AbstractSulphur emissions have significantly decreased in Europe since the 1980s. Consequently, atmospheric concentrations of sulphur dioxide and particle bound sulphate have decreased, too, but not to the same extent. The oxidation of sulphur dioxide has become more efficient over time, leading to an increased sulphate to sulphur dioxide ratio. The reasons for this were investigated in a long term CMAQ model run covering the period 1985–2007. Observations and model results show the same non-linear relation between sulphur dioxide and particle bound sulphate concentrations. An analysis of the sulphur dioxide oxidation pathways was performed in a box-model simulation using the same algorithms as implemented in the CMAQ model. The oxidation was accelerated over time due to an increase in the hydrogen peroxide concentrations. This was mainly caused by a reduction of the sulphur dioxide concentrations, themselves.