Seasonal Variation of Atmospheric Composition of Water-Soluble Inorganic Species at Rural Background Site in Tanzania, East Africa

Abstract

Samples of coarse, fine and PM10 aerosols were collected at a rural Morogoro sites, during the 2005 dry season and 2006 wet season campaigns using a “Gent” PM10 stacked filter unit sampler with sequential Nuclepore polycarbonate filters. A total of 80 aerosol samples were analyzed for water-soluble inorganic ions components using Ion Chromatography. The mean concentration for the anions Cl-, NO3 -, and SO4 2- and the cations Na+, NH4 +, K+, Mg2+, Ca2+, showed seasonal variation in all size fractions with higher levels during the 2005 dry season campaign than the 2006 wet season campaign. The mean concentrations and associated standard deviation of fine, coarse and PM10 mass were, 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, respectively. Ca2+ was the most important cation and the SO4 2- was the main acidifying anionic component in PM10 while NH4 + was the most abundant cation in the fine fraction and Cl- the main anionic species in the coarse fraction. The ion balance ratios were all larger than 1.0; they range from 1.30 to 1.44 for the fine fraction and from 2.01 to 3.14 for the coarse fraction. The carbonates were not measured by Ion Chromatography therefore; these missing carbonates are thought to be largely responsible for the observed deviation from 1.0. The study suggests that primary sources such as soil dust dispersion and biomass burning made a significant contribution to the atmospheric particulate pollution in Morogoro.

Keywords: Ion chromatography; Aerosol Characterization; coarse, fine and PM10 fractions; Meteorology