A pinch of salt is all it takes: chemistry at the frozen water surface.

Chemical interactions at the air-ice interface are of great importance to local atmospheric chemistry but also to the concentrations of pollutants deposited onto natural snow and ice. However, the study of such processes has been hampered by the lack of general, surface-specific probes. Even seemingly basic chemical properties, such as the local concentration of chemical compounds, or the pH at the interface, have required the application of assumptions about solute distributions in frozen media. The measurements that have been reported have tended for the most part to focus on entire ice or snow samples, rather than strictly the frozen interface with the atmosphere. We have used glancing-angle laser spectroscopy to interrogate the air-ice interface; this has yielded several insights into the chemical interactions there. The linear fluorescence and Raman spectra thus measured have the advantage of easy interpretability; careful experimentation can limit their probe depth to that which is relevant to atmospheric heterogeneous processes. We have used these techniques to show that the environment at the interface between air and freshwater ice surfaces is distinct from that at the interface between air and liquid water. Acids such as HCl that adsorb to ice surfaces from the gas phase result in significantly different pH responses than those at liquid water surfaces. Further, the solvation of aromatic species is suppressed at freshwater ice surfaces compared with that at liquid water surfaces, leading to extensive self-association of aromatics at ice surfaces. Photolysis kinetics of these species are much faster than at liquid water surfaces; this can sometimes (but not always) be explained by red shifts in the absorption spectra of self-associated aromatics increasing the extent to which solar radiation is absorbed. The environment presented by frozen saltwater surfaces, in contrast, appears to be reasonably well-described by liquid water. The extent of hydrogen bonding and the solvation of adsorbed species are similar at liquid water surfaces and at frozen saltwater surfaces. Adsorbed acids and bases evoke similar pH responses at frozen saltwater ice surfaces and liquid water surfaces, and photochemical kinetics of at least some aromatic compounds at frozen saltwater ice surfaces are well-described by kinetics in liquid water. These differences are not observed in experiments that interrogate the entire ice sample (i.e., that do not distinguish between processes occurring in liquid regions within bulk ice and those at the air-ice interface). Our work has shown that in general, the chemistry occurring at salty frozen interfaces is well described as being cold aqueous chemistry, whereas that seen at the pure ice interface is not. These findings have significant implications for heterogeneous atmospheric processes in ice-covered environments.