POLAR STRATOSPHERIC CLOUDS DUE TO VAPOR ENHANCEMENT - HALOE OBSERVATIONS OF THE ANTARCTIC VORTEX IN 1993

Aerosol measurements from the Halogen Occultation Experiment (HALOE) d uring the Antarctic spring of 1993 are compared with calculations of t he volume of different types of polar stratospheric clouds (PSCs) at e quilibrium. The observed volumes increased by a factor of similar to 3 0 coincident with water vapor enhancements of similar to 3 ppmv, sugge sting that the enhancement of water vapor was important in determining PSC growth. The enhanced water vapor was coincident with increased me thane mixing ratios, and trajectory analysis suggests that the vapor e nhancements were consistent with transport from lower latitudes. The n itric acid distribution was not measured and is treated either as cons tant or as positively correlated with the observed water. Comparing th e observed volumes with model calculations assuming constant nitric ac id suggests that the PSCs were composed of liquid ternary H2SO4-H2O-HN O3 aerosols (LTA) rather than solid nitric acid trihydrate (NAT). Howe ver, if the water vapor intrusions were accompanied by enhanced nitric acid, the observations closely match predictions for solid NAT, and t hose for LTA. These comparisons highlight the importance of the vapor distributions for modeling PSC growth and suggest that vapor resupply is important for late spring PSC growth. This work also highlights som e inherent limitations of large field of view limb-viewing instruments for the observation of PSCs.