Molecular distribution and stable carbon isotopic compositions of dicarboxylic acids and related SOA from biogenic sources in the summertime atmosphere of Mt. Tai in the North China Plain

Molecular distributions and stable carbon isotopic (inline-formulaδ¹³C values) compositions of dicarboxylic acids and related secondary organic aerosols (SOA) in PMinline-formula_2.5 aerosols collected on a day/night basis at the summit of Mt. Tai (1534 m a.s.l.) in the summer of 2016 were analyzed to investigate the sources and photochemical aging process of organic aerosols in the forested highland region of the North China Plain. The molecular distributions of dicarboxylic acids and related SOA are characterized by the dominance of oxalic acid (inline-formulaC₂), followed by malonic (inline-formulaC₃), succinic (inline-formulaC₄) and azelaic (inline-formulaC₉) acids. The concentration ratios of inline-formulaC₂ inline-formula∕ inline-formulaC₄, diacid-C inline-formula∕ OC and inline-formulaC₂ inline-formula∕ total diacids are larger in the daytime than in the nighttime, suggesting that the daytime aerosols are more photochemically aged than those in the nighttime due to the higher temperature and stronger solar radiation. Both ratios of inline-formulaC₂ inline-formula∕ inline-formulaC₄ (inline-formulaR²>0.5) and inline-formulaC₃ inline-formula∕ inline-formulaC₄ (inline-formulaR²>0.5) correlated strongly with the ambient temperatures, indicating that SOA in the mountaintop atmosphere are mainly derived from the photochemical oxidation of local emissions rather than long-range transport. The mass ratios of azelaic acid to adipic acid (inline-formulaC₉ inline-formula∕ inline-formulaC₆), azelaic acid to phthalic aid (inline-formulaC₉ inline-formula∕ inline-formulaPh) and glyoxal to methylglyoxal (Gly inline-formula∕ mGly) and the strong linear correlations of major dicarboxylic acids and related SOA (i.e., inline-formulaC₂, inline-formulaC₃, inline-formulaC₄, inline-formulaωinline-formulaC₂, Pyr, Gly and mGly) with biogenic precursors (SOA tracers derived from isoprene, inline-formulaα/inline-formulaβ-pinene and inline-formulaβ-caryophyllene) further suggest that aerosols in this region are mainly originated from biogenic sources (i.e., tree emissions).

inline-formulaC₂ concentrations correlated well with aerosol pH, indicating that particle acidity favors the organic acid formation. The stable carbon isotopic compositions (inline-formulaδ¹³C) of the dicarboxylic acids are higher in the daytime than in the nighttime, with the highest value (inline-formula $M38inlinescrollmathml-\mathrm{normal\; 16.5}\pm \mathrm{normal\; 1.9}$ 58pt10ptsvg-formulamathimgabc5b0d85663379f5b8a300b5d9dd9f7 acp-18-15069-2018-ie00001.svg58pt10ptacp-18-15069-2018-ie00001.png  ‰) found for inline-formulaC₂ and the lowest value (inline-formula $M40inlinescrollmathml-\mathrm{normal\; 25.2}\pm \mathrm{normal\; 2.7}$ 58pt10ptsvg-formulamathimg3650a3abb87cd8b1bffa0d762b2f830f acp-18-15069-2018-ie00002.svg58pt10ptacp-18-15069-2018-ie00002.png  ‰) found for Cinline-formula₉. An increase in inline-formulaδ¹³C values of inline-formulaC₂ along with increases in inline-formulaC₂ inline-formula∕ Gly and inline-formulaC₂ inline-formula∕ mGly ratios was observed, largely due to the isotopic fractionation effect during the precursor oxidation process.