Spatial and seasonal variations in CCN distribution and the aerosol-CCN relationship over southern Africa

[1] The sources and transport of cloud condensation nuclei ( CCN) over southern Africa have been investigated using in situ measurements collected during the Aerosol Recirculation and Rainfall Experiment ( ARREX) wet season projects and during the Southern African Regional Science Initiative ( SAFARI- 2000) intensive dry season campaign. CCN concentrations over the subcontinent are generally higher in the dry season than in the wet season and exceed 2000 cm(-3) in highly polluted air masses. In the late dry season, CCN concentrations are highest in the northern regions of the subcontinent due to the burning of savanna biomass. Emissions from industries and power plants on the South African Highveld are a prolific year- round source of CCN and are sufficient to account for CCN levels south of 20 degreesS throughout the year. Most CCN are contained within the mixing layer, which extends to an altitude of 3000 - 4000 m over the plateau and is capped by a temperature inversion. Multiple inversions and absolutely stable layers control the stratification of aerosols and CCN. Biomass burning particles are efficient CCN, and the median diameter of the accumulation mode is large ( up to 0.19 mm). Recently emitted industrial aerosols are less soluble and have a smaller median diameter ( 0.11 mum). Twice as many aerosols act as CCN in the dry season ( 68%) than in the wet season ( 34%). The fraction is highest in the dry season over the tropical regions (> 80%), where smoke aerosol predominates. Elevated aerosol and CCN concentrations are expected to have implications for direct radiative forcing, especially in the dry season, and for indirect forcing in the wet season.