Stable carbon isotopic compositions of low-molecular-weight dicarboxylic acids, oxocarboxylic acids, α-dicarbonyls, and fatty acids: implications for atmospheric processing of organic aerosols

Abstract:

Stable carbon isotopic compositions (δ¹³C) were measured for 23 individual organic species including 9 dicarboxylic acids, 7 oxocarboxylic acids, 1 tricarboxylic acid, 2 α-dicarbonyls and 4 fatty acids in the aerosols from Gosan background site in East Asia. δ¹³C of particle-phase glyoxal and methylglyoxal are significantly higher than those previously reported for isoprene and other precursors, associated with isotope fractionation during atmospheric oxidation. ¹³C is consistently more enriched for oxalic acid (C₂), glyoxylic acid, pyruvic acid, glyoxal and methylglyoxal compared to other organic compounds identified, which can be explained by the kinetic isotope effects during aqueous-phase processing and the subsequent gas-particle partitioning after clouds or wet aerosols evaporation δ¹³C of C₂ is positively correlated with C₂ and organic carbon ratio, indicating that a photochemical production of C₂ is more pronounced than its degradation process during long-range transport. The ¹³C results also suggest that aqueous-phase oxidation of glyoxal and methylglyoxal is major formation process of oxalic acid production via the major intermediates glyoxylic acid and pyruvic acid. This study provides evidence that organic aerosols are intensively photo-chemically aged in this region.