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 i nto 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 a erosol characteristics is of utmost concern owing to the seasonality i n the observed ozone depletion and climate implications. This study fo cuses on Antarctic stratospheric aerosol characteristics during three temporal periods: September 23-30, September 30 to October 13, and Nov ember 13-27, 1991, at latitudes poleward of 60 degrees S for verticall y averaged characteristics, and at latitudes poleward of 50 degrees S for temporal and spatial characteristics. Stratospheric aerosol charac teristics are inferred from the Stratospheric Aerosol and Gas Experime nt (SAGE) II measurements using a modified randomized minimization sea rch technique (RMST). Aerosol characteristics such as size distributio n, 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 instanc es for all three time periods and can be fitted with the sum of two lo gnormal distributions. Larger concentrations are observed for particle s of all sizes between 18 and 30 km during November 1991, signaling th e 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 be tween 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, 199 1, and 0.3 cm s(-1) during mid to late November 1991. The mass deposit ion flux was estimated to be 9 x 10(-13) g cm(-2) s(-1). The peeling o f segments of the Pinatubo aerosol was found near the outer edge of th e circumpolar vortex wail centered around 56 degrees S. Meridional wav e 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 diff erent segments of the vortex edge. In the first region we found waveli ke isopleths with rising motions between 55 degrees and 58 degrees S a nd sinking motions thereafter. In the second region, rising motions we re located between 60 degrees and 63 degrees S and sinking motions at latitudes poleward of 63 degrees S. In contrast to previous reports, w e found an indication of entrainment of volcanic aerosol into the vort ex edge. The Mount Pinatubo aerosol is an effective tracer of south po lar transport.