Seasonal variation of water-soluble inorganic species in the coarse and fine atmospheric aerosols at Dar es Salaam, Tanzania

The ionic composition of coarse, fine and total PM10 was investigated in aerosol samples collected from a kerbside in Dar es Salaam during the 2005 dry season and 2006 wet season. A “Gent” PM10 stacked filter unit sampler with sequential Nuclepore polycarbonate filters, providing coarse (8 μm) and fine (0.4 μm) size fractions, was deployed. The mean concentrations and associated standard deviation of fine, coarse and PM10 were, respectively, 17 ± 4, 52 ± 27, and 69 ± 29 μg/m3 during the 2005 dry season campaign and 13 ± 5, 34 ± 23 and 47 ± 25 μg/m3 for the 2006 wet season campaign. The higher PM mass concentrations during the dry season campaign are essentially due to soil dust dispersal, much biomass burning and temperature inversions. Chloride, Na+ and Mg2+ were the dominant ions in coarse fraction, indicating a significant influence of sea-salt aerosols. In the fine fraction, SO 4 2 - and NH 4 + and K+ were the most important ions. The mean equivalent PM2 NO 3 - concentration in the 2005 dry season campaign was two times higher than in the 2006 wet season campaign, probably due to reaction of NaCl (sea-salt) with HNO3 as a result of higher levels of NOx during the dry season and/or reduced volatilization of NH4NO3 due to lower temperature in the dry season. The results from our water-soluble ions study strongly suggests that biomass burning and secondary aerosols make a significant contribution to fine particulate mass in Dar es Salaam atmosphere. Thus, burning of waste and biomass are thought to be the major causes for the atmospheric particulate pollution in Dar es Salaam during the dry season.