Continuous observation of isoprene,α-pinene and β-pinene was carried out in a typical urban area of Beijing from March 2014 to February 2015,using an AirmoVOC online analyzer.Based on the analysis of the ambient level and variation characteristics of isoprene,α-pinene and β-pinene,the chemical reactivity was studied,and their sources were identified.Results showed that the concentrations of isoprene,α-pinene and β-pinene in the urban area of Beijing were lower than those in richly vegetated areas;the concentrations of isoprene were at a moderate level compared with those of previous studies of Beijing.Concentrations of isoprene,α-pinene and β-pinene showed different seasonal,monthly,daily and dial variations,and all of the three species showed higher level at night than those in the daytime as a whole,the variations of isoprene,α-pinene and β-pinene mainly influenced by emission of sources,photochemical reaction,and meteorological parameters.Isoprene was the largest contributor to the total OFP values than α-pinene and β-pinene.α-Pinene was the largest contributor to the total SOAFP values than isoprene and β-pinene in autumn,while isoprene was the largest one in other seasons.Isoprene,α-pinene and β-pinene were derived mainly from biological sources;and α-pinene level were also affected by industrial sources.To reduce the concentrations of isoprene,α-pinene and β-pinene,it is necessary to scientifically select urban green plant species,and more strict control measures should be taken to reduce the emission of α-pinene from industrial sources,such as artificial flavors and resins synthesis processes.