The Aerosol Recirculation and Rainfall Experiment (ARREX): an initial study on aerosol-cloud interactions over South Africa

South Africa, and especially its industrial highveld, is a significant sour ce of anthropogenic and natural aerosols and trace gases. This region produ ces more than 1 million tonnes of sulfur dioxide (SO2) a year, mainly from the burning of coal. Through atmospheric gas-to-particle conversion, SO2 is transformed into hygroscopic sulfates, which are potentially active as clo ud condensation nuclei (CCN). These additional nuclei can affect rainfall f ormation in clouds. The Aerosol Recirculation and Rainfall Experiment (ARRE X) was started in 1998 to investigate the distribution of aerosols and trac e gases over southern Africa and their possible effect on rainfall producti on efficiency. In southern Africa this is an important issue as water is a scarce resource. This paper presents preliminary findings on continental-ma ritime contrast in aerosol, CCN and cloud droplet properties under typical summer-time conditions that could explain the differences in rainfall produ ction efficiency between the highveld and the eastern seaboard. These resul ts form the basis for evidence to support the hypothesis that SO2 sources a ffect cloud properties that influence precipitation production efficiency. The complex nature of the processes involved is highlighted and suggestions made for research that should be conducted in future experiments in the re gion.