions. These simulations suggest that aqSOA might contribute almost as much mass as gasSOA to the SOA budget, with highest contributions from biogenic emissions of volatile organic compounds (VOC) in the presence of anthropogenic pollutants (i.e. NOx) at high relative humidity and cloudiness. Gaps in the current understanding of aqSOA processes are discussed and further studies (laboratory, field, model) are outlined to complement current data sets.

Ted Hullar
Cort Anastasio

Hydrogen peroxide (HOOH) is a significant oxidant in atmospheric condensed phases (e.g., cloud and fog drops, aqueous particles, and snow) that also photolyzes to form hydroxyl radical (•OH). •OH can react with organics in aqueous phases to form organic peroxyl radicals and ultimately reform HOOH, but the efficiency of this process in atmospheric aqueous phases, as well as snow and ice, is not well understood. We investigate HOOH formation from •OH attack on 10 environmentally relevant organic compounds: formaldehyde, formate, glycine, phenylalanine, benzoic acid, octanol, octanal, octanoic acid, octanedioic acid, and 2-butoxyethanol. Liquid and ice samples with and without nitrate (as an •OH source) were illuminated using simulated solar light, and HOOH formation rates were measured as a function of pH and temperature. For most compounds, the formation rate of HOOH without nitrate was the same as the background formation rate in blank water (i.e., illumination of the organic species does not produce HOOH directly), while formation rates with nitrate were greater than the water control (i.e., reaction of •OH with the organic species forms HOOH). Yields of HOOH, defined as the rate of HOOH production divided by the rate of •OH production, ranged from essentially zero (glycine) to 0.24 (octanal), with an average of 0.12 ± 0.05 (95 % CI). HOOH production rates and yields were higher at lower pH values. There was no temperature dependence of the HOOH yield for formaldehyde or octanedioic acid between −5 to 20 °C and ice samples had approximately the same HOOH yield as the aqueous solutions. In contrast, HOOH yields in formate solutions were higher at 5 and 10 °C compared to −5 and 20 °C. Yields of HOOH in ice for solutions containing nitrate and either phenylalanine, benzoate, octanal, or octanoic acid were indistinguishable from zero. Our HOOH yields were approximately half those found in previous studies conducted using γ-radiolysis, but this difference might be due to the much lower (and more environmentally relevant) •OH formation rates in our experiments.

Wanmin Gong
Craig Stroud
Leiming Zhang

The representations of cloud processing of gases and aerosols in some of the current state-of-the-art regional air quality models in North America and Europe are reviewed. Key processes reviewed include aerosol activation (or nucleation scavenging of aerosols), aqueous-phase chemistry, and wet deposition/removal of atmospheric tracers. It was found that models vary considerably in the parameterizations or algorithms used in representing these processes. As an emerging area of research, the current understanding of the uptake of water soluble organics by cloud droplets and the potential aqueous-phase reaction pathways leading to the atmospheric secondary organic aerosol (SOA) formation is also reviewed. Sensitivity tests using the AURAMS model have been conducted in order to assess the impact on modeled regional particulate matter (PM) from: (1) the different aerosol activation schemes, (2) the different below-cloud particle scavenging algorithms, and (3) the inclusion of cloud processing of water soluble organics as a potential pathway for the formation of atmospheric SOA. It was found that the modeled droplet number concentrations and ambient PM size distributions were strongly affected by the use of different aerosol activation schemes. The impact on the modeled average ambient PM mass concentration was found to be limited in terms of averaged PM 2.5 concentration (~a few percents) but more significant in terms of PM 1.0 (up to 10 percents). The modeled ambient PM was found to be moderately sensitive to the below-cloud particle scavenging algorithms, with relative differences up to 10% and 20% in terms of PM 2.5 and PM 10 , respectively, when using the two different algorithms for the scavenging coefficient (Λ) OPEN ACCESS Atmosphere 2011, 2 568 corresponding to the lower and upper bounds in the parameterization for Λ. The model simulation with the additional cloud uptake and processing of water-soluble organic gases was shown to improve the evaluation statistics for modeled PM 2.5 OA compared to the IMPROVE network data, and it was demonstrated that the cloud processing of water-soluble organics can indeed be an important mechanism in addition to the traditional secondary organic gas uptake to the particle organic phase.

