This project is directed to identifying the mechanisms and quantifying the rates of atmospherically important heterogeneous processes. The full range of these processes includes the transfer of molecules from the gaseous to the condensed state by new particle formation or by adsorption onto or dissolution into a microdroplet or aerosol particle, together with concomitant chemical conversion. These processes can serve to remove species from the gas phase, thereby altering their lifetimes, transport and removal processes, and at the same time influencing their further participation in gas phase reactions and enhancing those possible in the condensed phase. The thrust of the present proposal is on studies designed to elucidate the mechanisms of reactions which involve aqueous systems comprising the liquid media associated with (some) aerosols, cloud droplets and ice crystals through an investigation of clusters. Some critical experiments will be conducted in each of the aforementioned areas, including work designed to elucidate transformations in aqueous phases and the influence of various neutral and ionic species. The proposed studies are subdivided into four general areas. The first, to which major emphasis will be given, constitutes an in-depth investigation of reactions involving water clusters, both pure and containing other constituents, along with important atmospheric molecules and free radicals. An investigation of mixed cluster systems between water and other molecules such as salts and acids, along with a determination of the role of various co-clustered species on the structure and bonding, will also be undertaken. The second area, which is related to the aforementioned, involves an investigation of the thermochemistry of mixed clusters. The primary objective of this aspect of the work is to provide data from which to determine the partial pressure of constituents interacting with droplets (related to activity coefficients). The third aspect of the work involves studies of the influence of light on the reactions identified to be important in the first area. The fourth activity constitutes a calculational phase of the program devoted to elucidating the structure and bonding of cluster species identified in the other three phases of the program as being of critical importance to the reactions and the activity coefficients. Calculations of vibrational frequencies and related dynamical aspects of clusters will be of value in interpreting the results of the proposed studies in the first three areas.
