A new source of methylglyoxal in the aqueous phase

Carbonyl compounds are ubiquitous in atmospheric multiphase system participating in gas, particle, and aqueous-phase chemistry. One important compound is methyl ethyl ketone (MEK), as it is detected in significant amounts in the gas phase as well as in cloud water, ice, and rain. Consequently, it can be expected that MEK influences the liquid-phase chemistry. Therefore, the oxidation of MEK and the formation of corresponding oxidation products were investigated in the aqueous phase. Several oxidation products were identified from the oxidation with OH radicals, including 2,3-butanedione, hydroxyacetone, and methylglyoxal. The molar yields were 29.5 % for 2,3-butanedione, 3.0 % for hydroxyacetone, and 9.5 % for methylglyoxal. Since methylglyoxal is often related to the formation of organics in the aqueous phase, MEK should be considered for the formation of aqueous secondary organic aerosol (aqSOA). Based on the experimentally obtained data, a reaction mechanism for the formation of methylglyoxal has been developed and evaluated with a model study. Besides known rate constants, the model contains measured photolysis rate constants for MEK (kp= 5× 10 −5 s), 2,3-butanedione (kp= 9× 10 −6 s), methylglyoxal (kp= 3× 10 −5 s), and hydroxyacetone (kp= 2× 10 −5 s). From the model predictions, a branching ratio of 60 / 40 for primary/secondary H-atom abstraction at the MEK skeleton was found. This branching ratio reproduces the experiment results very well, especially the methylglyoxal formation, which showed excellent agreement. Overall, this study demonstrates MEK as a methylglyoxal precursor compound for the first time.


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In the last decades, carbonyl compounds have been a subject of intense research due to their ubiquitous abundance 19 and their effect on atmospheric chemistry and human health. They are emitted directly from biogenic and 20 anthropogenic sources or formed through the oxidation of hydrocarbons (e.g., Atkinson, 1997 One carbonyl compound that is emitted from numerous and mainly biological sources is methyl ethyl 23 ketone (MEK). It is released from grass, clover (Kirstine et al., 1998;de Gouw et al., 1999), different types of 24 forests, and biomass burning processes (Khalil and Rasmussen, 1992;Warneke et al., 1999;Isidorov et al., 1985). 27 Yokelson et al., 2013). In addition, MEK is emitted into the atmosphere through the application as solvent for the 28 production of glue, resins, cellulose, rubber, paraffin wax and lacquer (Ware, 1988).

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Tropospheric MEK gas-phase concentration was found to be in the range of 0.02 -15 ppbv, depending

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Furthermore, an enrichment of MEK in the surface micro layer was found with concentrations up to 2.28 nmol L -1 38 (Zhou and Mopper, 1997). MEK was also investigated in ice, fog, and rain samples (Grosjean and Wright, 1983).

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It was not found in fog but there were traces in rain water. In cloud water, a concentration of up to 650 nmol L -1 40 was measured. This is supported by van Pinxteren et al. (2005), who measured a concentration of 41 70 to 300 nmol L -1 in cloud water. These studies concluded that the liquid-phase fraction of MEK is higher than 42 the expected fraction calculated according to the Henry constant.

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The Henry constants at a temperature of 25 °C were found to vary between 7.7 and 21 M atm -1 in 44 numerous studies (Buttery et al., 1969;Snider and Dawson, 1985;Ashworth et al., 1988;Zhou and Mopper, 1990; 45 Morillon et al., 1999;Karl et al., 2002). However, Schütze and Herrmann (2004) estimated the Henry constant to 46 be between 23 and 50 M atm -1 , which is higher than the previous measured values found in the literature. This 47 higher Henry constant supports the conclusion from van Pinxteren et al. (2005) and tends to support the 48 investigation of MEK in the liquid phase as aqSOA precursor compound. AqSOA is formed through the oxidation 49 of organic compounds in the aqueous particle phase and is often related to missing SOA sources. These missing 50 sources are most likely responsible for the huge discrepancies between measured and calculated SOA burden. As

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In the present study, the reaction of MEK with OH radicals in water was investigated. Based on the 55 experimentally obtained data, a reaction mechanism was developed to explain methylglyoxal formation. The            hydroxyacetone. This is in good agreement with the experimental results obtained in the present study.

