DYNAMIC ICE SURFACE IN THE POLAR STRATOSPHERE

Abstract

Heterogeneous reactions on polar stratospheric clouds (PSCs), such as ClONO2+HCl — Cl2+HNO3 , are important for an understanding of the production of active chlorine species (HOCl, Cl2 ) and Antarctic ozone depletion. H2O -ice forms the Type II PSCs and the nature of H2O -ice surfaces under stratospheric conditions may affect the heterogeneous chemistry. This review focuses on recent measurements of H2O adsorption kinetics on ice, H2O desorption kinetics from ice, H2O surface diffusion on ice and H2O diffusion into ice. These measurements reveal that the ice surface is extremely dynamic under polar stratospheric conditions. For example, the residence time for an H2O molecule on an ice surface at 188 K is only ~20 milliseconds before desorption and only ~0.4 milliseconds before diffusion into the ice bulk. The dynamic nature of the ice surface may significantly affect the adsorption, solvation, diffusion and reaction of the chlorine reservoir molecules ( ClONO2 , HCl). The dynamic ice surface may also serve as a model for the surfaces of other molecular solids.