LIDAR OBSERVATIONS OF POLAR STRATOSPHERIC CLOUDS AT THE SOUTH-POLE .2. STRATOSPHERIC PERTURBED CONDITIONS, 1992 AND 1993

Observations of polar stratospheric clouds (PSCs), carried out at the Amundsen Scott South Pole Station by lidar from May 1992 through Octob er 1993, are reported and compared with previously obtained results. A t that time the Antarctic stratosphere was loaded with sulfuric acid a erosol due to the eruptions of Mount Pinatubo, primarily, and of Mount Hudson. The seasonal evolution of the backscatter profiles has been i nvestigated in relation to the presence of the volcanic aerosol and to the processes of PSC formation, particle sedimentation, and dehydrati on. During the first winter after the eruptions the PSC activity was m ore intense than in the following year, particularly above 12.5 km, wh ere the amount of volcanic aerosol was larger in 1992 than in 1993. At lower altitudes the volcanic aerosol loading as well as the PSC pheno menon were comparable during the 2 years. No substantial changes in th e signal due to the volcanic aerosol has been observed comparing the b ackscattering profiles before and after the PSC periods (June-Septembe r), except for a downward shift, attributed to the subsidence of the a ir inside the polar vortex. It is concluded that only a small fraction of the aerosol particles, probably those with the largest radii, were involved in the nucleation of PSC particles.