Abstract
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.
Highlights
Secondary organic aerosols (SOA) account for a substantial fraction (20 %–90 %) of the total PM2.5 mass in the troposphere, of which up to 80 % are water-soluble (Hallquist et al, 2009; Kroll and Seinfeld, 2008)
Our results showed that the levels of water-soluble organic compounds decreased by 30 %–80 % when it was rainy, suggesting that dicarboxylic acids and related SOA can be removed efficiently by wet deposition because these water-soluble compounds are washed out and can be efficiently removed www.atmos-chem-phys.net/18/15069/2018/
The concentrations of total dicarboxylic acids and ketocarboxylic acids are higher in the daytime than those in the daytime, but α-dicarbonyls present lower values in the daytime, suggesting the mountainous atmospheric environment is more photochemically aged in the daytime than in the nighttime
Summary
Secondary organic aerosols (SOA) account for a substantial fraction (20 %–90 %) of the total PM2.5 mass in the troposphere, of which up to 80 % are water-soluble (Hallquist et al, 2009; Kroll and Seinfeld, 2008). Tai pounds play important roles in atmospheric aqueous chemistry and influence radiative forcing of aerosols by acting as cloud condensation nuclei (CCN) (Hoque et al, 2017; Wang et al, 2012, 2016; Zhang et al, 2016) They can be emitted directly from sources such as incomplete combustion of fossil fuels (Kawamura and Kaplan, 1987) and biomass burning (Kawamura et al, 2013a, b; Narukawa et al, 1999), atmospheric dicarboxylic acids and related compounds are largely produced by photochemical oxidation of unsaturated fatty acids, PAHs (Kawamura et al, 1996), cyclic alkanes and other compounds (Kawamura and Usukura, 1993). It is necessary to investigate the abundances, compositions and formation mechanisms of oxalic acid and related SOA when vegetation emission is dominant, especially in the forested highland region where aerosols are more accessible to clouds due to higher elevation. We discussed the impact of temperature (T ), relative humidity (RH), particle acidity (pHIS), liquid water content (LWC) and O3 concentration on oxalic acid and related SOA to explore their sources and formation mechanisms in the forested highland region of the North China Plain
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