Molecular composition of dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids in atmospheric aerosols from Tanzania, East Africa during wet and dry seasons

Abstract. Atmospheric aerosol samples of PM_2.5 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/flame ionization detector (GC/FID) and GC/mass spectrometry. Here we report the molecular composition 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 ketoacid and α-dicarbonyl, respectively in both seasons in PM_2.5 and PM₁₀. Mean concentration of C₂ in PM_2.5 (121 ± 47 ng m⁻³) was lower in wet season than dry season (258 ± 69 ng m⁻³). Similarly, PM₁₀ samples showed lower concentration of C₂ (169 ± 42 ng m⁻³) in wet season than dry season (292 ± 165 ng m⁻³). Relative abundances of C₂ in total diacids were 65% and 67% in PM_2.5 and 65% and 64% in PM₁₀ in the wet and dry seasons, respectively. Total concentrations of diacids (289–362 ng m⁻³), ketoacids (37.8–53.7 ng m⁻³), and α-dicarbonyls (5.7–7.8 ng m⁻³) in Tanzania are higher than those reported at a rural background site in Nylsvley (South Africa) but comparable or lower than those reported from sites in Asia and Europe. Diacids and ketoacids were found to be present mainly in PM_2.5 in both seasons (total α-dicarbonyls in the dry season), suggesting a production of organic acids from pyrogenic sources and photochemical oxidations. Averaged contributions of total diacids to aerosol total carbon were 1.4% in PM_2.5 and 2.1% in PM₁₀ during wet season and 3.3% in PM_2.5 and 3.9% in PM₁₀ during dry season whereas those to water-soluble organic carbon were 2.2% and 4.7% in PM_2.5 during wet season and 3.1% and 5.8% in PM₁₀ during dry season. The higher ratios in dry season suggest an enhanced photochemical oxidation of organic precursors probably via heterogeneous reactions on aerosols under strong solar radiation. Strong positive correlations were found among diacids and related compounds as well as good relations to source tracers in both seasons, suggesting a mixed source from natural biogenic emissions, biomass burning, biofuel combustion, and photochemical production.