Year-round observations of stable carbon isotopic composition of carboxylic acids, oxoacids and α-Dicarbonyls in fine aerosols at Tianjin, North China: Implications for origins and aging.

To better understand the origins and photochemical processing (aging) of organic aerosols (OA), we studied fine aerosols (PM) collected at urban (Nankai District (ND)) and suburban (Haihe Education Park (HEP)) Tianjin, North China over a one-year period (2018-2019) for stable carbon isotopic composition (δC) of water-soluble diacids, oxoacids, α-dicarbonyls and fatty acids. Maleic (M, -18.3 ± 10.9‰ at ND and -23.5 ± 10.2‰ at HEP) and fumaric (F, -22.0 ± 12.1‰ at ND and -22.5 ± 10.5‰ at HEP) acids were found to be most enriched with C followed by oxalic acid (C, -24.7 ± 3.9‰ at ND and -25.9 ± 4.7‰ at HEP) during the campaign. Based on seasonal changes in δC of selected marker species: C and C diacids, phthalic, glyoxylic and pyruvic acids and glyoxal, and their comparison with the source signatures, we found that water-soluble OA in Tianjin were mainly originated from fossil fuel combustion and biomass burning emissions and were subjected for significant aging. The contribution from fossil fuel combustion including coal combustion was high in autumn and winter, especially at ND. Considering the enrichment of C in specific species together with linear relations of δC of selected species with their concentrations, with mass ratios and with the relative abundance of C diacid, we inferred that the photochemical transformations of longer-chain diacids, oxidation of α-dicarbonyls (Gly and mGly), preferably in gas phase, were important in warm period (March-September), whereas the oxidation of Gly, mGly and other precursors in aqueous phase were major in cold period (October-February).