ESTIMATION OF HETEROGENEOUS REACTION-RATES FOR STRATOSPHERIC TRACE GASES WITH PARTICULAR REFERENCE TO THE DIFFUSIONAL UPTAKE OF HCL AND CLONO2 BY POLAR STRATOSPHERIC CLOUDS

The stratosphere holds a variety of particulates like polar stratosphe ric clouds (PSCs) and sulphate aerosols which catalyse chemical reacti ons. These reactions cause changes in the composition of the stratosph ere, including the redistribution of active chlorine which might lead to ozone destruction. As a result during recent years a lot of effort has been directed towards the quantification of the uptake of trace ga ses like ClONO2, HCl, etc. into these particulates. However, it has be en observed that many of the two and three dimensional models used in such studies are constrained by the lack of adequate rate constant dat a. This paper describes a theoretical approach to estimate the reactio n rate constants for 23 gases on both types of polar stratospheric clo uds (type I and II). It is found that for gases like N2O5, ClONO2 and HCl, diffusional uptake is important and contributes significantly to the heterogeneous reaction rate. A complete Lennard-Jones calculation is used to accurately compute the trace gas diffusion coefficients.