Stratospheric SO₂ and sulphate aerosol, model simulations and satellite observations

Abstract. A multiyear study with the atmospheric chemistry general circulation model EMAC with the aerosol module GMXe at high altitude resolution demonstrates that the sulfur gases COS and SO₂, the latter from low-latitude volcanic eruptions, predominantly control the formation of stratospheric aerosol. The model consistently uses the same parameters in the troposphere and stratosphere for 7 aerosol modes applied. Lower boundary conditions for COS and other long-lived trace gases are taken from measurement networks, while estimates of volcanic SO₂ emissions are based on satellite observations. We show comparisons with satellite data for aerosol extinction (e.g. SAGE) and SO₂ in the middle atmosphere (MIPAS on ENVISAT). This corroborates the interannual variability induced by the Quasi-Biennial Oscillation, which is internally generated by the model. The model also realistically simulates the radiative effects of stratospheric and tropospheric aerosol including the effects on the model dynamics. The medium strength volcanic eruptions of 2005 and 2006 exerted a nonnegligible radiative forcing of up to −0.6 W m⁻² in the tropics, while the large Pinatubo eruption caused a maximum though short term tropical forcing of about −10 W m⁻². The study also shows that observed upper stratospheric SO₂ can be simulated accurately only when a sulphur sink on meteoritic dust is included and the photolysis of gaseous H₂SO₄ in the near infrared is higher than assumed previously.