Experimental Investigations of Physical and Chemical Processes at Air-ice Interfaces

Studies were performed to characterize the physical nature of the air-ice interface, and to clarify its role in processes that occur there. A glancing-angle Raman probe was developed to monitor hydrogen bonding at atmospheric interfaces; we saw enhanced hydrogen-bonding on ice compared to on water. Using glancing-angle laser-induced fluorescence (LIF), we determined that small acids and bases dissociated to similar extents at air-water and air-ice interfaces, but aromatic compounds were less well solvated at air-ice interfaces, resulting in self-association even at low surface coverages. We measured uptake kinetics of organic compounds using LIF and Raman spectroscopy. The uptake kinetics can be adequately fit by a single-exponential growth equation, but in order to properly describe the self-association of aromatics observed at the air-ice interface, equations accounting for self-association should be incorporated into the uptake model. A simple model was developed for naphthalene which included terms for self-association; good fits to the observed growth of intensity from monomeric and self-associated naphthalene were obtained. Direct photolysis of aromatics was faster at air-ice interfaces than in bulk ice or aqueous solution. While red shifts in the absorption spectra of benzene and naphthalene at air-ice interfaces could explain their enhanced reactivity there, the enhanced anthracene photolysis kinetics on ice are likely due to enhanced absorption cross sections or photolysis quantum yields, or to a different photolysis mechanism there. Oxidation rates of aromatics by photo-formed hydroxyl radicals are suppressed at air-ice interfaces, but not in bulk ice. Similarly, gas-phase OH reacts rapidly with aromatics at air-water interfaces, but no reaction is observed at air-ice interfaces. Conversely, the reactivity of ozone toward phenanthrene is enhanced there. This is not due to temperature effects or to enhanced partitioning of ozone to ice. Ozonation of bromide is also more rapid at air-ice interfaces than at air-water interfaces at environmentally relevant bromide concentrations. This enhancement could be due to exclusion of bromide to the air-ice interface during freezing. The rapid reactions of ozone with bromide and phenanthrene at air-ice interfaces suggest that both reactions could be atmospherically important.Ph