Recent studies show that isoprene-derived organosulfates are an important fraction of ambient secondary organic aerosol (SOA), adding up to 20% to the organic mass. Organosulfates with m/z of 199 and 183 relating to C4 compounds are found in ambient and laboratory generated SOA and a sulfate radical induced oxidation of methacrolein (MACR) and methyl vinyl ketone (MVK) has been shown to be a possible formation mechanism. In the present study, experiments on the sulfate radical-induced oxidation of methacrolein and methyl vinyl ketone were performed in bulk aqueous phase, as well as in an aerosol chamber, and finally compared with ambient PM10 samples collected at a rural East German village during the summer 2008, to investigate their relevance in aqueous phase SOA formation. Samples from aqueous phase experiments and extracts from filters were analysed with UPLC/(-)ESI-IMS-QTOFMS. All the samples showed the abundance of highly oxidised organosulfates with m/z 153, 155, 167, 183 and 199 corresponding to the species found in ambient particle samples. In the bulk phase studies with laser-induced sulfate radical formation, the signal intensities increased with increasing number of laser pulses, indicating the sulfate radical-induced formation of these organosulfates. Additionally, the chamber experiments showed a particle mass growth of about 10 microg m(-3) and 4 microg m(-3) for experiments on the reactive uptake of MACR and MVK with a sulfate radical precursor (K2S2O8) in the seed particles. Correlations of the C2 to C5 organosulfate species (including the m/z 215, C5H11O7S-), detected in the ambient samples were found to be very strong (r > 0.8), indicating that these compounds are formed from similar mechanisms and under equal environmental conditions. This study shows that sulfate radical-induced oxidation in the aqueous particle phase provides a reasonable explanation for the formation of these organosulfates from methacrolein and methyl vinyl ketone.

The OH-initiated oxidation of acetone in aqueous solution is investigated because of its potential implications in atmospheric chemistry. The UV-spectrum of the transient acetonylperoxy radical was measured. Two characteristic absorption bands of the acetonylperoxy radical spectrum are found in the 220-400 nm wavelength region. The rate constant for the recombination reaction of the acetonylperoxy radical was determined as a function of temperature for the first time in aqueous solution with k(rec,298 K) = (7.3 ± 1.3) × 10(8) M(-1) s(-1), E(A) = 4.5 ± 3.3 kJ mol(-1), and A = (4.7 ± 2.7) × 10(9) M(-1) s(-1). Furthermore, kinetic investigations of the OH-initiated oxidation of methylglyoxal and pyruvic acid were performed with the following results: for methylglyoxal, k(second) = (6.2 ± 0.2) × 10(8) M(-1) s(-1), E(A) = 12 ± 2 kJ mol(-1), and A = (7.8 ± 0.2) × 10(9) M(-1) s(-1); for pyruvic acid (pH = 0), k(second) = (3.2 ± 0.6) × 10(8) M(-1) s(-1), E(A) = 15 ± 5 kJ mol(-1), and A = (1.1 ± 0.1) × 10(11) M(-1) s(-1); for pyruvate (pH = 6), k(second) = (7.1 ± 2.4) × 10(8) M(-1) s(-1), E(A) = 25 ± 19 kJ mol(-1), and A = (1.5 ± 0.4) × 10(13) M(-1) s(-1). Quantitative product studies were done as a function of the number of laser photolysis pulses for acetone and its oxidation products methylglyoxal, hydroxyacetone, pyruvic acid, acetic acid, and oxalic acid. After the recombination reaction of acetonylperoxy radicals, there are two possible decomposition reactions where the primary products methylglyoxal and hydroxyacetone are formed. From product analysis after a single photolysis laser shot, the ratio of the main product-forming reactions was determined as (A) 30% and (B) 56% for the methylglyoxal formation via channel A to yield two molecules of methylglyoxal and channel B to yield one molecule of methylglyoxal and one molecule of hydroxyacetone. The remaining product can be ascribed to channel C, the radical-retaining channel forming alkoxy radicals with a yield of 14%. Pyruvic acid and acetic acid were found to be the major intermediates estimated with concentrations in the same order of magnitude and a similar time profile, indicating that acetic acid is also a possible oxidation product of methylglyoxal.

