Molecular distribution and compound-specific stable carbon isotopic composition of dicarboxylic acids, oxocarboxylic acids and <i>α</i>-dicarbonyls in PM<sub>2.5</sub> from Beijing, China

Abstract

<p><strong>Abstract.</strong> This study investigates the seasonal variation, molecular distribution and stable carbon isotopic composition of diacids, oxocarboxylic acids and <i>α</i>-dicarbonyls to better understand the sources and formation processes of fine aerosols (PM_2.5) in Beijing. The concentrations of total dicarboxylic acids varied from 110 to 2580<span class="thinspace"></span>ng<span class="thinspace"></span>m⁻³, whereas oxoacids (9.50–353<span class="thinspace"></span>ng<span class="thinspace"></span>m⁻³) and dicarbonyls (1.50–85.9<span class="thinspace"></span>ng<span class="thinspace"></span>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 waste burning tracer. Ambient concentrations of phthalic acid (37.9<span class="thinspace"></span>±<span class="thinspace"></span>27.3<span class="thinspace"></span>ng<span class="thinspace"></span>m⁻³) and tPh (48.7<span class="thinspace"></span>±<span class="thinspace"></span>51.1<span class="thinspace"></span>ng<span class="thinspace"></span>m⁻³) were larger in winter than in other seasons, illustrating that fossil fuel combustion and plastic waste incineration contribute more to wintertime aerosols. The year-round mass concentration ratios of malonic acid to succinic acid (C₃<span class="thinspace"></span>∕<span class="thinspace"></span>C₄) were relatively low by comparison with those in other urban aerosols and remote marine aerosols. The values were less than or equal to unity in Beijing, implying that the degree of photochemical formation of diacids in Beijing is insignificant. Moreover, strong correlation coefficients of major oxocarboxylic acids and <i>α</i>-dicarbonyls with nss-K⁺ suggest that biomass burning contributes significantly to these organic acids and related precursors. The mean <i>δ</i>¹³C value of succinic acid is the highest among all species, with values of −17.1<span class="thinspace"></span>±<span class="thinspace"></span>3.9<span class="thinspace"></span>‰ (winter) and −17.1<span class="thinspace"></span>±<span class="thinspace"></span>2.0<span class="thinspace"></span>‰ (spring), while malonic acid is more enriched in ¹³C than others in autumn (−17.6<span class="thinspace"></span>±<span class="thinspace"></span>4.6<span class="thinspace"></span>‰) and summer (−18.7<span class="thinspace"></span>±<span class="thinspace"></span>4.0<span class="thinspace"></span>‰). The <i>δ</i>¹³C values of major species in Beijing aerosols are generally lower than those in the western North Pacific atmosphere, the downwind region, which indicates that stable carbon isotopic compositions of diacids depend on their precursor sources in Beijing. Therefore, our study demonstrates that in addition to photochemical oxidation, high abundances of diacids, oxocarboxylic acids and <i>α</i>-dicarbonyls in Beijing are largely associated with anthropogenic primary emissions, such as biomass burning, fossil fuel combustion and plastic waste burning.</p>