Vertical Evolution of Boundary Layer Volatile Organic Compounds in Summer over the North China Plain and the Differences with Winter

The vertical observation of volatile organic compounds(VOCs)is an important means to clarify the mechanisms of ozone formation.To explore the vertical evolution of VOCs in summer,a field campaign using a tethered balloon during summer photochemical pollution was conducted in Shijiazhuang from 8 June to 3 July 2019.A total of 192 samples were collected,23 vertical profiles were obtained,and the concentrations of 87 VOCs were measured.The range of the total VOC concentration was 41-48 ppbv below 600 m.It then slightly increased above 600 m,and rose to 58±52 ppbv at 1000 m.The proportion of alkanes increased with height,while the proportions of alkenes,halohydrocarbons and acetylene decreased.The proportion of aromatics remained almost unchanged.A comparison with the results of a winter field campaign during 8-16 January 2019 showed that the concentrations of all VOCs in winter except for halohydrocarbons were more than twice those in summer.Alkanes accounted for the same proportion in winter and summer.Alkenes,aromatics,and acetylene accounted for higher proportions in winter,while halohydrocarbons accounted for a higher proportion in summer.There were five VOC sources in the vertical direction.The proportions of gasoline vehicular emissions+industrial sources and coal burning were higher in winter.The proportions of biogenic sources+long-range transport,solvent usage,and diesel vehicular emissions were higher in summer.From the surface to 1000 m,the proportion of gasoline vehicular emissions+industrial sources gradually increased.