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 (delta C-13) 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 delta C-13 of diacids, glyoxylic acid (omega C-2) and glyoxal. delta C-13 of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, delta C-13 of C-2-C-4 diacids showed an increasing trend with decreasing chain length. Based on delta C-13 of diacids and related compounds and their relations to their concentrations, we found that C-2 and C-3 are enriched with C-13 during the photochemical decomposition and production from their higher homologues and oxoacids. Photochemical breakdown of higher (>= C-3) to lower diacids is also important in the enrichment of C-13 in C3-C9 diacids whereas their production from primary precursors causes depletion of C-13. In case of omega C-2 and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of C-13. This study reveals that delta C-13 of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport