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

Backscatter profiles were measured at 940 and 490 nm in the winter north polar vortex using balloon borne backscattersondes. There was no mid-season breakdown of the vortex, and temperatures were sufficiently cold to expect the formation of polar stratospheric clouds (PSCs). The mass loading from the Mount Pinatubo aerosols inside the vortex was observed to be larger than that of the previous year and equivalent to about 20 megatonnes if spread uniformly over the earth. The positive identification of typical PSCs was made more difficult by the presence of considerable volcanic aerosol background. An unusual scattering layer that can probably be identified as some type of PSC was not associated with as much backscatter as observed in previous non-volcanic years with similar stratospheric temperatures. The layer apparently was composed of comparatively large particles with a relatively low, highly variable number concentration and was easily identified by the enhancement in fine structure of the scattering properties rather than an outstanding increase in average scattering values. The arctic volcanic aerosol layer was found to produce a small but significant depolarized backscattered signal. This preliminary result could be interpreted as indicating 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.