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Abstract
Abstract. Aqueous phase chemical processes of organic compounds in the atmosphere have received increasing attention, partly due to their potential contribution to the formation of secondary organic aerosol (SOA). Here, we analyzed the aqueous OH-initiated oxidation of isoprene and its reaction products including carbonyl compounds and organic acids, regarding the acidity and temperature as in-cloudy conditions. We also performed a laboratory simulation to improve our understanding of the kinetics and mechanisms for the products of aqueous isoprene oxidation that are significant precursors of SOA; these included methacrolein (MACR), methyl vinyl ketone (MVK), methyl glyoxal (MG), and glyoxal (GL). We used a novel chemical titration method to monitor the concentration of isoprene in the aqueous phase. We used a box model to interpret the mechanistic differences between aqueous and gas phase OH radical-initiated isoprene oxidations. Our results were the first demonstration of the rate constant for the reaction between isoprene and OH radical in water, 1.2 ± 0.4) × 1010 M−1 s−1 at 283 K. Molar yields were determined based on consumed isoprene. Of note, the ratio of the yields of MVK (24.1 ± 0.8 %) to MACR (10.9 ± 1.1%) in the aqueous phase isoprene oxidation was approximately double that observed for the corresponding gas phase reaction. We hypothesized that this might be explained by a water-induced enhancement in the self-reaction of a hydroxy isoprene peroxyl radical (HOCH2C(CH3)(O2)CH = CH2) produced in the aqueous reaction. The observed yields for MG and GL were 11.4 ± 0.3 % and 3.8 ± 0.1 %, respectively. Model simulations indicated that several potential pathways may contribute to the formation of MG and GL. Finally, oxalic acid increased steadily throughout the course of the study, even after isoprene was consumed completely. The observed yield of oxalic acid was 26.2 ± 0.8 % at 6 h. The observed carbon balance accounted for ~50 % of the consumed isoprene. The presence of high-molecular-weight compounds may have accounted for a large portion of the missing carbons, but they were not quantified in this study. In summary, our work has provided experimental evidence that the availably abundant water could affect the distribution of oxygenated organic compounds produced in the oxidation of volatile organic compounds.
Highlights
Since the 1980s, scientific attention has focused on chemical processes of organic compounds found in the atmospheric aqueous phase, including clouds, fog, rain, and wet aerosols
We focused on the multifunctional carbonyl compounds, because they were considered to be the precursors of secondary organic aerosol (SOA)
We found that the YMVK (18.9 ± 0.8 %) and YMACR (9.0 ± 1.1 %) observed in the aqueous phase isoprene-OH reaction were significantly different from those observed in the corresponding gas phase reaction
Summary
Since the 1980s, scientific attention has focused on chemical processes of organic compounds found in the atmospheric aqueous phase, including clouds, fog, rain, and wet aerosols. The photochemical reactions of several VOCs, including isoprene (Boge et al, 2006) and aromatics (Kroll et al, 2007; Ng et al, 2007), in the presence of wet seed particles in the chamber were investigated for SOA formation Those works did not distinguish the aqueous phase chemical processes from those of the gas phase; they observed that the wet aerosols facilitated the degradation of the poorly-soluble VOCs. Currently, little information is available on the aqueous phase chemical processes of poorly-soluble VOCs. The rate constants and mechanism for the aqueous oxidation of VOCs are so limited that one cannot evaluate the relative importance of those aqueous processes. Using the determined aqueous rate constant, we evaluate the relative importance of aqueous surface OH oxidation of isoprene compared with the corresponding gas phase reaction
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