Organic matter is a major component of PM2.5 in megacities. In order to understand the detailed characteristics of organic compounds (≥C6) at a molecular level on non-haze and haze days, we determined more than 300 organic compounds in the PM2.5 from an urban area of Beijing collected in November–December?2016 using two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-ToF-MS). 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, polycyclic aromatic hydrocarbons – PAHs, levoglucosan, branched alkanes, n-alkenes and alkyl-benzenes) account for 66% of total identified organic compound mass. In addition, PAHs and oxygenated PAHs (O-PAHs) were abundant amongst the atmospheric organic compounds on both haze and non-haze days. The most abundant hydrocarbon groups were observed with a carbon atom range of C19–C28. In addition, the total concentration of unidentified compounds present in the chromatogram was estimated in the present study. The total identified compounds account for approximately 47% of total organic compounds (≥C6) in the chromatogram on both the non-haze and haze days. The total mass concentrations of organic compounds (≥C6) in the chromatogram were 4.0 and 7.4μgm?3 on the non-haze and haze days, respectively, accounting for 26.4% and 18.5% of organic matter, respectively, on those days estimated from the total organic carbon concentration. Ratios of individual compound concentrations between haze and non-haze days do not give a clear indication of the degree of oxidation, but the overall distribution of organic compounds in the chromatogram provides strong evidence that the organic aerosol is less GC volatile and hence more highly oxidized on haze days.
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