Long‐term Stratospheric Aerosol and Gas Experiment I and II measurements of upper tropospheric aerosol extinction

A detailed analysis has been made of Stratospheric Aerosol and Gas Experiment 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 mean extinction, at a wavelength of 1.02 μm, of the nonvolcanic component of the upper tropospheric aerosol is found to increase from approximately 1 × 10 -4 km - 1 at 70°S to about 7 times that value at 70°N. Maximum downward transfer of volcanic material into the upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70°S or greater in the southern hemisphere and at about 50°N in the northern hemisphere. The almost 20-year data sequence (1979-1981, 1984-1991, 1994-1998) has been examined for evidence of any long-term trends in the aerosol optical depth of the upper troposphere. It is unlikely that any change in the upper tropospheric 1-μm aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere.