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Abstract
Abstract. The aqueous phase reaction of volatile organic compounds (VOCs) has not been considered in most analyses of atmospheric chemical processes. However, some experimental evidence has shown that, compared to the corresponding gas phase reaction, the aqueous chemical processes of VOCs in the bulk solutions and surfaces of ambient wet particles (cloud, fog, and wet aerosols) may potentially contribute to the products and formation of secondary organic aerosol (SOA). In the present study, we performed a laboratory experiment of the aqueous ozonolysis of isoprene at different pHs (3–7) and temperatures (4–25 °C). We detected three important kinds of products, including carbonyl compounds, peroxide compounds, and organic acids. Our results showed that the molar yields of these products were nearly independent of the investigated pHs and temperatures, those were (1) carbonyls: 56.7 ± 3.7 % formaldehyde, 42.8 ± 2.5 % methacrolein (MAC), and 57.7 ± 3.4 % methyl vinyl ketone (MVK); (2) peroxides: 53.4 ± 4.1 % hydrogen peroxide (H2O2) and 15.1 ± 3.1 % hydroxylmethyl hydroperoxide (HMHP); and (3) organic acids: undetectable (<1 % estimated by the detection limit). Based on the amounts of products formed and the isoprene consumed, the total carbon yield was estimated to be 94.8 ± 4.1 %. This implied that most of the products in the reaction system were detected. The combined yields of both MAC + MVK and H2O2 + HMHP in the aqueous isoprene ozonolysis were much higher than those observed in the corresponding gas phase reaction. We suggest that these unexpected high yields of carbonyls and peroxides are related to the greater capability of condensed water, compared to water vapor, to stabilize energy-rich Criegee radicals. This aqueous ozonolysis of isoprene (and possibly other biogenic VOCs) could potentially occur on the surfaces of ambient wet particles and plants. Moreover, the high-yield carbonyl and peroxide products might provide a considerable source of aqueous phase oxidants and SOA precursors.
Highlights
Carbonyl and peroxide compounds are important trace components in the atmosphere
We determined the amounts of carbonyls, peroxides, and organic acids for this aqueous ozonation system
We identified and quantified five products, including formaldehyde (HCHO), MAC, methyl vinyl ketone (MVK), hydroxylmethyl hydroperoxide (HMHP), and H2O2
Summary
Carbonyl and peroxide compounds are important trace components in the atmosphere. In recent years, these two compounds are paid more and more attention, because they form components or precursors of secondary organic aerosol (SOA) (e.g., Mochida et al, 2006; Tong et al, 2006; Hallquist et al, 2009), and they produce reactive radical oxidants, including OH, HO2, and RO2 (e.g., Jacob, 1986; Atkinson and Arey, 2003). The gas phase ozonolysis of biogenic unsaturated volatile organic compounds (VOCs) was considered to be an important pathway that simultaneously formed carbonyls and peroxides (Atkinson and Carter, 1984; Becker et al, 1990; Gab et al, 1995; Neeb et al, 1997; Sauer et al, 1999; Zhang et al, 2002; Chen et al, 2008). Henry’s law constant was considered to be the key parameter that determined the Published by Copernicus Publications on behalf of the European Geosciences Union
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