Theoretical Studies of Heterogeneous Halogen Chemistry on Stratospheric and Tropospheric Ice

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9613802 Hynes In this project, quantum chemical electronic structure methods and molecular dynamics computer simulations will be carried out to study the rates and mechanisms of a number of reactions of chlorine and bromine species on the surface of atmospherically relevant particles. These range from pure water ice to more complex surfaces which include nitrate and/or sulfate ions. Heterogeneous reactions involving chlorine and bromine have been strongly implicated in the catalytic destruction of ozone in the Antarctic stratosphere and in the Arctic troposphee. Specific examples include the one-step reaction of chlorine nitrate with hydrochloric acid and the two- step version involving chlorine nitrate hydrolysis forming hypochlorous acid which then reacts with hydrochloric acid to form molecular chlorine. The bromine analogs of these reactions as well as the mixed bromine/chlorine cases will also be considered. This study will provide reaction mechanisms and reaction rates which are needed as input for global chemistry and climate models. It is hypothesized that the ice lattice plays an essential catalytic role in these reactions via proton transfers. It is often difficult to determine experimentally whether a process occurs in one or several steps, and whether reactions are ionic in character or not.