Ground-level ozone (O3) has become a critical pollutant impeding air quality improvement in Yangtze River Delta region of China.In this study,we present O3 pollution characteristics based on one-year online measurements during 2016 at an urban site in Nanjing,Jiangsu Province.Then,the sensitivity of O3 to its precursors during 2 O3 pollution episodes in August was analyzed using a box model based on observation (OBM).The relative incremental reactivity (RIR) of hydrocarbons was larger than other precursors,suggesting that hydrocarbons played the dominant role in O3 formation.The RIR values for NOx ranged from -0.41％/％ to 0.19％/％.The O3 sensitivity was also analyzed based on relationship of simulated O3 production rates with reductions of VOC and NOx derived from scenario analyses.Simulation results illustrate that O3 formation was between VOCs-limited and transition regime.Xylenes and light alkenes were found to be key species in O3 formation according to RIR values,and their sources were determined using the Positive Matrix Factorization (PMF) model.Paints and solvent use was the largest contributor to xylenes (54％),while petrochemical industry was the most important source to propene (82％).Discussions on VOCs and NOx reduction schemes suggest that the 5％ O3 control goal can be achieved by reducing VOCs by 20％.To obtain 10％ O3 control goal,VOCs need to be reduced by 30％ with VOCs/NOx larger than 3∶1.