Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: Implications for sources and formation mechanisms

PM10 aerosols from the summit of Mt. Hua (2060 m a.s.l) in central China during the winter and summer of 2009 were analyzed for dicarboxylic acids, ketocarboxylic acids and alpha-dicarbonyls. Molecular composition of dicarboxylic acids (C-2-C-11) in the free tropospheric aerosols reveals that oxalic acid (C-2, 399 +/- 261 ng m(-3) in winter and 522 +/- 261 ng m(-3) in summer) is the most abundant species in both seasons, followed by malonic (C-3) and succinic (C-4) acids, being consistent with that on ground levels. Most of the diacids are more abundant in summer than in winter, but adipic (C-6) and phthalic (Ph) acids are twice lower in summer, suggesting more significant impact of anthropogenic pollution on the wintertime alpine atmosphere. Moreover, glyoxal (Gly) and methylglyoxal (mGly) are also lower in summer (12 +/- 6.1 ng m(-3)) than in winter (22 +/- 13 ng m(-3)). As both dicarbonyls are a major precursor of C-2, their seasonal variation patterns, which are opposite to those of the diacids, indicate that the mountain troposphere is more oxidative in summer. C-2 showed strong linear correlations with levoglucosan in winter and oxidation products of isoprene and monoterpane in summer. PCA analysis further suggested that the wintertime C-2 and related SOA in the Mt Hua troposphere mostly originate from photochemical oxidations of anthropogenic pollutants emitted from biofuel and coal combustion in lowland regions. On contrast, the summertime C-2 and related SOA mostly originate from further oxidation of the mountainous isoprene and monoterpene oxidation products. The AIM model calculation results showed that oxalic acid concentration well correlated with particle acidity (R-2 = 0.60) but not correlated with particle liquid water content, indicating that particle acidity favors the organic acid formation because aqueous-phase C-2 production is the primary mechanism of C-2 formation in ambient aerosols and is driven by acid-catalyzed oxidation.</p

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 (&delta;13C 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 &gt; 0:5) and C3 = C4 (R2 &gt; 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, !C2, Pyr, Gly and mGly) with biogenic precursors (SOA tracers derived from isoprene, &alpha;/&beta;-pinene and &beta;-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 (&delta;13C) of the dicarboxylic acids are higher in the daytime than in the nighttime, with the highest value (&minus;16:5 &plusmn; 1:9 &permil;) found for C2 and the lowest value (&minus;25:2 &plusmn; 2:7 &permil;) found for C9. An increase in &delta;13C 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.</p