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.

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.

Although the air-ice interface is atmospherically important, it is difficult to model accurately because exclusion and precipitation of solutes during freezing, deposition of atmospheric species, and heterogeneous/photochemical processes all affect its properties. In this thesis, glancing-angle spectroscopic methods were developed to study ice surfaces. Glancing-angle Raman spectroscopy showed that nitrate is not strongly excluded to the ice surface during freezing, in contradiction with expectations based on equilibrium thermodynamics. Glancing-angle laser-induced fluorescence showed that hydronium ions are not strongly excluded when dilute acidic solutions (HNO3 or HCl) are frozen. These results suggest that solutes are not universally excluded and that care should be taken in modelling surface concentrations on ice. Deposition of HCl(g) was found to result in different pH responses at the "pure" vs. "salty" ice surfaces. Changes at the "salty" ice surface were consistent with the existence of a brine layer at the air-ice interface while changes at the "pure" ice surface were distinctly different, indicating that it may not be appropriate to model it as a cold, liquid layer. Significantly, results also suggest that the sea ice surface is buffered against pH changes, with important implications for interpreting pH-dependent chemistry. The conversion of sea-salt derived halides to reactive halogen species can lead to dramatic changes in the oxidative capacity of the overlying atmosphere. At ambient pH and naturally occurring halide concentrations, the dark ozonation of NaBr and NaI solutions was found to proceed more quickly on frozen vs. aqueous substrates, consistent with a freeze-concentration enhancement in halide concentration at the surface. A photochemical mechanism for halogen release from artificial saline snow was evidenced. The presence of ozone and light in the actinic region leads to accelerated production of Br2 and BrCl and the release of Cl2, in a process enhanced by high surface area, acidity and additional gas phase Br2. The results provide strong evidence for snowpack "halogen explosion" chemistry in which gas phase halogens are recycled back into a concentrated brine layer at the snow grain surface.

Although the air-ice interface is atmospherically important, it is difficult to model accurately because exclusion and precipitation of solutes during freezing, deposition of atmospheric species, and heterogeneous/photochemical processes all affect its properties. In this thesis, glancing-angle spectroscopic methods were developed to study ice surfaces. Glancing-angle Raman spectroscopy showed that nitrate is not strongly excluded to the ice surface during freezing, in contradiction with expectations based on equilibrium thermodynamics. Glancing-angle laser-induced fluorescence showed that hydronium ions are not strongly excluded when dilute acidic solutions (HNO3 or HCl) are frozen. These results suggest that solutes are not universally excluded and that care should be taken in modelling surface concentrations on ice. Deposition of HCl(g) was found to result in different pH responses at the "pure" vs. "salty" ice surfaces. Changes at the "salty" ice surface were consistent with the existence of a brine layer at the air-ice interface while changes at the "pure" ice surface were distinctly different, indicating that it may not be appropriate to model it as a cold, liquid layer. Significantly, results also suggest that the sea ice surface is buffered against pH changes, with important implications for interpreting pH-dependent chemistry. The conversion of sea-salt derived halides to reactive halogen species can lead to dramatic changes in the oxidative capacity of the overlying atmosphere. At ambient pH and naturally occurring halide concentrations, the dark ozonation of NaBr and NaI solutions was found to proceed more quickly on frozen vs. aqueous substrates, consistent with a freeze-concentration enhancement in halide concentration at the surface. A photochemical mechanism for halogen release from artificial saline snow was evidenced. The presence of ozone and light in the actinic region leads to accelerated production of Br2 and BrCl and the release of Cl2, in a process enhanced by high surface area, acidity and additional gas phase Br2. The results provide strong evidence for snowpack "halogen explosion" chemistry in which gas phase halogens are recycled back into a concentrated brine layer at the snow grain surface.