CHANGES IN ANTARCTIC STRATOSPHERIC AEROSOL CHARACTERISTICS DUE TO VOLCANIC-ERUPTIONS AS MONITORED BY THE STRATOSPHERIC AEROSOL AND GAS EXPERIMENT-II SATELLITE

An estimated 20-30 megatons of SO2 and crustal material was injected into the stratosphere during June 12-16, 1991, by the eruption of Mount Pinatubo (15.1 degrees N, 120.4 degrees E). The impact on Antarctic aerosol characteristics is of utmost concern owing to the seasonality in the observed ozone depletion and climate implications. This study focuses on Antarctic stratospheric aerosol characteristics during three temporal periods: September 23-30, September 30 to October 13, and November 13-27, 1991, at latitudes poleward of 60 degrees S for vertically averaged characteristics, and at latitudes poleward of 50 degrees S for temporal and spatial characteristics. Stratospheric aerosol characteristics are inferred from the Stratospheric Aerosol and Gas Experiment (SAGE) II measurements using a modified randomized minimization search technique (RMST). Aerosol characteristics such as size distribution, number concentration, mass loading, surface area concentration, and radial characteristics are derived between 15 and 30 km for particles having radii between 0.1 and 0.8 mu m. Results indicate that aerosol size distributions between 15 and 30 km are bimodal in several instances for all three time periods and can be fitted with the sum of two lognormal distributions. Larger concentrations are observed for particles of all sizes between 18 and 30 km during November 1991, signaling the arrival of;the Mount Pinatubo plume. An order of magnitude increase in concentration is observed for particles with radii between 0.1 and 0.2 mu m and between 0.7 and 0.8 mu m. Vertical aerosol profiles show that the peak in aerosol concentration shifted to a higher altitude between 21 and 26 km as compared to the preplume peak between 15 and 18 km. Using the displacement asa function of time for a mass loading of 1.7 mu g m(-3) isopleth, we estimated meridional velocity approximate to 0.9 m s(-1), zonal velocity approximate to 16 m s(-1) and downward vertical velocity of 0.5 cm s(-1) during September to mid-October, 1991, and 0.3 cm s(-1) during mid to late November 1991. The mass deposition flux was estimated to be 9 x 10(-13) g cm(-2) s(-1). The peeling of segments of the Pinatubo aerosol was found near the outer edge of the circumpolar vortex wail centered around 56 degrees S. Meridional wave activity was also observed poleward of 56 degrees S. Two meridional separation regimes were found: between 56 degrees and 62 degrees S and between 62 degrees and 66 degrees S. These two regions represent different segments of the vortex edge. In the first region we found wavelike isopleths with rising motions between 55 degrees and 58 degrees S and sinking motions thereafter. In the second region, rising motions were located between 60 degrees and 63 degrees S and sinking motions at latitudes poleward of 63 degrees S. In contrast to previous reports, we found an indication of entrainment of volcanic aerosol into the vortex edge. The Mount Pinatubo aerosol is an effective tracer of south polar transport.