FIELD STUDIES OF THE SO2 AQUEOUS S(IV) EQUILIBRIUM IN CLOUDS

Concentrations of S(IV) were measured in cloudwater at Great Dun Fell and compared with theoretical HSO3- assuming equilibrium between aqueous and gaseous phases in cloud. Detectable concentrations of S(IV) in the range of 1 × 10-6 to 17.2 × 10-6 mol dm-3 were observed only in samples which contained low H2O2 concentrations, generally <1 × 10-6 mol dm-3. Concentrations of S(IV) were below the detection limit of 1 × 10-6 mol dm-3 in samples which contained hign H2O2 levels (1 × 10-6-80 × 10-6 mol dm-3) confirming that either SO2 or H2O2 acts as the limiting reagent in the oxidation of SO2 in cloudwater. Equilibrium HSO3- concentrations were estimated from the measured cloudwater pH, the gas phase SO2 concentration and the ambient temperature and found to be on average about 5 times lower than the measured S(IV) concentrations. The possible role of formaldehyde in stabilizing S(IV) in cloudwater is discussed. The kinetic data available in the literature suggest that the complexation reaction between S(IV) and HCHO is too slow to account for the observed difference between measured and calculated S(IV) concentrations over the typical lifetime of clouds in our study. S(IV) accounted for up to 10% of the SO42- measured in stored cloudwater samples. © 1990.