How does deposition of gas phase species affect pH at frozen salty interfaces?

Chemical processes occurring on snow and ice surfaces play an important role in controlling the oxidative capacity of the overlying atmosphere. However, efforts to gain a better, mechanistic understanding of such processes are impeded by our poor understanding of the chemical nature of the air-ice interface. Here we use glancing-angle laser induced fluorescence in conjunction with harmine – a surface-active, pH-sensitive fluorescent dye – to investigate how the nature of the ice, whether frozen freshwater, salt water or seawater, influences pH changes at the surface. Deposition of HCl(g) leads to a very different pH response at the frozen freshwater surface than at the frozen salt water surface indicating that these two surfaces present different chemical environments. Importantly, the sea ice surface is buffered against pH changes arising from deposition of gas phase species. These results have important implications for understanding pH-sensitive processes occurring at the air-ice boundary, such as bromine activation.