The vertical sulfur dioxide distribution at the tropopause level

In 1978-1980 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 filter 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 import influence on the observed SO2 mixing ratio; (b) between the 500 mb and the actual tropopause level the SO2 mixing ratio is fount to be smaller than 100 pptv, and weak vertical gradients of SO2, suggest only a small flux of tropospheric SO2 into the stratosphere; (c) increasing SO2 mixing ratios within the first kilometers of the stratospheric 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 et al.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 SO2 mixing ratios measured at the tropopause level indicate that the contribution of tropospheric (man-made) SO2 to the stratospheric aerosol layer is of only minor importance. (IFU)