Organic peroxide and OH formation in aerosol and cloud water : laboratory evidence for this aqueous chemistry

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Introduction
Secondary organic aerosol (SOA) is a major component of atmospheric fine particulate matter [PM 2.5 ] (Zhang et al., 2007), contributes to adverse health, and affects climate by scattering (Seinfeld and Pandis, 1998) and sometimes by absorbing solar radiation (e.g., "brown carbon") (Andreae and Gelencser, 2006;Bones et al., 2010;Zhang et al., Introduction Conclusions References Tables Figures

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Full effects, the properties of SOA are poorly understood because SOA formation itself is poorly understood.Aqueous chemistry in atmospheric waters (e.g., cloud droplets or wet aerosols) is a potentially important pathway to produce SOA (SOA aq ; Blando and Turpin, 2000), and could be comparable in magnitude to "traditional" SOA, formed via partitioning of semivolatile organic products of gas-phase oxidation (SOA gas ) globally (Liu et al., 2012;Lin et al., 2012;Henze et al., 2008) and in locations where relative humidity and aerosol hygroscopicity are high (Carlton and Turpin, 2013;Carlton et al., 2008;Fu et al., 2008;Chen et al., 2007).Because SOA aq is formed from small watersoluble precursors with high O/C ratios, it forms SOA (e.g., oligomers, organic salts) with high O/C ratios and helps to explain the highly oxygenated nature of atmospheric organic aerosols, while SOA gas is less oxygenated (Aiken et al., 2008;Lim et al., 2010Lim et al., , 2013)).OH radicals are important oxidants in clouds.In the high solute concentrations present in wet aerosols a more complex system of organic radical and non-radical reactions occurs (Lim et al., 2010;McNeill et al., 2012;Ervens et al., 2014).Thus, an understanding of the availability of OH radicals is important to assessing the relative importance of radical and non-radical chemistry in aerosols.The uptake of gas-phase OH radicals into atmospheric waters (Faust and Allen, 1993) and Fenton reactions in the condensed/aqueous media (Arakaki and Faust, 1998) are considered the major oxidant sources for aqueous organic chemistry.Oxidant sources in organic-containing cloud, fog and aerosol waters and oxidant reactions with dissolved organic compounds have been documented (Arakaki et al., 2013;Weller et al., 2014;Long et al., 2013).Depending on sources of OH radicals, aqueous oxidation reactions could exhibit a surface area dependence (e.g., controlled by OH uptake), or a volume dependence (e.g., controlled by OH production through aqueous chemistry; Ervens et al., 2014).Herein, we explore the hypothesis that organic peroxides produce OH radicals within the atmospheric aqueous phase; we also demonstrate the formation of organic peroxides in the aqueous phase and their contribution to condensed phase chemistry.Organic peroxides (herein particularly, organic hydroperoxides = ROOH) are known to play an important role in gas phase chemistry.They are commonly found in the atmosphere with mixing ratios of 0.1-1 ppb (Lee et al., 1993;De Serves et al., 1994;Sauer et al., 2001;Grossmann et al., 2003;Lee et al., 2000;Guo et al., 2014).They are known to form through gas-phase reactions of volatile organic compounds (VOCs) with OH radical, NO 3 radical and O 3 (Atkinson and Arey, 2003).While their chemistry is not fully understood, these atmospheric organic species are "key" to peroxy radical/NO x chemistry (Dibble, 2007;Glowacki et al., 2012), lead to photochemical smog formation, important to the HO x -NO x -O 3 balance (Wennberg et al., 1998;Singh et al., 1995), contribute to O 3 formation or depletion in the upper troposphere, and form SOA (Tobias and Ziemann, 2000).Organic peroxides (formed from gas-phase ozonolysis of monoterpenes, e.g., α-and β-pinenes) are major constituents of SOA (Docherty et al., 2005).Monoterpenes have a global flux second only to isoprene and maybe the most efficient SOA gas precursor class (Kanakidou et al., 2005).Organic peroxides contribute to organic aerosol by forming peroxyhemiacetal oligomers with atmospherically abundant organic carbonyls (e.g., aldehydes and ketones) via acid catalysis in aerosols (Tobias andZiemann, 2000, 2002).Due to the characteristically weak O-O bonds of organic peroxides, the gas-phase decomposition of organic peroxides through photolysis or intermolecular radical reactions recycles OH radicals and can enhance gas-phase photooxidation of atmospheric organic compounds.Recent field studies demonstrate that gas-phase OH recycling enhances isoprene photooxidation (Paulot et al., 2009;Taraborrelli et al., 2012).
Organic peroxides are known to be moderately water soluble (Henry's law constant ∼ 100-1000 M atm −1 ).They are present in rainwater with concentrations of 0.1-10 µM (Lind et al., 1986;Hellpointer and Gab, 1989;Liang et al., 2013), presumably by uptake from the gas phase.In this work, we show that organic peroxides are also produced from aqueous-phase OH oxidation.We identify organic peroxide products from methylglyoxal and acid catalyzed oligomers (i.e., peroxyhemiacetals formed with methylglyoxal) by ultra-high resolution mass spectrometry.We simulate organic peroxide and Figures

