Abstract. Molecular distributions and stable carbon isotopic (δ13C values)
compositions of dicarboxylic acids and related secondary organic aerosols
(SOA) in PM2.5 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 (C2), followed by malonic (C3),
succinic (C4) and azelaic (C9) acids. The concentration
ratios of C2 ∕ C4, diacid-C ∕ OC and
C2 ∕ 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 C2 ∕ C4 (R2>0.5) and
C3 ∕ C4 (R2>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
(C9 ∕ C6), azelaic acid to phthalic aid
(C9 ∕ Ph) and glyoxal to methylglyoxal (Gly ∕ mGly)
and the strong linear correlations of major dicarboxylic acids and related
SOA (i.e., C2, C3, C4, ωC2, Pyr, Gly
and mGly) with biogenic precursors (SOA tracers derived from isoprene,
α/β-pinene and
β-caryophyllene) further suggest that aerosols in this region are
mainly originated from biogenic sources (i.e., tree emissions). C2 concentrations correlated well with aerosol pH, indicating that
particle acidity favors the organic acid formation. The stable carbon
isotopic compositions (δ13C) of the dicarboxylic acids are higher
in the daytime than in the nighttime, with the highest value (-16.5±1.9 ‰) found for C2 and the lowest value (-25.2±2.7 ‰) found for C9. An increase in δ13C values of
C2 along with increases in C2 ∕ Gly and
C2 ∕ mGly ratios was observed, largely due to the isotopic
fractionation effect during the precursor oxidation process.
Stable carbon isotopic compositions of low-molecular-weight dicarboxylic acids, glyoxylic acid and glyoxal in tropical aerosols: implications for photochemical processes of organic aerosols
