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 important influence on the observed SO₂ mixing ratio; (b) between the 500 mb and the actual tropopause level the SO₂ mixing ratio is found to be <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 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 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.