Characterisation of PM 10 atmospheric aerosols for the wet season 2005 at two sites in East Africa

Abstract

Ambient daily PM 10 aerosol samples were collected at two sites in Tanzania in May and June 2005 (during the wet season), and their chemical characteristics were studied. The sites were a rural site in Morogoro and an urban kerbside site in Dar es Salaam. A Gent PM 10 stacked filter unit sampler with sequential Nuclepore polycarbonate filters, providing fine and coarse size fractions, and a PM 10 sampler with quartz fibre filters were deployed. Parallel collections of 24 h were made with the two samplers and the number of these collections was 13 in Morogoro and 16 in Dar es Salaam. The average mass concentration of PM 10 was 27 ± 11 μg/m 3 in Morogoro and 51 ± 21 μg/m 3 in Dar es Salaam. In Morogoro, the mean concentrations of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were 6.8, 0.51, and 2.8 μg/m 3, respectively. In contrast, higher mean concentrations (11.9, 4.6, and 3.3 μg/m 3, respectively) were obtained for Dar es Salaam. At both sites, species and elements, such as black carbon, NH 4 +, non-sea-salt SO 4 2−, K, and Ni (and at Dar es Salaam also V, As, Br, and Pb) were mainly present in the fine size fraction. The common crustal and sea-salt elements, including Na, Mg, Al, Si, Cl, Ca, Ti, Mn, Fe, and Sr, and also NO 3 − and P (and to a lesser extent Cu and Zn) were concentrated in the coarse particles. Aerosol chemical mass closure indicated that the PM 10 mass in Morogoro consisted, on average, of 48% organic matter (OM), 44% crustal matter, 4% sea salt, and 2% EC, while in Dar es Salaam OM, crustal matter, sea salt, and EC represented 37%, 32%, 9%, and 9% of the PM 10 mass. The contributions of the secondary inorganic aerosol (non-sea-salt sulphate, nitrate, and ammonium) were small, i.e., only 5% in total at each site. Carbonaceous materials and crustal matter were thus the most important components of the PM 10 mass. It is suggested that biomass burning is a major contributor to the OM; at Dar es Salaam there is also a very substantial contribution from traffic. A source apportionment calculation indicated that 68% of the OC at this site originated from traffic exhaust versus 32% from charcoal burning. The crustal matter at Morogoro is likely mainly attributable to soil dust resuspension, whereas in Dar es Salaam it is likely mostly resuspended road dust.