To better understand the origins and photochemical processing of water-soluble organic aerosols in the Tianjin region, North China, we studied the stable isotopic composition (δ13C) of diacids, oxoacids, α-dicarbonyls, citric acid and fatty acids in fine aerosols (PM2.5) collected at an urban (Nankai District, ND; n = 121) and a suburban (Haihe Education Park, HEP; n = 40) sites in Tianjin from 5 July 2018 to 4 July 2019. The δ13C of diacids and related compounds were measured using a capillary gas chromatography (Agilent 7890B) coupled with isotope ratio mass spectrometry (GC-irMS) system. Based on seasonal variations in δ13C of selected species and linear relations with their concentrations and mass ratios, we found that the diacids and related compounds were mainly derived from fossil fuel including coal combustion and biomass burning and photochemical processing, preferably in gaseous- and aqueous-phases in warm (March-September) and cold (October-February) periods, respectively, in Tianjin, North China.

Acknowledgements. This study was partly supported by the National Key R&D Program of China (2017YFC0212700) and National Natural Science Foundation of China (NSFC) through Grant-in-Aid number: 41775120.