Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and alpha-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C-2) is the dominant dicarboxylic acid, followed by succinic acid (C-4) and malonic acid (C-3), and glyoxylic acid (omega C-2) is the most abundant ketocarboxylic acids. Concentrations of C2, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM2.5 in the NCP. Moreover, C-2 and Gly and mGly linearly correlated with SO42-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pH(is)), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO42, NO3 and NH4+), dicarboxylic acids, and aerosol LWC in PM2.3 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (delta C-13) of major dicarboxylic acids and related SOA and the mass ratios of C-2/cliacids, C-2/Gly and C-2/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species. (C) 2019 The Author(s). Published by Elsevier B.V.