Volcanic eruptions reach the stratosphere, on average, at least once every two years (Simkin 1993). Plinian eruptions contribute to the stratospheric sulphate aerosol via injection of sulphur gases that are subsequently oxidized to form sulphate aerosol. Volcanic sulphur is emitted primarily as sulphur dioxide (SO2) and hydrogen sulphide (H2S). The H2S is oxidized within days to SO2 (e.g. McKeen 1984), which, in turn, is oxidized to sulphate, with a lifetime of approximately 35 days in the dry stratosphere (Bluth et al. 1992). The stratospheric aerosol burden can be significantly enhanced in the years following major volcanic eruptions. Sulphate aerosols in the stratosphere have radiative effects, altering the Earth’s radiation balance (e.g. Charlson et al. 1991, 1992; Stenchikov et al. 1998), and hence can influence the global climate. A reduction of stratospheric ozone after large volcanic eruptions has also been observed. The column ozone reduction after the 1991 Mount Pinatubo eruption, which could be attributed to the volcanic effect, ranged from about 2% in the tropics, to about 7% at mid latitudes (Angell 1997; Solomon et al. 1998). The observed ozone changes are a combined effect of perturbations in heating and photolysis rates, and in strato- spheric chemistry. Volcanic hydrated sulphate aerosols can serve as sites for heterogeneous reactions, which destroy ozone in the presence of halogens by converting passive halogen compounds into active ones (e.g. Hofmann & Solomon 1989; Granier & Brasseur 1992; Solomon et al. 1996). Hence, the increase in stratospheric halogens caused by anthropogenic activities has caused the observed decrease in stratospheric ozone after major volcanic erup- tions. Since the human-induced increase of chlorine concentration in the stratosphere has peaked, the effect of ozone destruction by volcanic aerosol will probably decrease in the next few decades (Brasseur et al. 1990; Tie & Brasseur 1995). RUNNING HEAD 307 The 12 900 years BP Laacher See eruption: estimation of volatile yields and simulation of their fate in the plume C. TEXTOR 1, * , P. M. SACHS 2,† , H.-F. GRAF 3 & T. H. HANSTEEN 2 1 Max-Planck Institute for Meteorology, Bundestraße 55, D-20146 Hamburg, Germany. (e-mail: textor@dkrz.de) 2 Forschungszentrum GEOMAR, Vulkanologie und Petrologie, Wischhofstraße 1–3, D-24148, Kiel, Germany. 3 Max-Planck Institute for Meteorology, Bundestraße 55, D-20146 Hamburg, Germany. Abstract: We estimated the volatile emissions of the 12 900 years BP eruption of Laacher See volcano (Germany), using a modified petrological method. Glass inclusions in phenocrysts and matrix glasses sampled over the Laacher See tephra profile were analysed by synchrotron X-ray fluorescence microprobe and electron microprobe to obtain the emitted masses of halogens, sulphur, and water. These data were used to initialize the numerical plume model ATHAM in order to investigate the fate of volcanic gases in the plume, and to estimate volatile masses injected into the stratosphere. The scavenging efficiency of each volatile component depends on its interactions with both liquid water and ice. We found a scavenging efficiency of c.5% for the sulphur species, and of only c.30% for hydrogen halides, despite their high water solubility. Our simulations showed that the greatest fraction of hydrometeors freeze to ice, due to the fast plume rise and great height of the eruption column. For the dry atmospheric conditions of the Laacher See eruption, the amount of liquid water was not sufficient to completely scavenge HCl and HBr,so that a large proportion could reach the stratosphere. From:OPPENHEIMER, C., PYLE, D.M. & BARCLAY , J. (eds) 2003 Volcanic Degassing. Geological Society, London, Special Publications, 213, 307–328. 0305–8719/03/$15.00 © The Geological Society of London 2003. *Author to whom correspondence should be addressed. † Deceased.