In 1978
In -1980
nine aircraft flights to an altitude of up to 15 km were made over western Europe. Sulfur dioxide was measured with a sensitive chemiluminescence method consisting of separate sampling and analysis stages and application of a wet chemical fitter procedure (detection limit: 8 pptv SO2).
The measurements performed in the upper troposphere and lower stratosphere lead to some unexpected results: (a) the meteorological conditions at the tropopause level have an important influence on the observed SO 2 mixing ratio; (b) between the 500 mb and the actual tropopause level the SO z mixing ratio is found to be < 100 pptv, and weak vertical gradients of SO 2 suggest only a small flux of tropospheric SO 2 into the stratosphere; (c) increasing SO: mixing ratios within the first kilometers of the stratosphere give strong support to a stratospheric source of SO2.
In the light of improved one-dimensional models considering the vertical distribution of stratospheric sulfur compounds (Crutzen, 1981; Turco etal. 1981) it can be shown that the oxidation of organic sulfur compounds (e.g., OCS, CS2) seems to be a stratospheric source of SO2. Furthermore, the flux calculations based on the SO 2 mixing ratios measured at the tropopause level indicate that the contribution of tropospheric (man-made) SO: to the stratospheric aerosol layer is of only minor importance.