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 coefficient at wavelengths of 1064 and 532 nm and the depolarization ratio at 532 nm of PSCs were measured by the lidar system. The stratospheric temperature was below the estimated frost point of nitric acid tri-hydrate (NAT) in the winter of 1994/1995. PSCs were observed more frequently in this low-temperature period than in previous winters since 1991. The characteristics of the PSCs were very variable but had a noticeable vertical ''sandwich'' structure in January in which a layer of liquid PSC particles at the altitude around 20 km existed between the two solid particle layers. The wavelength dependence of the backscattering shows that the size of both liquid and solid particles was larger than the average size of background stratospheric aerosols. Lidar observations of the liquid layer particles show characteristics in qualitative agreement with those expected from model PSC particles grown in ternary solutions of H2SO4, HNO3, and H2O with a temperature decrease. However, the observed backscattering coefficient and its wavelength dependence indicate that PSC particles require further growth than that predicted by the ternary solution model at temperature where most HNO3 molecules in the surrounding atmosphere are considered to be condensed on PSCs.