pH at the Air-Ice Interface: Connections to Halogen Activation Chemistry

Abstract:

Halogen activation – the conversion of relatively inert sea-salt derived halides, into reactive halogen species – has significant implications for perturbing the oxidative capacity of the boundary layer, as well as for mercury oxidation. Developing a good, mechanistic understanding of halogen activation chemistry – and particularly its pH dependence – requires knowledge of surface pH. Here we described the development and use of a surface-sensitive spectroscopic approach to investigate pH at frozen surfaces. The approach, which involves glancing-angle laser-induced fluorescence in conjunction with pH-sensitive fluorescent dyes (acridine and harmine), was used to study pH at frozen surfaces following the deposition of gas phase acids or bases. We demonstrate that frozen ‘pure water’ and frozen ‘salt water’ surfaces exhibit different responses. The responses are consistent with the presence of a chemically unique, disordered region at the frozen ‘pure water’ surface and the presence of liquid brine at the frozen ‘salt water’ surface. Significantly, we show that a frozen sea water surface is resistant to changes in pH, suggesting that some buffering capacity is maintained upon freezing. Finally the pH dependence of bromine and chlorine activation from artificial saline snow is presented. Bromine release is found to depend less strongly on pre-freezing snow pH, but the release of both species is favoured for low pre-freezing pH values. In the context of these pH and halogen activation studies, the potential importance of various frozen sea ice substrates for promoting halogen activation chemistry will be discussed.