Stable carbon isotopic compositions of low-molecular-weight dicarboxylic acids, glyoxylic acid and glyoxal in tropical aerosols: implications for photochemical processes of organic aerosols

Tropical aerosols of PM2.5 and PM10 were collected at a rural site in Morogoro, Tanzania (East Africa), and analysed for stable carbon isotopic composition (C-13) of dicarboxylic acids (C-2-C-9), glyoxylic acid (C-2) and glyoxal (Gly) using gas chromatography/isotope ratio mass spectrometer. PM2.5 samples showed that C-13 of oxalic (C-2) acid are largest (mean, -18.3 +/- 1.7 parts per thousand) followed by malonic (C-3, -19.6 +/- 1.0 parts per thousand) and succinic (C-4, -21.8 +/- 2.2 parts per thousand) acids, whereas those in PM10 are a little smaller: -19.9 +/- 3.1 parts per thousand (C-2), -20.2 +/- 2.7 parts per thousand (C-3) and -23.3 +/- 3.2 parts per thousand (C-4). The C-13 of C-2-C-4 diacids showed a decreasing trend with an increase in carbon numbers. The higher C-13 values of oxalic acid can be explained by isotopic enrichment of C-13 in the remaining C-2 due to the atmospheric decomposition of oxalic acid or its precursors. C-13 of C-2 and Gly that are precursors of oxalic acid also showed larger values (mean, -22.5 parts per thousand and -20.2 parts per thousand, respectively) in PM2.5 than those (-26.7 parts per thousand and -23.7 parts per thousand) in PM10. The C-13 values of C-2 and Gly are smaller than those of C-2 in both PM2.5 and PM10. On the other hand, azelaic acid (C-9; mean, -28.5 parts per thousand) is more depleted in C-13, which is consistent with the previous knowledge; that is, C-9 is produced by the oxidation of unsaturated fatty acids emitted from terrestrial higher plants. A significant enrichment of C-13 in oxalic acid together with its negative correlations with relative abundance of C-2 in total diacids and ratios of water-soluble organic carbon and organic carbon further support that a photochemical degradation of oxalic acid occurs during long-range transport from source regions.