The water-soluble fractions of aerosol samples and cloud water collected during Whistler Aerosol and Cloud Study (WACS 2010) were analyzed using an Aerodyne aerosol mass spectrometer (AMS). This is the first study to report AMS organic spectra of re-aerosolized cloud water, and to make direct comparison between the AMS spectra of cloud water and aerosol samples collected at the same location. In general, the aerosol and cloud organic spectra were very similar, indicating that the cloud water organics likely originated from secondary organic aerosol (SOA) formed nearby. By using a photochemical reactor to oxidize both aerosol filter extracts and cloud water, we find evidence that fragmentation of aerosol water-soluble organics increases their volatility during oxidation. By contrast, enhancement of AMS-measurable organic mass by up to 30% was observed during aqueous-phase photochemical oxidation of cloud water organics. We propose that additional SOA material was produced by functionalizing dissolved organics via OH oxidation, where these dissolved organics are sufficiently volatile that they are not usually part of the aerosol. This work points out that water-soluble organic compounds of intermediate volatility (IVOC), such as cis-pinonic acid, produced via gas-phase oxidation of monoterpenes, can be important aqueous-phase SOA precursors in a biogenic-rich environment.

Recent experimental findings indicate that secondary organic aerosol (SOA) represents an important and, under many circumstances, the major fraction of the organic aerosol burden. Here, we use a global 3-D model (IMPACT) to test the results of different mechanisms for the production of SOA. The basic mechanism includes SOA formation from organic nitrates and peroxides produced from an explicit chemical formulation, using partition coefficients based on thermodynamic principles together with assumptions for the rate of formation of low-volatility oligomers. We also include the formation of low-volatility SOA from the reaction of glyoxal and methylglyoxal on aqueous aerosols and cloud droplets as well as from the reaction of epoxides on aqueous aerosols. A model simulation including these SOA formation mechanisms gives an annual global SOA production of 120.5 Tg. The global production of SOA is decreased substantially to 90.8 Tg yr-1 if the HOx regeneration mechanism proposed by Peeters et al. (2009) is used. Model predictions with and without this HOx (OH and HO2 regeneration scheme are compared with multiple surface observation datasets, namely: the Interagency Monitoring of Protected Visual Environments (IMPROVE) for the United States, the European Monitoring and Evaluation Programme (EMEP), and aerosol mass spectrometry (AMS) data measured in both the Northern Hemisphere and tropical forest regions. All model simulations show reasonable agreement with the organic carbon mass observed in the IMPROVE network and the AMS dataset, however observations in Europe are significantly underestimated, which may be caused by an underestimation of primary organic aerosol emissions (POA) in winter and of emissions and/or SOA production in the summer. The modeled organic aerosol concentrations tend to be higher by roughly a factor of three when compared with measurements at three tropical forest sites. This overestimate suggests that more measurements and model studies are needed to examine the formation of organic aerosols in the tropics. The modeled organic carbon (OC) in the free troposphere is in agreement with measurements in the ITCT-2K4 aircraft campaign over North America and in pollution layers off Asia during the INTEX-B campaign, although the model underestimates OC in the free troposphere in comparison with the ACE-Asia campaign off the coast of Japan.

Clemens von Sonntag
Peter Dowideit
Fang Xingwang
Heinz-Peter Schuchmann

The reactions of peroxyl radicals occupy a central role in oxidative degradation. Under the term Advanced Oxidation Processes in drinking-water and wastewater processing, procedures are summarized that are based on the formation and high reactivity of the OH radical. These react with organic matter (DOC). With O2, the resulting carbon-centered radicals O2 give rise to the corresponding peroxyl radicals. This reaction is irreversible in most cases. An exception is hydroxycyclohexadienyl radicals which are formed from aromatic compounds, where reversibility is observed even at room temperature. Peroxyl radicals with strongly electron-donating substituents eliminate O2.−, those with an OH-group in a-position HO2.. Otherwise organic peroxyl radicals decay bimolecularly. The tetroxides formed in the first step are very short-lived intermediates and decay by various pathways, leading to molecular products (alcohols, ketones, esters and acids, depending on the precursor), or to oxyl radicals, which either fragment by scission of a neighbouring C-C bond or, when they carry an a-hydrogen, undergo a (water-assisted) 1,2-H-shift.

