AbstractThe mechanism of SO2oxidation in diluted catalytic solutions was studied. It was found that the oxidation stops after a certain pH value is reached. In the same catalytic solutions, the maximum possible SO4−formation is a linear function of the SO2partial pressure in the air.This mechanism of SO4−formation can account for observed SO4−concentrations in rain, fog, and smog only if some neutralizing cations in the aerosols or traces of NH3in the air are present. It is shown that in the latter case the system SO2NH3‐catalytic solution can explain the main features of SO4−formation in our