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 source of anthropogenic and natural aerosols and trace gases. This region produces 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 cloud condensation nuclei (CCN). These additional nuclei can affect rainfall formation in clouds. The Aerosol Recirculation and Rainfall Experiment (ARREX) was started in 1998 to investigate the distribution of aerosols and trace gases over southern Africa and their possible effect on rainfall production efficiency. In southern Africa this is an important issue as water is a scarce resource. This paper presents preliminary findings on continental-maritime contrast in aerosol, CCN and cloud droplet properties under typical summer-time conditions that could explain the differences in rainfall production efficiency between the highveld and the eastern seaboard. These results form the basis for evidence to support the hypothesis that SO₂ sources affect 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 region.