Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 1 2003 10.5194/acpd-3-551-2003 http://www.atmos-chem-phys-discuss.net/3/551/2003/ http://www.atmos-chem-phys-discuss.net/3/551/2003/acpd-3-551-2003.html http://www.atmos-chem-phys-discuss.net/3/551/2003/acpd-3-551-2003.pdf 551 596 2003-02-06 Atmospheric impact of the 1783&ndash;1784 Laki eruption: Part I Chemistry modelling D. S. Stevenson C. E. Johnson E. J. Highwood V. Gauci W. J. Collins R. G. Derwent Institute for Meteorology, University of Edinburgh, Edinburgh, U Met Office, Bracknell, UK Department of Meteorology, University of Reading, UK Department of Earth Sciences, The Open University, UK Results from the first chemistry-transport model study of the impact of the 1783&ndash;1784 Laki fissure eruption (Iceland: 64&deg; N, 17&deg; W) upon atmospheric composition are presented. The eruption released an estimated 122 Tg(SO<sub>2</sub>) into the troposphere and lower stratosphere. The model has a high resolution tropopause region, and detailed sulphur chemistry. The simulated SO<sub>2</sub> plume spreads over much of the Northern Hemisphere, polewards of ~40&deg; N. About 70% of the SO<sub>2</sub> gas is directly deposited to the surface before it can be oxidised to sulphuric acid aerosol. The main SO<sub>2</sub> oxidants, OH and H<sub>2</sub>O<sub>2</sub> , are depleted by up to 40% zonally, and the lifetime of SO<sub>2</sub> consequently in-creases. Zonally averaged tropospheric SO<sub>2</sub> concentrations over the first three months of the eruption exceed 20 ppbv, and sulphuric acid aerosol reaches ~2 ppbv. A total aerosol yield of 51&ndash;66 Tg(H<sub>2</sub>SO<sub>4</sub> ) is produced. The mean aerosol lifetime is only 6&ndash;9</font> days, and the peak aerosol loading of the atmosphere is only ~7 Tg(H<sub>2</sub>SO<sub>4</sub>.2H<sub>2</sub>O). Due to the relatively short atmospheric residence times of both the SO<sub>2</sub> and sulphate, the aerosol loading approximately mirrors the temporal evolution of emissions associated with the eruption. The model produces a reasonable simulation of the acid deposition found in Greenland ice cores. These results appear to be relatively insensitive to the vertical profile of emissions assumed, although if more of the emissions reached higher levels (&gt;12 km), this would give longer lifetimes and larger aerosol yields. This study suggests that most previous estimates of the global aerosol loading associated with Laki have been generally too large in magnitude, and too long-lived. Environmental effects following the Laki eruption may have been dominated by the widespread deposition of SO<sub>2</sub> gas rather than sulphuric acid aerosol.