ON THE DROP AND AEROSOL SIZE DEPENDENCE OF AQUEOUS SULFATE FORMATION IN A CONTINENTAL CUMULUS CLOUD

An entraining air parcel model including microphysical and chemical processes is used to investigate the drop size dependence of pollutant concentrations in a growing cloud under continental background conditons. For the calculation of the chemical processes, the drop size of the cloud drops and their initial dry particle size before activation is taken into account. The size dependence of the pollutant concentration in drops smaller than 8-mu-m radius is mainly influenced by the activation and fast condensational growth of recently entrained particles. In drops larger than 20-mu-m radius the main process influencing the drop size dependence of the pollutant concentration is coalescence between drops. The size dependence in drops between 8 and 20-mu-m radius is determined by the drops activated at the cloud base and the drops growing on particles that are entrained at higher altitudes, the influence of the latter rapidly increasing with height. Drops that are growing on large particles are polluted with initial aerosol matter to such an extent that sulfate production by oxidation is not significant in them. In drops growing on small particles the initial aerosol load has relatively little influence on the chemical processes in the drop. Oxidation of S(IV), especially by ozone, takes place mainly in these drops. The dependence of chemical processes on initial dry particle size holds throughout most of the cloud, until coalescence starts playing a role. If the cloud evaporates before producing precipitation-sized drops, the increase of aerosol mass due to cloud chemical processes is found to have taken place in the aerosol accumulation mode. If precipitation-sized drops were produced during the cloud stage, the soluble volume fraction of the aerosol matter has increased throughout the aerosol size distribution, but mainly in the accumulation mode.