EXPERIMENTAL-EVIDENCE FOR IN-CLOUD PRODUCTION OF AEROSOL SULFATE

The modification of physical and chemical properties of aerosols passi ng through clouds has received considerable attention over recent year s. Some of these transformations are related to in-cloud chemical reac tions, particularly the oxidation of sulphur dioxide (SO2) to sulphate (SO42-). The Great Dun Fell experiment provided an opportunity to inv estigate the connection between the chemistry within cloud droplets an d the processing of an aerosol population. We have noted significant i ncreases in SO42- in the aerosol population downstream of the cloud co mpared to the aerosol entering the cloud. These increases are connecte d to both S(IV) oxidation in the liquid phase and to the entrainment o f new air into the cloud, supplying reactants such as H2O2 to the syst em. The addition of SO42- mass to the aerosol is also associated with changes in the NH4+ aerosol concentrations, possibly as a result of ne utralisation of the acidified cloud droplets by NH3. The study was per formed taking into account dynamical mixing of air masses as well as p ossible sampling artefacts. (C) 1997 Elsevier Science Ltd.