Abstract. Recent studies indicate that isoprene and its gasphase oxidation products could contribute a considerable amount of aerosol through aqueous-phase acid-catalyzed oxidation with hydrogen peroxide (H 2 O 2 ), although the source of H 2 O 2 is unclear. The present study revealed a potentially important route to the formation of aqueous oxidants, including H 2 O 2 , from the aqueous-phase ozonolysis of methacrolein (MAC) and methyl vinyl ketone (MVK). Laboratory simulation was used to perform the atmospheric aqueous-phase ozonolysis at different pHs and temperatures. Unexpectedly high molar yields of the products, including hydroxylmethyl hydroperoxide (HMHP), formaldehyde (HCHO) and methylglyoxal (MG), of both of these reaction systems have been seen. Moreover, these yields are almost independent of pH and temperature and are as follows: (i) for MAC-O 3 , 70.3±6.3% HMHP, 32.3±5.8% HCHO and 98.6±5.4% MG; and (ii) for MVK-O 3 , 68.9±9.7% HMHP, 13.3±5.8% HCHO and 75.4±7.9% MG. A yield of 24.2±3.6% pyruvic acid has been detected for MVK-O 3 . HMHP is unstable in the aqueous phase and can transform into H 2 O 2 and HCHO with a yield of 100%. We suggest that the aqueous-phase ozonolysis of MAC and MVK can contribute a considerable amount of oxidants in a direct and indirect mode to the aqueous phase and that these compounds might be the main source of aqueous-phase oxidants. The formation of oxidants in the aqueous-phase ozonolysis of MAC and MVK can lead to substantial aerosol formation from the aqueous-phase acid-catalyzed reaction of H 2 O 2 with MAC, even if there are no other sources of oxidants.

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 (H 2 O 2 ) 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 H 2 O 2 + 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.