Chengdu is a megacity in the southwest of China with high ozone (O3) mixing ratio.Observation of volatile organic compounds (VOCs),NO2 and O3 with high temporal resolution was conducted in Chengdu to investigate the chemical processes and causes of high O3 levels.The hourlyixing ratios of VOCs,NO2,and O3 were monitored by an online system from 28 August to 7 October,2016.According to meteorological conditions,Chengdu,with relative warm weather and low wind speed,is favorable to O3 formation.Part of the O3 in Chengdu may be transported from the downtown area.In O3 episodes,the average mixing ratios of NO2 and O3 were 20.20 ppbv and 47.95 ppbv,respectively.In non-O3 episodes,the average mixing ratios of NO2 and O3 were 16.38 ppbv and 35.15 ppbv,respectively.The average mixing ratio of total VOCs (TVOCs) was 40.29 ppbv in non-O3 episodes,which was lower than that in O3 episodes (53.19 ppbv).Alkenes comprised 51.7％ of the total O3 formation potential (OFP) in Chengdu,followed by aromatics which accounted for 24.2％.Ethylene,trans-pentene,propene,and BTEX (benzene,ethylbenzene,toluene,m/p-xylene,o-xylene) were also major contributors to the OFP in Chengdu.In O3 episodes,intensive secondary formations were observed during the campaign.Oxygenated VOCs (OVOCs),such as acetone,Methylethylketone (MEK),and Methylvinylketone (MVK) were abundant.Isoprene rapidly converted to MVK and Methacrolein (MACR) during O3 episodes.Acetone was mainly the oxidant of C3-C5 hydrocarbons.