Volatile Organic Compounds In Wintertime North China Plain:Insights From Measurements Of Proton Transfer Reaction Time-of-Flight Mass Spectrometer(PTR-ToF-MS)

Volatile organic compounds(VOCs)are precursors of surface ozone and secondary organic aerosol(SOA),which may play an important role in the formation of haze in China.The characteristics of wintertime volatile organic compounds(VOCs)in the North China Plain(NCP)region are complicated and remain obscure.Comprehensive understandings of the characteristics of VOC emission sources have become an urgent task in controlling severe atmospheric pollution in China.In this study,VOC measurements were conducted by a proton transfer reaction time-of-flight mass spectrometer(PTR-ToF-MS)at a rural site in the NCP from November to December 2018.Uncalibrated ions measured by PTR-ToF-MS were quantified and the overall VOC compositions were investigated by combining the measurements of PTR-ToF-MS and gas chromatography-mass spectrometer/flame ionization detector(GC-MS/FID).The measurement showed that although atmospheric VOCs concentrations are often dominated by primary emissions,the secondary formation of oxygenated VOCs(OVOCs)is non-negligible in the wintertime,i.e.,OVOCs accounts for 42% ± 7% in the total VOCs(151.3 ± 75.6 ppb V).We demonstrated that PTR-MS measurements for isoprene are substantially overestimated due to the interferences of cycloalkanes.The chemical changes of organic carbon in a pollution accumulation period were investigated,the changes of average element(carbon and oxygen)number for VOCs suggests the direction of chemical evolution.The changes of emission ratios of VOCs between winter 2011 and winter 2018 in the NCP support the positive effect of ’coal to gas’ strategies in curbing air pollutants.Based on the mass spectra of PTR-ToF-MS,we identify many key species(e.g.oxygenated aromatics)in wintertime of NCP.The high abundances of oxygenated aromatics(e.g.phenols)and aromatics are indicative of strong emissions of coal combustion in wintertime of NCP.We also discussed the application of VOCs pairs for distinguishing coal combustion from other sources.The ratio of naphthalene to C8 aromatics was proposed as a potential indicator of the influence of coal combustion on VOCs.