VOLCANIC PERTURBATION OF THE ATMOSPHERE IN BOTH POLAR-REGIONS - 1991-1994

Long-term measurements by sunphotometers of the spectral dependence of aerosol optical depth are reported for several sites in the Arctic an d Antarctic for the period January 1991 through December 1994. In the Antarctic a pronounced increase of atmospheric turbidity was observed at the end of September 1991. The observed wavelength dependence in ae rosol optical depth indicated that the increase was due to the presenc e of fresh and therefore small stratospheric aerosol particles associa ted with the eruption of Cerro Hudson in August 1991. After the breakd own of the polar vortex in mid-November 1991 we measured a second sign ificant increase of the aerosol optical depth. At this time the 1.0-mu m aerosol optical depth was approximately 0.12 or about 10 times back ground levels. This second increase is shown to be the result of the i nflux of Mount Pinatubo aerosols. A similar perturbation of the aeroso l optical depth was observed in the Arctic with the return of sunlight in March 1992. However, the increased loading of the Arctic stratosph ere by the Pinatubo aerosols was already evident al high northern lati tudes in satellite measurements at the end of 1991. Stratospheric Aero sol and Gas Experiment II stratospheric 1.0-mu m optical depth measure ments show that meridional transport of Pinatubo aerosol from equatori al to middle and higher latitudes is greatest in the winter/spring hem isphere. This observation explains the observed seasonal trend of aero sol optical depth during the posteruption. A significant decrease of t he perturbation by Mount Pinatubo aerosol was observed in both polar r egions by the end of 1994. The measured 1.0-mu m aerosol optical depth s at this time were only approximate to 0.04; these values exceed the background level by about 0.01-0.02. Therefore the aerosol optical dep th values are still slightly higher than during undisturbed conditions . In addition, we show that the occurrence of volcanic aerosols caused changes in the spectral dependence of the aerosol optical depth in th e Arctic and the Antarctic. These variations, including the changes in the aerosol size distribution, derived from the aerosol optical depth , are discussed in comparison to undisturbed conditions.