Kinetic and mechanistic studies were conducted on the isoprene oxidation products methacrolein, methyl vinyl ketone, methacrylic and acrylic acid reacting with hydroxyl and nitrate radicals and sulfate radical anions in aqueous solution by use of the laser flash photolysis technique and a reversed-rate method for kinetics. High-performance liquid chromatography/mass spectrometry was applied for product analysis. The kinetic investigations show the highest reactivity of the hydroxyl radical followed by sulfate and nitrate radicals. For methacrolein and methyl vinyl ketone the following rate constants have been determined at 298 K: k(OH+methacrolein) = (9.4 ± 0.7) × 10(9) M(-1) s(-1), k(OH+methyl vinyl ketone) = (7.3 ± 0.5) × 10(9) M(-1) s(-1), k(NO3+methacrolein) = (4.0 ± 1.0) × 10(7) M(-1) s(-1), k(NO3+methyl vinyl ketone) = (9.7 ± 3.4) × 10(6) M(-1) s(-1), k(SO4(-)+methacrolein) = (9.9 ± 4.9) × 10(7) M(-1) s(-1) and k(SO4(-)+methyl vinyl ketone) = (1.0 ± 0.2) × 10(8) M(-1) s(-1). Temperature and pH dependencies of the reactions of OH, NO3 and SO4(-) with methacrolein, methyl vinyl ketone, methacrylic and acrylic acid as well as Arrhenius parameters have been obtained and discussed. Product studies were performed on the OH radical induced oxidation of methacrolein and methyl vinyl ketone. In the reaction of methacrolein + OH methylglyoxal and hydroxyacetone were identified as first oxidation products with yields of 0.099 and 0.162. Methylglyoxal was primarily produced in the oxidation of methyl vinyl ketone with a yield of 0.052. For both precursor compounds the formation of glycolaldehyde was observed for the first time with yields of 0.051 and 0.111 in the oxidation of methacrolein and methyl vinyl ketone, respectively. Furthermore, highly functionalised C4 compounds were determined from the oxidation of both precursor compounds, but for the first time for methyl vinyl ketone. Reaction schemes were developed based on known peroxyl radical reaction mechanisms. The aqueous phase conversion of the first generation isoprene oxidation products can potentially contribute to tropospheric aqueous phase budgets of important carbonyl and dicarbonyl components which are expected to be conducive to the formation of aqSOA.

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Humic substances (HS) acting as photosensitizers can generate a variety of reactive species, such as OH radicals and excited triplet states ((3)HS*), promoting the degradation of organic compounds. Here, we apply compound-specific stable isotope analysis (CSIA) to characterize photosensitized mechanisms employing fuel oxygenates, such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), as probes. In oxygenated aqueous media, Λ (Δδ(2)H/Δδ(13)C) values of 23 ± 3 and 21 ± 3 for ETBE obtained by photosensitization by Pahokee Peat Humic Acid (PPHA) and Suwannee River Fulvic Acid (SRFA), respectively, were in the range typical for H-abstraction by OH radicals generated by photolysis of H2O2 (Λ = 24 ± 2). However, (3)HS* may become a predominant reactive species upon the quenching of OH radicals (Λ = 14 ± 1), and this process can also play a key role in the degradation of ETBE by PPHA photosensitization in deoxygenated media (Λ = 11 ± 1). This is in agreement with a model photosensitization by rose bengal (RB(2-)) in deoxygenated aqueous solutions resulting in one-electron oxidation of ETBE (Λ = 14 ± 1). Our results demonstrate that the use of CSIA could open new avenues for the assessment of photosensitization pathways.

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Abstract. 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−1), 2,3-butanedione (kp  =  9  ×  10−6 s−1), methylglyoxal (kp  =  3  ×  10−5 s−1), and hydroxyacetone (kp  =  2  ×  10−5 s−1). 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.