Characterization and transport of aerosols over equatorial eastern Africa

Abstract

Measurements of the composition of aerosol partitioned into two size fractions, fine (particle aerodynamic equivalent diameter, dp ≤ 2.5 μm) and coarse (2.5 < dp ≤ 10 μm) were made 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 interhemispheric transport at the equator. The two size fractions allow long‐range transport aerosol to be distinguished from those more locally derived. Background midtropospheric elemental aerosol concentrations observed on Mount Kenya, for example, S = 0.0157 μg m−3, are lower than commensurate findings 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 stable conditions characterized by downslope katabatic flow. Nocturnal conditions favor the measurement of long‐range, midtropospheric transport products 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 km. So‐called pristine marine air from the central Indian Ocean is shown to contain aged and recirculated Si that could only have been derived from land areas after transport over long distances. The nature of the aerosols measured on Mount Kenya depends critically on regional patterns of aerosol transport. Interregional transfers seem to be a feature of the transport climatology. Likewise, interhemispheric transport across the equator in east Africa is observed.