CORRELATED MILLIMETER-WAVE MEASUREMENTS OF CLO, N2O, AND HNO3 FROM MCMURDO, ANTARCTICA, DURING POLAR SPRING 1994

Ground-based observations of stratospheric ClO, N2O, and HNO3 were mad e almost continuously at McMurdo Station, Antarctica (77.9 degrees S, 166.6 degrees E), during the austral spring of 1994, using two separat e microwave receivers. Vertical profiles of these trace gases have bee n retrieved from the pressure broadened emission spectra between Septe mber 4 and October 8, 1994. In early September, McMurdo was located we ll inside the polar vortex, and high mixing ratios of chlorine monoxid e (up to 1.8 ppbv) were measured in the lower stratosphere. Because of vortex movement, later measurements were taken in edge regions, where ClO was found to be quite variable. This vortex movement also provide d an opportunity to study relative changes between all three species. Almost no HNO3 was seen below 20 km during the measurement period, ind icating that stratospheric air had been efficiently denitrified by pol ar stratospheric cloud formation. A significant increase of the nitric acid column was observed only around September 20, when McMurdo was c loser to the outer edge of the vortex. At the beginning of the measure ments, the vertical profiles of the inert tracer N2O had already desce nded so far that very little N2O was present above 20 km. During the o bservation period, the N2O distribution did not show strong changes ex cept for a slight downward trend which increased with altitude. This i ndicates, as noted in previous years, that subsidence continued in the stratosphere over McMurdo Station until at least early October, when measurements were stopped. The temporal correlations between the behav ior of ClO, N2O, HNO3, altitude, and temperature at the 50-hPa level, and of ozone measured by local ozonesondes show that changes in the at mospheric composition were partly due to dynamic effects. A backward t rajectory analysis was performed to interpret the ClO data in an attem pt to clarify some irregularities.