INTERHEMISPHERIC DIFFERENCES IN POLAR STRATOSPHERIC HNO3, H2O, CIO, AND O-3

Simultaneous global measurements of nitric acid (HNO3), water (H2O), c hlorine monoxide (ClO), and ozone (O-3) in the stratosphere have been obtained over complete annual cycles in both hemispheres by the Microw ave Limb Sounder on the Upper Atmosphere Research Satellite. A sizeabl e decrease in gas-phase HNO3 was evident in the lower stratospheric vo rtex over Antarctica by early June 1992, followed by a significant red uction in gas-phase H2O after mid-July. By mid-August, near the time o f peak ClO, abundances of gas-phase HNO3 and H2O were extremely low. T he concentrations of HNO3 and H2O over Antarctica remained depressed i nto November, well after temperatures in the lower stratosphere had ri sen above the evaporation threshold for polar stratospheric clouds, im plying that denitrification and dehydration had occurred. No large dec reases in either gas-phase HNO3 or H2O were observed in the 1992-1993 Arctic winter vortex. Although ClO was enhanced over the Arctic as it was over the Antarctic, Arctic O-3 depletion was substantially smaller than that over Antarctica, A major factor currently limiting the form ation of an Arctic ozone ''hole'' is the lack of denitrification in th e northern polar vortex, but future cooling of the lower stratosphere could lead to more intense denitrification and consequently larger los ses of Arctic ozone.