Spatial and temporal distribution of tropospheric ozone over Southern Africa

Abstract

Tropospheric ozone over southern Africa is characterised using the Southern Africa Regional Science Initiative (SAFARI) 2000 aircraft datasets collected from surface to just above the boundary layer, and satellite remotely sensed ozone column depth. Ozonesonde datasets for Irene, South Africa; Lusaka, Zambia; Nairobi and Malindi, both Kenyan stations were, used to supplement the aircraft and satellite data. The period studied is the dry biomass burning season between August and September, 2000 Source areas of ozone precursors have been identified by trajectory analysis. Fivepoint, back trajectory clusters in the region reveal well-defined air mass transport corridors. Enhanced ozone concentrations, above 70 ppb, were regularly encountered in the lower troposphere (3.5 km above ground level) in air masses distributed over the sub-region by distinct re-circulation pathways, which prevailed during the study period. Occasional, episodes of significant higher ozone pollution, above 100 ppbv are identified. Savannah fire products transported from seasonal biomass burning regions of western Zambia, northern Zimbabwe and Mozambique have been found to be the major sources of ozone pollution over the sub-region. Another source, though on a lesser scale, is the Highveld industrial region of South Africa. In situ air observations established that the western and eastern sectors of the southern Africa, north of 23°S latitude, were the major sources of regional ozone precursor gases. The regions southwards of 23°S were observed to have lower ozone pollution.