LONG-TERM STRATOSPHERIC-AEROSOL-AND-GAS-EXPERIMENT-I AND STRATOSPHERIC-AEROSOL-AND-GAS-EXPERIMENT-II MEASUREMENTS OF UPPER-TROPOSPHERIC AEROSOL EXTINCTION

A detailed analysis has been made of Stratospheric Aerosol and Gas Exp eriment I and II aerosol extinction data for the upper troposphere (6- km altitude to the seasonally averaged tropopause) taken between 1979 and 1998. An improved method of separation of the volcanic and surface -derived components of the aerosol optical depth has been used. The me an extinction, at a wavelength of 1.02 mu m, of the nonvolcanic compon ent of the upper tropospheric aerosol is found to increase from approx imately 1 x 10(-4) km(-1) at 70 degrees S to about 7 times that value at 70 degrees N. Maximum downward transfer of volcanic material into t he upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70 degrees S or greater in the southern hemisphere and at about 50 degrees N in the northern hemisph ere. The almost 20-year data sequence (1979-1981, 1984-1991, 1994-1998 ) has been examined for evidence of any long-term trends in the aeroso l optical depth of the upper troposphere. It is unlikely that any chan ge in the upper tropospheric 1-mu m aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere.