VOLCANIC AEROSOL AND POLAR STRATOSPHERIC CLOUDS IN THE WINTER 1992 93NORTH POLAR VORTEX

Backscatter profiles were measured at 940 and 490 nm in the winter nor th polar vortex using balloon borne backscattersondes. There was no mi d-season breakdown of the vortex, and temperatures were sufficiently c old to expect the formation of polar stratospheric clouds (PSCs). The mass loading from the Mount Pinatubo aerosols inside the vortex was ob served to be larger than that of the previous year and equivalent to a bout 20 megatonnes if spread uniformly over the earth. The positive id entification of typical PSCs was made more difficult by the presence o f considerable volcanic aerosol background. An unusual scattering laye r that can probably be identified as some type of PSC was not associat ed with as much backscatter as observed in previous non-volcanic years with similar stratospheric temperatures. The layer apparently was com posed of comparatively large particles with a relatively low, highly v ariable number concentration and was easily identified by the enhancem ent in fine structure of the scattering properties rather than an outs tanding increase in average scattering values. The arctic volcanic aer osol layer was found to produce a small but significant depolarized ba ckscattered signal. This preliminary result could be interpreted as in dicating the presence of some non-spherical particles (perhaps due to freezing effects) and/or spherical particles containing inclusions of irregular shapes with differing refractive index.