This award will continue support of research on the wintertime north polar stratospheric vortex, which is focused on phenomena that are believed to be related to polar stratospheric ozone depletion. The measurements are made by balloon borne instruments, some of which are especially designed for this purpose. The instruments will measure ozone concentration, water vapor concentration by the frost point method, temperature, pressure, nitric acid, and aerosols and polar stratospheric clouds. The aerosol and cloud measurements will be made by a new instrument called a backscattersonde that has been developed by the principal investigator for this work. The balloons will be flown from Alert, NWT, Canada; Heiss Island, USSR; the Kola Peninsula, USSR; and Frobisher Bay, NWT, Canada. The first two sites are located well within the normal position of the polar vortex while the remaining two are on the edge of the vortex and in a region where there is some sunlight at mid-winter. The goal of the research is to elucidate the role that polar stratospheric clouds and aerosols play in the destruction of ozone. This project involves a collaboration between U.S., Canadian and Soviet scientists. The project is very unusual in that it includes a Soviet Co-Principal Investigator, though there is no exchange of funds. The project has been recommended for approval by COMEX of the CIA.