THE SCAVENGING OF HIGH-ALTITUDE AEROSOL BY SMALL ICE CRYSTALS

There have been several global models developed for the theoretical investigation of the removal of high altitude aerosol from the atmosphere, following concern about the injection of particulate material by nuclear explosions and volcanic events. These models lack a knowledge of the scavenging efficiencies of the small ice crystals associated with cirrus clouds and storm ice anvils. These are the only hydrometeors that could remove the injected particles. In the past there have been a number of practical studies into the scavenging efficiencies of large ice crystals and snowflakes. A comparison of the extrapolated results of these findings and the theoretical models of Martin et al. (1980, Pure appl. Phys. 188, 1109-1129, J. atmos. Sci. 37, 1628-1638) for the small crystal situation has been made. It was found that in general the extrapolated results gave efficiencies that were significantly higher than the predicted value. This difference was found to be enhanced as the crystal diameter decreased. Experiments used small ice plates grown at approximately -18.5-degrees-C in a cloud chamber, which were then permitted to fall through a dense aerosol cloud, to provide the first direct measurements of the scavenging efficiencies of these small crystals under cloud conditions. Initial results are presented for mono-disperse NaCl aerosol particles of size 4-6-mu-m.