Atmospheric aerosol particles and transport: a climatological perspective for Zimbabwe

Abstract

Atmospheric aerosol particles were collected in the coarse (2-10 µm diameter) and fine (< 2 µm diameter) size fractions at Rukomechi research station, (16.1 °S, 29.4 °E, 500 m a.s.l.), northern Zimbabwe from September 1994 to January 2000. The collected samples were analysed for particulate mass (PM), black carbon and 47 elements at the Gent Institute for Nuclear Sciences. The main sources of coarse and fine aerosols were found to be crustal matter, sea salt and biomass burning for both wet and dry seasons. However, additional sources for copper and biogenic aerosols were detected during the wet and dry seasons, respectively, while anthropogenic influences were only found in the fine aerosols. The Absolute Principal Component Analysis (APCA) attributed 29% of the total wet season coarse particulate mass to the mixed biogenic/biomass burning component, while the major contribution (32%) was attributed to biomass burning during the dry season. The biomass burning component also provided the major contribution to the total fine particulate mass, accounting for 44% and 79% in the wet and dry seasons, respectively.

The climatology of air mass transport to northern Zimbabwe was examined through an analysis of 5-day kinematic back trajectories arriving daily at Rukomechi at ~ 850 hPa for the period of 1994 to 1999. Classification of trajectories into different flow regimes was done using a non-hierarchical cluster algorithm. The dominant transport features include the eastern corridor, the south-east corridor, southern and north north-westerly flows, as well as regionally re-circulated air. The eastern corridor is comprised of fast and slow air masses that contribute 35% of the total number of trajectories. The air masses in these corridors bring late dry season to wet season air masses driven by an anticyclone that wraps around the subcontinent and stretches into the Mozambique Channel. The south-east corridor is also composed of fast and slow winds that contribute late wet season to dry season air masses and account for 44% of the total number of trajectories. These winds are associated with the Atlantic Ocean anticyclone and the Indian Ocean tropical depression. The fast dry season southern flow contributes 8% of the total number of trajectories and is driven by a continental anticyclone over South Africa coupled to an Atlantic Ocean anticyclone. The north north-westerly flow contributes least to the total number of trajectories (6%) and is associated with Inter-Tropical Convergence Zone while the regionally re-circulated air, as a result of differential heating at the sur¬face, is the slowest.

Of the seven air flows, only four were found carrying air significantly loaded with aerosols to northern Zimbabwe. These are namely: the fast easterly that carries aerosols mainly from biomass burning related sources; the slow south easterly that is associated with aerosols from various sources; the southerly containing mineral dust, sea salt and anthropogenic aerosols; and the regional flows that contain aerosols of anthropogenic origin. The potential sources of biomass burning aerosols were found to be due to fire activities from northern Zimbabwe and central Mozambique. The other pathways carry anthropogenic aerosols, especially copper (from areas around the Copper-belt regions), sulphur (from South African industrial areas), and lead (from the major Southern African road network).