Tzung-May Fu
Daniel J. Jacob

We present the first global budgets of atmospheric glyoxal and methylglyoxal with the goal of quantifying their potential for global secondary organic aerosol (SOA) formation via irreversible uptake by aqueous aerosols and clouds. Our explicit simulation of glyoxal and methylglyoxal is based on the best current knowledge of source and sink processes. Global sources of glyoxal and methylglyoxal are 45 Tg y-1 and 140 Tg y-1, respectively. Isoprene is the largest global source of both dicarbonyls, producing 45% of glyoxal and 85% methylglyoxal. Acetylene and acetone act as background dicarbonyl sources due to their long lifetime. Atmospheric lifetimes of glyoxal and methylglyoxal are 2.9 hours and 1.6 hours, respectively, mainly due to removal by photolysis. Our simulated dicarbonyl concentrations at northern mid-latitudes during the growing season are in the range 10-100 ppt, consistent with in situ measurements. On a global scale, the highest simulated dicarbonyl concentrations are over biomass burning regions, in agreement with satellite observations. The global source of SOA from the irreversible uptake of dicarbonyls is estimated to be 11 Tg C y-1, including 2.6 Tg C y-1 from glyoxal and 8 Tg C y-1 from methylglyoxal. The dicarbonyl pathway could make a large contribution to SOA in the free troposphere over source regions, particularly under low-NOx conditions.

John H. Seinfeld
John F. Pankow

Carbonaceous compounds comprise a substantial fraction of atmospheric particulate matter (PM). Particulate organic material can be emitted directly into the atmosphere or formed in the atmosphere when the oxidation products of certain volatile organic compounds condense. Such products have lower volatilities than their parent molecules as a result of the fact that adding oxygen and/or nitrogen to organic molecules reduces volatility. Formation of secondary organic PM is often described in terms of a fractional mass yield, which relates how much PM is produced when a certain amount of a parent gaseous organic is oxidized. The theory of secondary organic PM formation is outlined, including the role of water, which is ubiquitous in the atmosphere. Available experimental studies on secondary organic PM formation and molecular products are summarized.

Secondary organic aerosols (SOA) constitute a significant fraction of ambient aerosols, but their global source is only beginning to be understood. Substantial evidence has shown that oxidation of water-soluble organic species in the liquid cloud leads to the formation of SOA. To evaluate this global source and explore its sensitivity to various assumptions concerning cloud properties, we simulate in-cloud SOA (IC-SOA) formation based on detailed multiphase chemistry incorporated into the newly developed Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry-climate model AM3. We find global IC-SOA production is around 20-30 Tg·yr-1between 1999 and 2001. Depending on season and location, oxalic acid accounts for 40-90% of the total IC-SOA source (particularly between 800 hPa-400 hPa), and glyoxylic acid and oligomers (formed by glyoxal and methylglyoxal in evaporating clouds) each contribute an additional 10-20%. Besides glyoxal and methylglyoxal (extensively studied by previous research), glycolaldehyde and acetic acid are among the most important precursors leading to the formation of IC-SOA, particularly oxalic acid. Different implementations of cloud fraction or cloud lifetime in global climate models could potentially modify estimates of IC-SOA mass production by 20-30%. Dense IC-SOA production occurs in the tropical and midlatitude regions of the lower troposphere (surface to 500 hPa). In DJF, IC-SOA production is concentrated over the western Amazon and southern Africa. In JJA, substantial IC-SOA production occurs over southern China and boreal forest regions. This study confirms a significant in-cloud source of SOA, which will directly and indirectly influence global radiation balance and regional climate.

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Source: https://www.researchgate.net/publication/260830633_Chemical_insights_explicit_chemistry_and_yields_of_secondary_organic_aerosol_from_OH_radical_oxidation_of_methylglyoxal_and_glyoxal_in_the_aqueous_phase

Andres Cuba

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