Enrichment of (13)C 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 (δ(13)C) 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 δ(13)C of diacids, glyoxylic acid (ωC(2)) and glyoxal. δ(13)C of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, δ(13)C of C(2)-C(4) diacids showed an increasing trend with decreasing chain length. Based on δ(13)C of diacids and related compounds and their relations to their concentrations, we found that C(2) and C(3) are enriched with (13)C 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 (13)C in C(3)-C(9) diacids whereas their production from primary precursors causes depletion of (13)C. In case of ωC(2) and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of (13)C. This study reveals that δ(13)C of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport.