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

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 values of particle phase glyoxal and methylglyoxal are significantly larger than those previously reported for isoprene and other precursors. The values are consistently less negative in oxalic acid (C₂, average -14.1‰), glyoxylic acid (-13.8‰), pyruvic acid (-19.4‰), glyoxal (-13.5‰), and methylglyoxal (-18.6‰) compared to other organic species (e.g., palmitic acid, -26.3‰), which can be explained by the kinetic isotope effects during atmospheric oxidation of pre-aged precursors (e.g., isoprene) and the subsequent gas-particle partitioning after the evaporation of clouds or wet aerosols. The δ¹³C values of C₂ is positively correlated with C₂ to organic carbon ratio, indicating that photochemical production of C₂ is more pronounced than its degradation during long-range atmospheric transport. The isotopic results also suggest that aqueous phase oxidation of glyoxal and methylglyoxal is a major formation process of oxalic acid via the intermediates such as glyoxylic acid and pyruvic acid. This study provides evidence that organic aerosols are intensively photochemically aged in the western North Pacific rim.