Aqueous-Phase Photochemical Formation of Hydrogen Peroxide and Hydroxyl Radical in Clouds and Fogs

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9307171 Faust Aqueous-phase reactions of hydrogen peroxide (HOOH) and hydroxyl radical (OH) in atmospheric water drops play crucial roles in the chemistry of the troposphere, yet knowledge of the sources of these oxidants to the water drops is incomplete. While gas-to- drop partitioning of HOOH and OH has received much study, comparatively little attention has been focussed on aqueous-phase photochemical reactions as sources of these oxidants to atmospheric water drops. The objectives of this research are: (a) determination of quantum yields and sunlight rates of HOOH and OH photo-formation in authentic cloud and fog waters collected over several years from several locations, (b) determination of the seasonal dependence of HOOH and OH photo-formation, (c) comparisons of the relative importance of aqueous-phase photochemistry versus gas-to-drop partitioning as sources of HOOH and OH to atmospheric water drops, (d) to evaluate chemical and Physical parameters of authentic atmospheric waters as predictors (based on mechanistic considerations) of the HOOH and OH photo-formation rates and of the OH lifetime, (e) to determine rates and quantum yields of HOOH photo-formation from compounds of relevance to cloud and fog chemistry, and (f) to increase the understanding of mechanisms responsible for photochemical formation of the oxidants in authentic atmospheric waters and form the individual compounds. The project will characterize the photo-formation of HOOH and OH in authentic cloud and fog waters obtained form several high elevation sites. Additionally, survey studies will be carried out on the HOOH photo-formation form well-defined aqueous solutions of individual organic compounds. Overall, the results form this proposal will allow for a more accurate understanding to the sources of HOOH and OH to cloud and fog drops. This will improve our understanding of the atmospheric chemical cycles of peroxides, sulfur, carbon, iron, and other elements. This new knowledge will enhance our predictions of sulfuric acid and sulfate aerosol formation that affect acid deposition and the global cooling effect exerted by sulfate aerosols.