Molecular distribution and stable carbon isotopic composition of dicarboxylic acids, ketocarboxylic acids, and α-dicarbonyls in size-resolved atmospheric particles from Xi'an City, China

Size-resolved airborne particles (9-stages) in urban Xi'an, China, during summer and winter were measured for molecular distributions and stable carbon isotopic compositions of dicarboxylic acids, ketocarboxylic acids, and α-dicarbonyls. To our best knowledge, we report for the first time the size-resolved differences in stable carbon isotopic compositions of diacids and related compounds in continental organic aerosols. High ambient concentrations of terephthalic (tPh, 379 ± 200 ng m^-3) and glyoxylic acids (ωC₂, 235 ± 134 ng m^-3) in Xi'an aerosols during winter compared to those in other Chinese cities suggest significant emissions from plastic waste burning and coal combustions. Most of the target compounds are enriched in the fine mode (<2.1 μm) in both seasons peaking at 0.7-2.1 μm. However, summertime concentrations of malonic (C₃), succinic (C₄), azelaic (C₉), phthalic (Ph), pyruvic (Pyr), 4-oxobutanoic (ωC₄), and 9-oxononanoic (ωC ₉) acids, and glyoxal (Gly) in the coarse mode (>2.1 μm) are comparable to and even higher than those in the fine mode (<2.1 μm). Stable carbon isotopic compositions of the major organics are higher in winter than in summer, except oxalic acid (C₂), ωC₄, and Ph. δ¹³C of C₂ showed a clear difference in sizes during summer, with higher values in fine mode (ranging from -22.8‰ to -21.9‰) and lower values in coarse mode (-27.1‰ to -23.6‰). The lower δ¹³C of C₂ in coarse particles indicate that coarse mode of the compound originates from evaporation from fine mode and subsequent condensation/adsorption onto pre-existing coarse particles. Positive linear correlations of C₂, sulfate and ωC₂ and their δ¹³C values suggest that ωC₂ is a key intermediate, which is formed in aqueous-phase via photooxidation of precursors (e.g., Gly and Pyr), followed by a further oxidation to produce C₂.