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

Abstract

Tropical aerosols of PM_2.5 and PM₁₀ were collected at a rural site in Morogoro, Tanzania (East Africa), and analysed for stable carbon isotopic composition (δ¹³C) of dicarboxylic acids (C₂–C₉), glyoxylic acid (ωC₂) and glyoxal (Gly) using gas chromatography/isotope ratio mass spectrometer. PM_2.5 samples showed that δ¹³C of oxalic (C₂) acid are largest (mean, −18.3±1.7‰) followed by malonic (C₃, −19.6±1.0‰) and succinic (C₄, −21.8±2.2‰) acids, whereas those in PM₁₀ are a little smaller: −19.9±3.1‰ (C₂), −20.2±2.7‰ (C₃) and −23.3±3.2‰ (C₄). The δ¹³C of C₂–C₄ diacids showed a decreasing trend with an increase in carbon numbers. The higher δ¹³C values of oxalic acid can be explained by isotopic enrichment of ¹³C in the remaining C₂ due to the atmospheric decomposition of oxalic acid or its precursors. δ¹³C of ωC₂ and Gly that are precursors of oxalic acid also showed larger values (mean, −22.5‰ and −20.2‰, respectively) in PM_2.5 than those (−26.7‰ and −23.7‰) in PM₁₀. The δ¹³C values of ωC₂ and Gly are smaller than those of C₂ in both PM_2.5 and PM₁₀. On the other hand, azelaic acid (C₉; mean, −28.5‰) is more depleted in ¹³C, which is consistent with the previous knowledge; that is, C₉ is produced by the oxidation of unsaturated fatty acids emitted from terrestrial higher plants. A significant enrichment of ¹³C in oxalic acid together with its negative correlations with relative abundance of C₂ 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.

• PM_2.5 and PM₁₀
• stable carbon isotope ratios
• oxalic acid
• diacids
• glyoxylic acid
• glyoxal
• organic aerosols
• Tanzania
• East Africa

6.Acknowledgements

We acknowledge the financial support by the Japan Society for the Promotion of Science (JSPS) to S.L.M. and the Environment Research and Technology Development Fund (B-0903) from the Ministry of the Environment, Japan. We thank Mr. Filbert T. Sogomba of the Department of Physical Sciences (SUA) for aerosol sample collection. The authors also thank the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa.gov/ready.php) used in this publication.