Using balloon soundings, Hoffman and Deshler [1991] found that particle distributions in Antarctic PSC's were bimodal. The small particle mode ranged from 0.05 to 0.10 microns and represented the sulfate layer. The large particle mode was between 1.5 and 3.5 microns radius and had concentrations 3 to 4 orders of magnitude less than the small particle mode. The large particle mode was observed at temperatures above the frost point, and is believed to be associated with NAT condensation on larger particles in the sulfate layer, similar to the findings of Kawa [1992] and Dye et al. [1992] in the Arctic. Similar observations of a bimodal size spectra were made in 1990 by Deshler et al. [1991] and Lin and Saxena [1992]. Lidar observations of PSC's by Collins et al. [1993] show downward motions in the cloud layers similar to the phase progressions of upwardly propagating gravity waves, suggesting that the kilometer scale vertical structure of the PSC's is maintained by low frequency gravity waves propagating through the cloud layers.
Microphysical model calculations of the heterogeneous chemistry occurring in type I PSC's by Drdla et al. [1993] show that the presence and surface area of type I PSCs early in winter are crucial in determining ozone depletion.
A number of other factors have been suggested to effect the formation of PSC's. Peter et al. [1991] showed that exhaust from aircraft in the stratosphere can increase the partial pressures of nitric acid and water vapor, and thus allow the formation of ozone depleting PSC's at warmer temperatures than currently observed. Solomon et al. [1993] suggested that sulfuric acid droplets produced by volcanic eruptions may also lead to ozone depletion in the stratosphere, since similar reactions take place on sulfuric acid droplets as on the surfaces of PSC's. They suggest that the Mt. Pinatubo aerosols probably contributed to the unprecedented depth and areal extent of the Antarctic ozone depletion in 1992.
Due to the important role played by PSCs in the depletion of ozone near the poles, it is critical that more details on the microphysics of these clouds be obtained through future field programs.