Atmos. Chem. Phys. Discuss., 12, 25657-25701, 2012
www.atmos-chem-phys-discuss.net/12/25657/2012/
doi:10.5194/acpd-12-25657-2012
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under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Size distributions of dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids in atmospheric aerosols from Tanzania, East Africa during wet and dry seasons
S. L. Mkoma1,2 and K. Kawamura1
1Institute of Low Temperature Science, Hokkaido University, N19 W08, Kita-ku, Sapporo 060-0819, Japan
2Department of Physical Sciences, Faculty of Science, Sokoine University of Agriculture, P.O. Box 3038, Chuo Kikuu, Morogoro, Tanzania

Abstract. Atmospheric aerosol samples of PM2.5 and PM10 were collected during the wet and dry seasons in 2011 from a rural site in Tanzania and analysed for water-soluble dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids using a gas chromatography (GC) and GC/mass spectrometry. Here we report the size distribution and sources of diacids and related compounds for wet and dry seasons. Oxalic acid (C2) was found as the most abundant diacid species followed by succinic and/or malonic acids whereas glyoxylic acid and glyoxal were the dominant ketoacids and α-dicarbonyls, respectively in both seasons in PM2.5 and PM10. Mean concentration of C2 in PM2.5 (121.5± 46.6 ng m−3) was lower in wet season than dry season (258.1± 69.5 ng m−3). Similarly, PM10 samples showed lower concentration of C2 (168.6 ± 42.4 ng m−3) in wet season than dry season (292.4± 164.8 ng m−3). Relative abundances of C2 in total diacids were 65.4% and 67.1% in PM2.5 and 64.6% and 63.9% in PM10 in the wet and dry seasons, respectively. Total concentrations of diacids (289–362 m−3), ketoacids (37.8–53.7ng m−3), and α-dicarbonyls (5.7–7.8 ng m−3) in Tanzania are higher to those reported at a rural background site in Nylsvley (South Africa) but comparable or lower to those reported from sites in Asia and Europe. Diacids and ketoacids were found to be present mainly in the fine fraction in both seasons (total α-dicarbonyls in the dry season), suggesting a production of organic aerosols from pyrogenic sources and photochemical oxidations. The averaged contributions of total diacid carbon to aerosol total carbon were 1.4% in PM2.5 and 2.1% in PM10 in wet season and 3.3% in PM2.5 and 3.9% in PM10 in dry season whereas those to water-soluble organic carbon were 2.2% and 4.7% inPM2.5 and 3.1% and 5.8% in PM10 during the wet and dry seasons, respectively. These ratios suggest an enhanced photochemical oxidation of organic precursors and heterogeneous reactions on aerosols under strong solar radiation and high humidity. Correlations among organic components and relations between source tracers with diacids and related compounds in both seasons showed influence of mixed sources from natural biogenic emissions, biomass burning, biofuel combustion, and enhanced photochemical production.

Citation: Mkoma, S. L. and Kawamura, K.: Size distributions of dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids in atmospheric aerosols from Tanzania, East Africa during wet and dry seasons, Atmos. Chem. Phys. Discuss., 12, 25657-25701, doi:10.5194/acpd-12-25657-2012, 2012.
 
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