Heterogeneous chemistry on Antarctic polar stratospheric clouds: A microphysical estimate of the extent of chemical processing

A detailed model of polar stratospheric clouds (PSCs), which includes nucleation, condensational growth, and sedimentation processes, has been applied to the study of heterogeneous chemical reactions. For the first time, the extent of chemical processing during a polar winter has been estimated for an idealized air parcel in the Antarctic vortex by calculating in detail the rates of heterogeneous reactions on PSC particles. The resulting active chlorine and NO x concentrations at first sunrise are analyzed with respect to their influence upon the Antarctic ozone hole, using a photochemical model. It is found that the species present at sunrise are primarily influenced by the relative values of the heterogeneous reaction rate constants (and thus the “sticking coefficients”) and the initial gas concentrations. However, the extent of chlorine activation is also influenced by whether N 2 O 5 is removed by reaction with HCl or H 2 O. The reaction of N 2 O 5 with HCl, which occurs rapidly on type 1 PSCs, activates the chlorine contained in the reservoir species HCl. Hence the presence and surface area of type 1 PSCs early in the winter are crucial in determining ozone depletion.