Characterization and transport of aerosols over equatorial eastern Africa

Measurements of the composition of aerosol partitioned into two size fracti ons, fine (particle aerodynamic equivalent diameter, d(p) less than or equa l to 2.5 mum) and coarse (2.5 < d(p) less than or equal to 10 mum) were mad e at a high-altitude site over equatorial eastern Africa on Mount Kenya to study long-range transport of aerosol and to determine the extent of interh emispheric transport at the equator. The two size fractions allow long-rang e transport aerosol to be distinguished from those more locally derived. Ba ckground midtropospheric elemental aerosol concentrations observed on Mount Kenya, for 3 example, S = 0.0157 mug m-(3), are lower than commensurate fi ndings for remote areas in industrialized countries and elsewhere in Africa but are 3 factors higher than concentrations reported from the South Pole. Locally derived aerosols are frequently transported up the slopes of Mount Kenya by day in valley winds but are rarely transported at night in the st able conditions characterized by downslope katabatic, flow. Nocturnal condi tions favor the measurement of long-range, midtropospheric transport produc ts to Kenya from afar. Case studies reveal that anthropogenically derived S and Fe aerosols from both southernmost and central southern Africa may be observed on Mount Kenya after transport over distances > 6000 kin. So-calle d pristine marine air from the central Indian Ocean is shown to contain age d and recirculated Si that could only have been derived from land areas aft er transport over long distances. The nature of the aerosols measured on Mo unt Kenya depends critically on regional patterns of aerosol transport. Int erregional transfers seem to be a feature of the transport climatology. Lik ewise, interhemispheric transport across the equator in east Africa is obse rved.