POLAR STRATOSPHERIC CLOUDS OBSERVED BY LIDAR OVER SPITSBERGEN IN THE WINTER OF 1994 1995 - LIQUID PARTICLES AND VERTICAL SANDWICH STRUCTURE/

Polar stratospheric clouds (PSCs) were observed by lidar at Ny-Alesund , Spitsbergen, in December 1994 and January 1995. The backscattering c oefficient at wavelengths of 1064 and 532 nm and the depolarization ra tio at 532 nm of PSCs were measured by the lidar system. The stratosph eric temperature was below the estimated frost point of nitric acid tr i-hydrate (NAT) in the winter of 1994/1995. PSCs were observed more fr equently in this low-temperature period than in previous winters since 1991. The characteristics of the PSCs were very variable but had a no ticeable vertical ''sandwich'' structure in January in which a layer o f liquid PSC particles at the altitude around 20 km existed between th e two solid particle layers. The wavelength dependence of the backscat tering shows that the size of both liquid and solid particles was larg er than the average size of background stratospheric aerosols. Lidar o bservations of the liquid layer particles show characteristics in qual itative agreement with those expected from model PSC particles grown i n ternary solutions of H2SO4, HNO3, and H2O with a temperature decreas e. However, the observed backscattering coefficient and its wavelength dependence indicate that PSC particles require further growth than th at predicted by the ternary solution model at temperature where most H NO3 molecules in the surrounding atmosphere are considered to be conde nsed on PSCs.