Experimental evidence for in-cloud production of aerosol sulphate

The modification of physical and chemical properties of aerosols passing through clouds has received considerable attention over recent years. Some of these transformations are related to in-cloud chemical reactions, particularly the oxidation of sulphur dioxide (SO2) to sulphate (SO42−. The Great Dun Fell experiment provided an opportunity to investigate the connection between the chemistry within cloud droplets and the processing of an aerosol population. We have noted significant increases in SO42− in the aerosol population downstream of the cloud compared to the aerosol entering the cloud. These increases are connected to both S(IV) oxidation in the liquid phase and to the entrainment of new air into the cloud, supplying reactants such as H2O2 to the system. The addition of SO42− mass to the aerosol is also associated with changes in the NH4+ aerosol concentrations, possibly as a result of neutralisation of the acidified cloud droplets by NH3. The study was performed taking into account dynamical mixing of air masses as well as possible sampling artefacts.