SIMULTANEOUS OZONE AND POLAR STRATOSPHERIC CLOUD OBSERVATIONS AT SOUTH-POLE STATION DURING WINTER AND SPRING 1991

Simultaneous polar stratospheric cloud (PSC) and ozone measurements were made over South Pole Station using a two-wavelength backscattersonde. This instrument produces aerosol profiles similar to those obtained with a ground-based lidar system but with higher vertical resolution. In one sounding, depolarization of the PSCs was also measured. The backscattersondes were supplemented with occasional frost point soundings. The measurements made before the appearance of PSCs do not show clear evidence of particle deliquescence, suggesting that the background sulfate particles may be frozen solids rather than liquids. PSCs began appearing at approximately 20 km when the temperature at that altitude dropped to -80-degrees-C (193 K). Initially, there was apparent evidence of supersaturation (with respect to nitric acid trihydrate) associated with some type I PSCs, while other examples indicated that the condensation of nitric acid was in quantitative agreement with that expected from the saturation vapor pressure and available nitric acid vapor. The apparent supersaturated layers (which occurred within the first 2 weeks of the onset of PSCs) can alternatively be interpreted as denitrified regions. The wavelength dependence of the backscatter is used to deduce rough particle sizes, and in particular, type Ia and Ib population types can be readily identified by the backscattersonde when not occurring as mixed systems. The mode radius of the first observed PSCs of the season was approximately 0.5 mum. In the polarization sensitive sounding, two varieties of type I PSCs were observed, one of which exhibited significant depolarization and another which produced very little depolarization. This observation would be consistent with the classification of types Ia and Ib, respectively. At the precise time that sunlight was returning to the stratosphere near South Pole Station, a strong inverse correlation in the structure of PSCs and ozone mixing ratio was observed. Using trajectory analysis, it is argued that the effect is probably the result of chemical depletion rather than transport processes. This chance observation is consistent with enhanced ozone depletion occurring in association with sunlit PSCs during the early spring.