The Importance of Halogen Atom Chemistry in the Arctic Troposphere

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9631572 Shepson Recent observations in the Arctic indicate that there is a poorly understood process occurring in which ozone is rapidly destroyed in the lower troposphere near the surface of the Arctic Ocean. A number of field measurement studies have resulted in the hypothesis that ozone is being destroyed by a catalytic cycle involving Br atoms. If this were important in the marine environments in general, it would have substantial implications for our understanding and modeling of the global budget of tropospheric ozone. However, there are several areas of the chemistry and dynamics of the process that are poorly understood, for example: What is the source of bromine atoms? What is the fate of the BrO radicals produced? What are the important removal mechanisms for reactive bromine radicals? What is the mechanism for recycling HBr? What are the spatial and temporal characteristics of ozone destruction in the Arctic troposphere? Purdue University has brought together four groups of researchers with complementary capabilities to enable them to uniquely pursue the answers to these questions. The specific objectives, and associated approaches, of this research effort are: (1) Measurement of the kinetics of the operative reactions, presumably occurring on the surface of or within sea salt or acidic sulfate aerosol, whereby bromide ions are oxidized to molecular bromine or other photolytically active bromine compounds, under conditions relevant to the Arctic. These measurements will be conducted using stopped-flow and pulsed accelerated flow spectroscopic techniques. (2) Investigation of the competitive reaction paths for Br and Cl atom reactions with aldehydes and ketones, the reactions between BrO and ClO radicals, and the role of anions in catalyzing aqueous phase oxidation of the bromide ion. This work will be conducted using ab initio computational methods. (3) Development and application of highly sensitive and selective methods for the mea surement of carbonyl compounds. This instrument will be used in future field projects in the Arctic. The results of the laboratory, field and computational studies will be integrated into a box model of the detailed chemistry occurring in the Arctic marine boundary layer. (4) The Purdue Mesoscale Model will be refined for application to the Arctic. The modified and optimized model will then be tested against observational data from the Arctic, followed by incorporation of the box-model chemistry. ***