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Since 2,3-butanedione is the main oxidation product, it was necessary to investigate the contribution of 157 2,3-butanedione to the product distribution, especially for the formation of methylglyoxal. In the oxidation of 2,3-158 butanedione (Set 5), no methylglyoxal was detected in the GC/MS chromatogram over a reaction period of 159 240 min (Fig. 2). Consequently, a contribution of 2,3-butanedione to the methylglyoxal formation could be 160 excluded.

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Despite the low molar yield of hydroxyacetone during MEK oxidation, the oxidation of hydroxyacetone

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this source is of atmospheric relevance. Due to the molar yields of 9.5% for methylglyoxal, 29.5% for 2,3-179 butanedione, and 3.0% for hydroxyacetone, ≈ 42% of the oxidation products of MEK could be elucidated in the 180 present study with only these carbonyl compounds (Table 2). This highlights the importance of carbonyl 181 compounds for the aqueous phase chemistry. Based on the experimental findings, a reaction mechanism was 182 developed to describe the formation of methylglyoxal (Fig. 4).

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The primary and secondary alkyl radicals react rapidly with oxygen to form alkylperoxy radicals. The alkylperoxy that can react further, as described before through pathways i-iii. The described mechanism was included in a 198 COPASI model to examine the developed oxidation mechanism, the decomposition of the precursor compound, 199 and the formation of the observed products. Table 3 shows the considered reactions, the rate constants, and their 200 references. Only the reactions leading to the formation of the products identified are discussed in detail.
201 Surprisingly, the products 3-oxobutanal and hydroxybutanone were not observed during the experiments. Since

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The results are shown in Fig. 6 and discussed based on the molar yields of the products. As can be seen 225 for 2,3-butanedione (Fig. 6A), a branching ratio of 60% for the primary H-atom abstraction and 40% for the 226 secondary H-atom abstraction leads to lower molar yields, whereas the molar yields start to increase with an 227 increasing fraction of secondary H-atom abstraction. According to the mechanism (Fig. 4)

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The oxidation of hydroxyacetone with OH radicals was also considered in the model study with a rate 302 constant of k = 8 × 10 8 M -1 s -1 (R33; Stefan and Bolton, 1999). During the experiment, methylglyoxal was formed 303 with 100% molar yield. Thus, the reaction of hydroxyacetone to methylglyoxal was included in the model study.

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There is also good agreement for the formation of methylglyoxal and limited agreement for the molar yields of 314 2,3-butanedione. The initial high molar yield of 2,3-butanedione is reflected well (Fig. 7B). Thus, after 60 minutes 315 of reaction time, molar yields of 23.7% in the model study and 29.5 ± 6.0% in the experiment were reached. Under 316 consideration of the standard deviation, this is in good agreement with the COPASI model.

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The determined molar yields up to a reaction time of 120 minutes showed very good conformity with the  The sources of methylglyoxal in the aqueous phase are thus-far not fully elucidated. Methylglyoxal can originate 328 in the atmospheric aqueous phase through i) uptake from the gas phase, and/or ii) formation in the aqueous phase.

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The importance of the uptake from the gas into the aqueous phase is discussed in the literature but large 330 discrepancies can be found. Kroll et al. (2005) investigated the uptake of methylglyoxal on inorganic seed particles 331 under varying relative humidity. It was found that the uptake was not relevant for methylglyoxal even under high 332 relative humidity. Contrary, Zhao et al. (2006) measured an uptake coefficient of γ = 7.6 × 10 -3 on acidic solution.

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They found an irreversible uptake, which decreases with increasing acidity. Fu et al. (2008Fu et al. ( , 2009) determined an 334 uptake coefficient on aqueous particles and cloud droplets γ = 2.9 × 10 -3 , which is in good agreement with the

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The in-situ formation of methylglyoxal in the aqueous phase could be an important source as well (Blando and

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Further carbonyl compounds could be identified and quantified. 2,3-Butanedione was found as the main 358 oxidation product (molar yield ≈ 29.5%) and was formed during the photolysis of MEK as well. As a further 359 oxidation product, hydroxyacetone was identified and was formed with a molar yield of ≈ 3.0% during the 360 oxidation of MEK.

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The oxidation mechanism of MEK in aqueous solution was elucidated, and MEK was demonstrated to 362 be as a precursor compound for methylglyoxal in the aqueous phase. Regarding the important role of 363 methylglyoxal for the aqSOA formation, MEK has to be considered for aqSOA as well, which could be a next