Enrichment of 13C in diacids and related compounds during photochemical processing of aqueous aerosols: New proxy for organic aerosols aging

To investigate the applicability of compound specific stable carbon isotope ratios (δ13C) of organics in assessment of their photochemical aging in the atmosphere, batch UV irradiation experiments were conducted on two ambient (anthropogenic and biogenic) aerosol samples in aqueous phase for 0.5-120 h. The irradiated samples were analyzed for δ13C of diacids, glyoxylic acid (ωC2) and glyoxal. δ13C of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, δ13C of C2-C4 diacids showed an increasing trend with decreasing chain length. Based on δ13C of diacids and related compounds and their relations to their concentrations, we found that C2 and C3 are enriched with 13C during the photochemical decomposition and production from their higher homologues and oxoacids. Photochemical breakdown of higher (≥C3) to lower diacids is also important in the enrichment of 13C in C3-C9 diacids whereas their production from primary precursors causes depletion of 13C. In case of ωC2 and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of 13C. This study reveals that δ13C of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport.