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. This study investigates the seasonal variation, molecular distribution andstable carbon isotopic composition of diacids, oxocarboxylic acids and α -dicarbonyls to better understand the sources and formationprocesses of fine aerosols (PM 2.5) in Beijing. The concentrations oftotal dicarboxylic acids varied from 110 to 2580 ng m −3 , whereasoxoacids (9.50–353 ng m −3) and dicarbonyls (1.50–85.9 ng m −3) were less abundant. Oxalic acid was found to be the most abundant individualspecies, followed by succinic acid or occasionally by terephthalic acid(tPh), a plastic waste burning tracer. Ambient concentrations of phthalicacid (37.9  ±  27.3 ng m −3) and tPh (48.7  ±  51.1 ng m −3) were larger in winter than in other seasons, illustrating that fossil fuelcombustion and plastic waste incineration contribute more to wintertimeaerosols. The year-round mass concentration ratios of malonic acid tosuccinic acid (C 3 ∕  C 4) were relatively low by comparison withthose in other urban aerosols and remote marine aerosols. The values wereless than or equal to unity in Beijing, implying that the degree ofphotochemical formation of diacids in Beijing is insignificant. Moreover,strong correlation coefficients of major oxocarboxylic acids and α -dicarbonyls with nss-K + suggest that biomass burning contributessignificantly to these organic acids and related precursors. The mean δ13 C value of succinic acid is the highest among all species, withvalues of − 17.1  ±  3.9 ‰ (winter) and − 17.1  ±  2.0 ‰ (spring), while malonic acid is more enriched in 13 C than others in autumn ( − 17.6  ±  4.6 ‰) andsummer ( − 18.7  ±  4.0 ‰). The δ13 C valuesof major species in Beijing aerosols are generally lower than those in thewestern North Pacific atmosphere, the downwind region, which indicates thatstable carbon isotopic compositions of diacids depend on their precursorsources in Beijing. Therefore, our study demonstrates that in addition tophotochemical oxidation, high abundances of diacids, oxocarboxylic acids and α -dicarbonyls in Beijing are largely associated with anthropogenicprimary emissions, such as biomass burning, fossil fuel combustion andplastic waste burning.

atmospheric chemistry and physics

Molecular distribution and compound-specific stable carbon isotopic composition of dicarboxylic acids, oxocarboxylic acids and &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-dicarbonyls in PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; from Beijing, China