The effect of sulphur chemistry on the scattering properties of particles

A model has been developed of the interaction of aerosol with shallow layer clouds. The model includes aqueous phase chemical processes, particularly the oxidation of sulphur dioxide (SO 2 ) occurring in droplets formed on cloud condensation nuclei of known chemical composition. The cloud microphysics as a function of height above cloud base is predicted along with changes to the aerosol size distribution and hygroscopic properties after a single cycle through cloud. The predictions of the model are compared with observations from a hill cap cloud experiment for a single cloud pass. A stratocumulus cloud model then examines the changes in the cloud microphysics and aerosol population arising as the modified aerosol distribution undergoes a further passage through cloud. The cycling process is repeated a further nine times. We show that the cloud processing of the aerosol results in a strongly bimodal aerosol size distribution which can significantly affect the direct radiative forcing of the aerosol. The processing is strongly oxidant-limited and forced entrainment of hydrogen peroxide through cloud top is required to maintain the oxidation. A strong surface source of ammonia substantially enhances the observed modification. The processing affects the cloud condensation nucleus spectrum, initially increasing the number of droplets activated. However, multiple cycling has a tendency to stabilize the droplet number.