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

Abstract

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.