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Full peroxyhemiacetal formation under atmospheric conditions and explore organic peroxide contributions to aqueous-phase OH production and to SOA aq formation.

Cuvette chamber reactions
Reactions of methylglyoxal with OH radicals in the aqueous phase were conducted in a cuvette chamber, which holds 10 cuvettes (3 mL each; Spectrocell) equidistant from a 254 nm Hg UV lamp (Strahler).Cuvettes were immersed in a water bath to maintain the temperature at 25  (Lim et al., 2013).The OH radical concentrations were estimated via modeling (Lim et al., 2013).It should be noted that using 20 mM of H 2 O 2 and the 254 nm UV lamp was not intended to simulate tropospheric photolysis, rather to provide a source of OH radicals.According to our previous control experiments (i.e., methylglyoxal + UV; methylglyoxal + H 2 O 2 ), small amounts of pyruvic, acetic and formic acids form slowly in control experiments.However, dicarboxylic acids, the major products, did not form in the absence of OH radicals (i.e., in control experiments; Tan et al., 2010).Photooxidation of methylglyoxal was allowed to proceed for 1 h.After being removed from the chamber, the cuvettes were kept frozen until analysis.No catalase was added in order to preserve organic peroxide products.Introduction

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Organic peroxide and peroxyhemiacetal analysis
Ultra high resolution Fourier Transform Ion Cyclotron Resonance Electrospray Ionization Mass Spectrometer (FTICR-MS; Thermo-Finnigan LTQ-XL, Woods Hole Oceanographic Mass Spectrometer Facility) was used to determine the elemental composition of organic products as described previously (Altieri et al., 2008;Tan et al., 2012).The capillary voltage and a capillary temperature were −30.00 V and 300 • C, respectively for negative mode analyses.Positive mode analyses were conducted with a capillary voltage of 20.00 V and a capillary temperature of 260 • C. Both FTICR-MS and FTICR-MS/MS were used to analyze organic peroxide products from aqueous photooxidation of methylglyoxal and a standard solution, which was prepared by adding 10 mM of tert-butyl hydroperoxide (Sigma-Aldrich) and 10 mM of methylglyoxal (Sigma-Aldrich).These samples were diluted 100 fold with water (by volume), and diluted again with methanol (MeOH) by 2 fold (by volume).Thus, the mobile phase consisted of 50 % water and 50 % MeOH; 0.1 % of formic acid (by volume) was also added.These diluted samples were immediately introduced into the electrospray ionization source by direct infusion at 5 µL min −1 .Photooxidation products of methylglyoxal were expected in both negative and positive modes due to a carboxylic group (negative mode) and a hydroxyl group (positive mode) in their structure (Table 1), whereas tert-butyl hydroperoxide is found only in the positive mode.

Organic peroxide chemistry
We hypothesize that aqueous-phase OH radical reactions of methylglyoxal lead to organic peroxide formation as shown in Fig. 1.OH radical reactions are initiated by Hatom abstraction.Subsequent O 2 addition and HO 2 decomposition mainly lead to the formation of pyruvic acid and acetic acid (Lim et al., 2013).Both pyruvic and acetic acid react further with OH radicals and O 2 , forming peroxy radicals (RO 2 ), which undergo bimolecular RO 2 -RO 2 reactions (Lim et al., 2013).However, substantial amounts of Introduction

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Full peroxy radicals could also react with HO 2 forming organic peroxides (as indicated by a bold arrow) since HO 2 is a common byproduct of aqueous photooxidation (Lim et al., 2010 and2013) and is also water soluble (Henry's law constant = 4 × 10 3 M atm −1 ; this is ∼ 100 times higher than that of OH radicals).
We further expect organic peroxides to form peroxyhemiacetals with aldehydes via acid catalysis in the aqueous phase (Fig. 2a), as they do in dry aerosols (Tobias and Ziemann, 2000;Docherty et al., 2005).Below we document the formation of peroxyhemiacetals from a commercially available organic peroxide, tert-butyl hydroperoxide and methylglyoxal in aqueous solution (Fig. 2b).Then we argue that organic peroxide products (R 1 OOH and R 2 OOH in Fig. 1) from the aqueous OH oxidation of methylglyoxal react further with methylglyoxal in water to produce peroxyhemiacetals.Briefly, a carbonyl group (aldehyde) in methylglyoxal is protonated by H + , then a hydroperoxyl group (-OOH) attacks a protonated carbonyl group forming peroxyhemiacetal.This peroxyhemiacetal chemistry is a well established oligomerization mechanism for SOA from gas-phase ozone reactions of alkenes in smog chamber studies (Tobias and Ziemann, 2000).In Tobias and Ziemann study, organic peroxides are first formed in the gas phase and become particles through gas-particle partitioning.Then organic peroxides form peroxyhemiacetals with by-product aldehydes through acid-catalyzed heterogeneous reactions on the particle surface.In current study, the detection of peroxyhemiacetals in our aqueous chemistry experiments (see below) provides evidence for organic peroxide formation through aqueous photochemistry.).In electron impact (hard ionization), fragmentation of organic peroxides results in the loss of HO 2 (Tobias and Ziemann, 2000;Docherty et al., 2005).However, O 2 loss is expected for soft ionization, IRMPD fragmentation in FTICR-MS/MS (M.Soule and E. Kujawinski, personal communication, 2013).In Fig. 5, the ion m/z + 81.06971 indicates the loss of O 2 from t-butyl hydroperoxide.We also observed the O 2 loss (m/z + 153.07158) from PHA std .FTICR-MS/MS and theoretical readings are provided in Table 2.Note that no organic peroxide peak was observed in the standard solution (nor in methylglyoxal + OH samples).This is not surprising because (1) high temperature of the capillary in an electrospray chamber (∼ 250 • C) is likely to decompose organic peroxides (M.Soule, personal communication, 2013), (2) it is difficult to ionize organic peroxides (Witkowski and Gierczak, 2013) and organic peroxides react with methylglyoxal to form peroxyhemiacetals.These peroxyhemiacetals are much more stable (lesser volatile) than organic peroxides (Tobias and Ziemann, 2000).These peroxy-Introduction

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Full hemiacetal peaks (and fragments) appear in FTICR-MS (and FTICR-MS/MS) analysis of standard solutions and samples (see below), providing evidence for the presence (and formation) of organic peroxides from methylglyoxal + OH.FTICR-MS/MS of peroxyhemiacetal peaks show corresponding organic peroxide fragments, methylglyoxal and other fragments as expected (Tobias and Ziemann, 2000;Docherty et al., 2005).

Aqueous photooxidation products of methylglyoxal
A FTICR mass spectrum of an aqueous methylglyoxal solution exposed to OH radicals for 60 min is shown in Fig. 6 1).
Fragmentation of these peaks by FTICR-MS/MS supports their identification as peroxyhemiacetals.In Fig. 7a, m results from the loss of O 2 from R 2 OOH, which is the organic peroxide constituent of PHA 2 .Note, as was the case with the mixed peroxide-aldehyde standard, the organic peroxides themselves were not observed (see previous section).Detected and theoretical readings are provided in Table 2.

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Atmospheric implications
Using ultra-high resolution FTICR-MS and FTICR-MS/MS, we observed the presence of peroxyhemiacetals, after aqueous photooxidation of methylglyoxal and in aqueous methylglyoxal-organic peroxide standard solutions.The presence of stable peroxyhemiacetals is an indicator of the existence of the less stable organic peroxides.Thus, this work provides evidence for the formation of organic peroxides through aqueous phase OH radical oxidation of methylglyoxal.

Organic peroxide production in clouds and wet aerosols
Below we demonstrate through chemical modeling that organic peroxides photochemically form from organics present both in clouds and wet aerosols.We used the full kinetic model for glyoxal and methylglyoxal (Lim et al., 2013) to simulate the formation of organic peroxides and peroxyhemiacetals.The following updates were made to the model: (1) the rate constant for the bimolecular reactions of RO 2 and HO 2 was given as 3 × 10 6 M −1 s −1 (Reactions R213-R219 in Table S1 in the Supplement) based on the rate constant for [HO 2 + HO 2 ] ∼ 1 × 10 6 M −1 s −1 , (2) the concentration of OH in the aqueous phase was set to ∼ 10 −14 (previously ∼ 10 −12 ) according to recent estimations (Arakaki et al., 2013) (Fig. S1a in the Supplement), (3) the concentration of HO 2 in the aqueous phase was estimated to be ∼ 10 −8 M maintained by the Henry's law equilibrium; therefore, the excess HO 2 produced by photooxidation in the aqueous phase was transported to the gas phase (Fig. S1b).All the reactions included in the model are listed in Table S1.For wet aerosol simulations, 1 M (the initial concentration) of methylglyoxal was used in the aqueous phase.Note that we do not expect that methylglyoxal is present at 1 M in aerosols.However, water-soluble organic matter is present at 1-10 M.So this analysis treats all water-soluble organic matter as if it behaves like methylglyoxal.Under wet aerosol conditions ([methylglyoxal]  The model also includes the sinks of aqueous-phase organic peroxides: OH radical reactions (Reactions R220-R225), photolysis (Reaction R230), and the evaporation to the gas phase (Reaction R234) in Table S1.Note that organic peroxide (ROOH) formation in Fig. 10a and b does not change within the Henry's law constant, 100 to 1000 M atm −1 , and the evaporation rate is assumed to be a diffusion-controlled transfer coefficient (Lelieveld and Crutzen, 1991;Lim et al., 2005), which is the upper limit based on the equation provided by Lelieveld and Crutzen (1991).Here, the gas-phase [ROOH] is assumed to be 1 ppb (R234 in Table S1).In atmospheric cloud conditions ∼ 0.4 µM of organic peroxide formation during the 12 h daytime is expected (Fig. 10b) while all the sinks of organic peroxides listed above are included.This concentration of aqueous-phase photochemically produced organic peroxides is within the range of measured rainwater concentrations (0.1-10 µM) and similar to the concentration expected by Henry's law equilibrium from gas-phase organic peroxides (0.1-1 ppb).

Peroxyhemiacetal formation in wet aerosol
The formation of peroxyhemiacetals competes with (1) hydration of methylglyoxal and (2) photolysis and OH reactions of organic peroxides (Fig. 9).Competing with methylglyoxal hydration means that only a dehydrated methylglyoxal (DeMGLY), not hydrated methylglyoxal (MGLY), forms a peroxyhemiacetal (PHA) with an organic peroxide (ROOH), since the aldehyde reacts with peroxides.The dehydration equilibrium for methylglyoxal is included in the model (R226 in Table S1).In wet aerosols, ∼ 0.4 mM of DeMGLY out of 1 M MGLY will undergo peroxyhemiacetal formation and react with OH radicals (Reactions R227 and R228 in Table S1) at the same time (Fig. S2a).The main sink for peroxyhemiacetals is expected to be OH reaction (no evaporation is expected).The peroxyhemiacetal formation equilibrium (Reaction R229) and the OH reaction of peroxyhemiacetals (Reaction R231) are listed in Table S1.The modified model simulates ∼ 0.4 µM of peroxyhemiacetal formation during the 12 h daytime and the minor increase during the nighttime (Fig. 10b).Under cloud conditions, peroxyhemiacetal for- mation is negligible (Note that the model simulates ∼ 4 × 10 −15 M peroxyhemiacetal formation during the daytime from 10 µM of methylglyoxal photooxidation in Fig. 10b).

OH recycling due to the photolysis of organic peroxides in atmospheric waters
In both cloud and wet aerosol conditions, 7.5 × 10 −15 M of aqueous-phase OH production is expected from the photolysis of organic peroxides ([ROOH] initial ∼ 400 µM in wet aerosols and [ROOH] initial ∼ 0.4 µM in cloud droplets) formed by aqueous photooxidation during the 12 h daytime (Fig. S3) while the sink of ROOH is OH reaction.Note that ∼ 10 −14 M of OH was recently estimated in atmospheric waters (Arakaki et al., 2013) and ∼ 10 −14 -10 −15 M of OH was previously estimated in the core of the bulk phase (Jacob, 1986).Thus, the aqueous production of organic peroxides in atmospheric waters could be an important source of aqueous OH through organic peroxide photolysis.
The Supplement related to this article is available online at doi:10.5194/acpd-15-17367-2015-supplement.Introduction

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b
MeOH = Methanol.Note that the mobile phase contains 50% water (with 0.05% formic acid) and 50% MeOH.c MeO is a deprotonated MeOH. 17

b
MeOH = Methanol.Note that the mobile phase contains 50% water (with 0.05% formic acid) and 50% MeOH.c MeO is a deprotonated MeOH.

a
Theoretical reading is based on actual atomic/molecular weights obtained by online software, "Molecular Isotopic Distribution Analysis (MIDAs)FTICR-MS and theoretical readings for methylglyoxal, organic peroxides and peroxy hemicetals.