HETEROGENEOUS CHLORINE CHEMISTRY IN THE TROPOPAUSE REGION

Satellite observations of cloud optical depths and occurrence frequenc ies are used as input to a two-dimensional numerical model of the chem istry and dynamics of the atmosphere to study the effects of heterogen eous reactions on cloud surfaces upon chemical composition and ozone d epletion in the tropopause region, Efficient reactions of ClONO2 with HCl and H2O, and of HOCl with HCl, are likely to take place on the sur faces of cirrus clouds [Borrmann et al., 1996] and perturb chlorine ch emistry, much as they do on polar stratospheric clouds present at high er altitudes and colder temperatures, Because of the very low predicte d background abundances of ClO near the tropopause, such reactions cou ld enhance the local ClO mixing ratios by up to 30-fold at midlatitude s. Substantial perturbations are also predicted for related chemical s pecies (e.g., HCl, HOCl, ClONO2, NO2, HO2) in the midlatitude and trop ical tropopause regions due to these heterogeneous reactions. If cirru s clouds occur with sufficient frequency and spatial extent, they coul d influence not only the chemical composition but also the ozone deple tion in the region near the tropopause. Because of variations in obser ved cloud occurrence frequency and in photochemical and dynamical time scales, the presence of cirrus clouds likely has its largest effect on ozone near the midlatitude tropopause of the northern hemisphere in s ummer.