Ion Exclusion, pH, and Halogen Activation at the Air-Ice Interface

• Main Author: Wren, Sumi
• Other Authors: Donaldson, D. James
• Format: Thesis
• Language: English
• Published: 2013
• Subjects: air-ice interface; nitrate; glancing-angle spectroscopy; halogen activation; pH; snow chemistry; 0494; 0725; Sea ice
• Online Access: http://hdl.handle.net/1807/43757

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.