Molecular distribution and compound-specific stable carbon isotopic composition of dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls in PM_2.5 from Beijing, China

This study investigates the seasonal variation, molecular distribution and stable carbon isotopic composition of diacids, oxocarboxylic acids and α-dicarbonyls to better understand the sources and formation processes of fine aerosols (PM2.5) in Beijing. The concentrations of total dicarboxylic acids varied from 110 to 2580 ng m−3, whereas oxoacids (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 individual species, followed by succinic acid or occasionally by terephthalic acid (tPh), a plastic burning tracer. Ambient concentrations of phthalic acid (37.9±27.3 ng m−3) and tPh (48.7±51.1 ng m−3) are larger in winter than in other seasons, illustrating that fossil fuel combustion and plastic waste incineration contribute more significantly to wintertime aerosols. The year-round mass concentration ratios of malonic acid to succinic acid (C3/C4) are relatively by comparison with those in other urban aerosols and remote marine aerosols, most of which are less than or equal to unity in Beijing; thus, the degree of photochemical formation of diacids in Beijing is insignificant. Moreover, positive correlations of some oxocarboxylic acids and α-dicarbonyls with nss-K+, a tracer for biomass burning, suggest biogenic combustion activities accounting for a large contribution of these organic acids and related precursors. The mean δ13C value of succinic acid is highest among all species with values of −17.1±3.9‰ (winter) and −17.1±2.0‰ (spring), while malonic acid is less enriched in 13C than others in autumn (−17.6±4.6‰) and summer (−18.7±4.0‰). The δ13C values of major species in the Beijing aerosols are generally lower with a wider range than those in downwind regions in the western North Pacific, which indicates that Beijing has diverse emission sources with weak photooxidation. Thus, our study demonstrates that in addition to photochemical oxidation, high abundances of diacids, oxocarboxylic acids and α-dicarbonyls in Beijing are largely associated with anthropogenic primary emissions, such as biomass burning, fossil fuel combustion, and plastic burning.