Influence of NO2 on S(IV) oxidation in aqueous suspensions of aerosol particles from two different origins

The influence of soluble compounds leached from real atmospheric aerosol particles (size range Dae: 0.17–1.6 μm) and dissolved NO2 on S(IV) oxidation in aqueous solution is presented. Experiments were conducted with aerosol particles of two different origins (i.e., urban and industrial) and at concentrations of trace gases in the gas mixtures (SO2/air and SO2/NO2/air) typical for a polluted atmosphere. During the introduction of SO2/air into the aqueous aerosol suspensions under dark conditions at pH 4, the formation of SO42− was very slow with a long induction period. However, in the presence of NO2 the oxidation rate of dissolved SO2 in suspensions of aerosols from both origins increased substantially (about 10 times). The results suggest that soluble compounds eluted from atmospheric aerosols have not only a catalytic (e.g. Fe, Mn), but also a pronounced inhibiting effect (e.g., oxalate, formate, acetate, glycolate) on S(IV) autoxidation. When NO2 was also introduced into the aerosol suspensions, the inhibition was not so highly expressed. An explanation for this is that the radical chain mechanism is mainly initiated by the interaction of dissolved NO2 and HSO3−. Therefore, at conditions typical for a polluted atmosphere dissolved NO2 can have a significant influence on the secondary formation of SO42−.