Characterization of oxalic acid-containing particles in summer and winter seasons in Chengdu, China

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Oxalic acid is the most abundant dicarboxylic acid in the atmosphere, but the processes involved in its formation in ambient aerosols are still uncertainty. We used single particle aerosol mass spectrometer to investigate the sources and formation mechanisms of oxalic acid-containing (C2) particles in ambient aerosols in Chengdu during summer and winter seasons. C2 particles accounted for 2.9 and 0.7% of the total amount of fine particles detected in summer and winter, respectively. The C2 particles in each season were classified as nine different types of particle with significant seasonal differences in their contribution to total C2 particles. The oxalic acid-sulfate (C2-SO4) particles and particles related to biomass burning (i.e., oxalic acid-organic nitrogen (C2-CN) and oxalic acid-elemental carbon (C2-EC)) represented 78% of the total C2 particles in summer, whereas the contributions of oxalic acid-dust (C2-D), oxalic acid-sulfate and nitrate (C2-SN), and oxalic acid-nitrate (C2-NO3) particles clearly increased in winter. Although the peak diameter of the average size distribution of total C2 particles was larger in winter than in summer, there was no significant seasonal difference in the number fraction of the size-resolved particles for each type of particle. The peak concentration of total C2 particle in summer occurred in the afternoon, but there was no obvious diurnal variation in winter. We concluded that local photochemical oxidation in the daytime favored the production of C2 particles in summer, whereas in-cloud processing with subsequent long-distance transport had an important effect on C2 particles in winter. The potential source contribution function (PSCF) analysis indicated that local sources from the eastern, southeastern, and southern regions of Sichuan Province dominated the C2 particles in summer, whereas the contribution from the long-distance transport of aerosols originating outside Sichuan Province was significantly enhanced in winter. These findings will help us to understand the formation of secondary organic aerosols in the atmosphere over the Sichuan Basin.