MICROPHYSICS AND HETEROGENEOUS CHEMISTRY OF POLAR STRATOSPHERIC CLOUDS

Liquid and solid particles in polar stratospheric clouds are of centra l importance for the depletion of stratospheric ozone. Surface-catalyz ed reactions on these particles, and diffusion-controlled processes in the bulk of the particles, convert halogens, which derive from compou nds of mainly anthropogenic origin, from relatively inert reservoir sp ecies into forms that efficiently destroy ozone. The microphysics of t hese particles under cold stratospheric conditions is still uncertain in many respects, in particular concerning phase transitions such as f reezing nucleation and deposition nucleation. Furthermore, there are i ndications that the rates of key heterogeneous reactions have not yet been established with sufficient accuracy to enable a reliable diagnos is of observed ozone losses by means of global models. The present pap er reviews the current (late 1996) knowledge of the physico-chemistry of polar stratospheric clouds and evaluates the remaining uncertaintie s with respect to their ozone depletion potential.