STRATOSPHERIC CLOUD OBSERVATIONS DURING FORMATION OF THE ANTARCTIC OZONE HOLE IN 1989

The results of six balloon flights at McMurdo Station, Antarctica, under varying temperature conditions, are used in a study of polar stratospheric clouds during September 1989. A new particle counter, with size resolution in the 0.5-mu-m radius region, indicated that size distributions observed in the clouds were bimodal. Mode radii ranging from 0.05 to 0.10-mu-m were observed for the small particle mode, representing the sulfate layer or condensational growth enhancements of it. Mode radii generally ranged from 1.5 to 3.5-mu-m for the large particle mode at concentrations 3 to 4 orders of magnitude lower than the small particle mode. The large particle mode, when observed at temperatures above the water ice point, is believed to be the result of nitric acid trihydrate (NAT) condensation on larger particles of the sulfate layer. In this case the HNO3 condensed mass mixing ratios were 1 to 5 ppbv for most of the cloud layers. Generally, the large particle NAT concentrations were higher in the lower stratosphere, indicating the redistribution of HNO3 through particle sedimentation. On several occasions, distributions were observed with mode radii as high as 7-mu-m, and correspondingly large inferred mass, indicating water ice clouds in the 12 to 15 km region. On other occasions, absence of such clouds at very low temperatures inferred water vapor mixing ratios of less than 3 ppmv.