Atmos. Chem. Phys. Discuss., 12, 6419-6445, 2012
© Author(s) 2012. This work is distributed
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Unexpected high yields of carbonyl and peroxide products of aqueous isoprene ozonolysis and implications
H. L. Wang1, D. Huang1, X. Zhang1,*, Y. Zhao1, and Z. M. Chen1
1State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
*now at: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, USA

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. These products included (1) carbonyls: 56.7 ± 6.7% 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 95 ± 4%. This implied that most of the products in the reaction system were detected. Of note, 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 suggested that these unexpected high yields of carbonyls and peroxides were 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. Thus, aqueous ozonolysis on the surface of plants, where carbonyls and peroxides form, might affect biogenic VOC emissions and the deposition of O3 and SO2 onto leaves to different extents in clean and polluted regions.

Citation: Wang, H. L., Huang, D., Zhang, X., Zhao, Y., and Chen, Z. M.: Unexpected high yields of carbonyl and peroxide products of aqueous isoprene ozonolysis and implications, Atmos. Chem. Phys. Discuss., 12, 6419-6445, doi:10.5194/acpd-12-6419-2012, 2012.
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