The oxidation of SO 2 in atmospheric water (cloud, rain, liquid aerosol and fog) is influenced by the presence of NH 3. The enhancing effect of NH 3 is especially pronounced if the oxidation occurs with an oxidant such as O 3 for which the reaction rate increases strongly with increasing pH, because NH 3 (i) codetermines the pH of the water and thus in turn the solubility of SO 2, and (ii) provides acid neutralizing capacity as well as buffer intensity to the heterogeneous atmosphere-water system in counteracting the acidity produced by the oxidation of SO 2. At low buffer intensity, the acidity production leads to the alleviation of further SO 2-oxidation. A computer model is used to assess the influence of SO 2, NH 3 and other potential acids and bases, of aerosols and of the liquid water content on the composition and its temporal variation of closed or open atmospheric systems as a consequence of SO 2 oxidation by O 3. An essential corollary to this model is a definition of atmospheric alkalinity (or acidity). Model results are compared with field data obtained in measuring the temporal variation in urban/rural fog composition.