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. Organic matter is a major component of PM 2.5 in megacities. In order tounderstand the detailed characteristics of organic compounds ( ≥  C 6 )at a molecular level on non-haze and haze days, we determined more than 300organic compounds in the PM 2.5 from an urban area of Beijingcollected in November–December 2016 using two-dimensional gas chromatographycoupled to time-of-flight mass spectrometry (GC  ×  GC-ToF-MS). Theidentified organic compounds have been classified into groups, andquantitative methods were used to calculate their concentrations. Primaryemission sources make significant contributions to the atmospheric organiccompounds, and six groups (including n-alkanes, polycyclic aromatichydrocarbons – PAHs, levoglucosan, branched alkanes, n-alkenes andalkyl-benzenes) account for 66 % of total identified organic compoundmass. In addition, PAHs and oxygenated PAHs (O-PAHs) were abundant amongstthe atmospheric organic compounds on both haze and non-haze days. The mostabundant hydrocarbon groups were observed with a carbon atom range ofC 19 –C 28 . In addition, the total concentration of unidentifiedcompounds present in the chromatogram was estimated in the present study.The total identified compounds account for approximately 47 % of totalorganic compounds ( ≥  C 6 ) in the chromatogram on both the non-hazeand haze days. The total mass concentrations of organic compounds ( ≥  C 6 ) in the chromatogram were 4.0 and 7.4  µ g m −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 thetotal organic carbon concentration. Ratios of individual compoundconcentrations between haze and non-haze days do not give a clear indicationof the degree of oxidation, but the overall distribution of organiccompounds in the chromatogram provides strong evidence that the organicaerosol is less GC volatile and hence more highly oxidized on haze days.

atmospheric chemistry and physics

Insight into the composition of organic compounds ( ≥ C&amp;lt;sub&amp;gt;6&amp;lt;/sub&amp;gt;) in PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; in wintertime in Beijing, China