Insight into the Composition of Organic Compounds (≥ C₆) in PM_2.5 in Wintertime in Beijing, China

Abstract

<p><strong>Abstract.</strong> Organic matter is a major component of PM<sub>2.5</sub> in megacities. In order to understand the detailed characteristics of organic compounds (> C<sub>6</sub>) at a molecular level on non-haze and haze days, we determined more than 300 organic compounds in the PM<sub>2.5</sub> from an urban area of Beijing in November-December 2016 using two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS). The identified organic compounds have been classified into groups, and quantitative methods were used to calculate their concentrations. Primary emission sources make significant contributions to the atmospheric organic compounds and six groups (including n-alkanes, PAHs, levoglucosan, branched-alkanes, n-alkenes and alkyl-benzenes) account for 66 % of total identified organic compound mass. In addition, polycyclic aromatic hydrocarbons (PAHs), and oxygenated PAHs (OPAHs) were abundant amongst the atmospheric organic compounds on both haze and non-haze days. A near-unimodal molecular distribution, peaking approximately within the range of C<sub>19</sub>-C<sub>28</sub>, was observed in most hydrocarbon groups. In addition, the concentrations of unidentified compounds were also estimated in the present study. The total identified compounds account for approximately 47 % of total organic compounds (> C<sub>6</sub>) in the chromatogram on both the non-haze and haze days. The total mass concentrations of organic compounds (> C<sub>6</sub>) in the chromatogram were 4.0 μg m<sup>−3</sup> and 7.4 μg m<sup>−3</sup> on the non-haze and haze days respectively, accounting for 26.5 % and 18.5 % of OM respectively on those days. There is strong evidence that the organic aerosol is more highly oxidised, and hence less GC-volatile on haze days.</p>