Molecular distributions and stable carbon isotopic compositions of dicarboxylic acids and related compounds in aerosols from Sapporo, Japan: Implications for photochemical aging during long-range atmospheric transport

Molecular and stable carbon isotopic (delta C-13 value) compositions of dicarboxylic acids, ketoacids, and dicarbonyls in aerosol samples (i.e., total suspended particles) collected in Sapporo, northern Japan during spring and summer were determined to better understand the photochemical aging of organic aerosols during long-range transport from East Asia and Siberia. Their molecular distributions were characterized by the predominance of oxalic acid (C-2) followed by malonic (C-3) or occasionally succinic (C-4) acids. Concentrations of total diacids ranged from 106-787 ng m(-3) with ketoacids (13-81 ng m(-3)) and dicarbonyls (2.6-28 ng m(-3)) being less abundant. Water-soluble organic carbon (WSOC) comprised 23-69% of aerosol organic carbon (OC). OC to elemental carbon (EC) ratios were high (3.6-19, mean: 8.7). The ratios of C-3/C-4 and WSOC/OC did not show significant diurnal changes, suggesting that the Sapporo aerosols were not seriously affected by local photochemical processes and instead they were already aged. delta C-13 values of the dominant diacids (C-2 - C-4) ranged from -14.0 to -25.3 parts per thousand. Largest delta C-13 values (-14.0 to -22.4 parts per thousand, mean: -18.8 parts per thousand) were obtained for C-2, whereas smallest values (-25.1 to -31.4 parts per thousand, mean: -28.1 parts per thousand) were for azelaic acid (C-9). In general, delta C-13 values of C-2 - C-4 diacids became less negative with aerosol aging (i.e., WSOC/OC), presumably due to isotopic fractionation during photochemical degradation of diacids. By comparing the delta C-13 values of diacids in the Sapporo aerosols with different air mass source regions, we suggest that although initial delta C-13 values of diacids depend on their precursor sources, the enrichment in C-13 can be ascribed to aerosol photochemical aging.