To identify the diurnal variation and formation mechanism of dicarboxylic acids and related compounds in PM2.5 from Heze City, PM2.5 samples were collected in the winter (December) of 2017, which were subsequently analyzed for dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls, and levoglucosan (Levo). The results showed that the total concentrations of dicarboxylic and ketocarboxylic acids were higher during daytime than those during nighttime. In contrast to the diurnal variation of dicarboxylic and ketocarboxylic acids, the total concentrations of α-dicarbonyls exhibited higher concentrations in nighttime than in daytime. Because α-dicarbonyls are the major precursors of dicarboxylic acids, the opposing patterns suggest that the photochemical oxidation in daytime is stronger than that in nighttime. Oxalic acid (C2) is the dominant species during both day-and nighttime, followed by phahalic acid (Ph), succinic acid (C4), and malonic acid (C3), which is consistent with that in other urban regions. The mass ratios of C3/C4 (R2>0.7) correlated strongly with temperature, indicating that organic compounds in the atmosphere of Heze City are mainly derived from the photochemical oxidation of local emissions rather than long-range transport in winter. C2 correlated with in-situ pH and SO42-, suggesting that aqueous-phase oxidation was the major formation pathway of C2, which is driven by acid-catalyzed oxidation. Since the major SOA (C2, glyoxal, and methyglyoxal, secondary organic aerosol) correlated with Levo and the average mass rations of K+/organic carbon was 0.06 (ranging from 0.03 to 0.13), it can be concluded that the dicarboxylic acids and related SOA and K+ in Heze City were significantly influenced by biomass burning in winter.
Keywords: Heze City; PM2.5; aqueous formation mechanism; dicarboxylic acids; diurnal variations.