Atmospheric Impact of the 1783 - 1784 Laki

3 of basalt, and released an estimated 122 Tg of SO 2. The impact of this release upon the chemical composition of the atmosphere has been simulated with a global 3-D chemical-transport model. The model suggests that the SO 2 generated a sulphate aerosol veil over much of the northern hemisphere during the summer of 1783, in agreement with observations of a widespread`dry fog'. In the model, over 70 % of the emitted Laki SO 2 is deposited to the surface before oxidation to aerosol can occur. Although aqueous-phase oxidation of SO 2 to sulphate aerosol dominates over gas-phase oxidation (by OH) in simulations of both the present-day and pre-industrial atmospheres, reaction with OH converts just over half of the non-deposited Laki SO 2 to aerosol, with aqueous-phase oxidation (mainly SO 2(aq) + H 2 O 2(aq)) accounting for the remainder. Modelled global annual mean atmospheric lifetimes for SO 2 increase from 1.5 days in the clean pre-industrial atmosphere to 6.6 days following the eruption. Sulphate lifetimes increase to a lesser extent, from 4.0 days to 5.2 days. These changes are largely due to the fact that more SO 2 and sulphate are present at higher altitudes and latitudes, where their residence times are longer, but also partly because the massive injection of SO 2 seriously depletes its main oxidants. The simulated aerosol elds are used in a companion paper Highwood and Stevenson, in preparation] to calculate the climatic impact of the eruption. 3