Air quality has always been a public concern,especially when regional air pollution episodes occurred in several typical regions of China,such as the Beijing-Tianjin-Hebei region,the Yangtze River Delta (YRD),and the Pearl River Delta.However,the decrease of fine particulate matter (PM_2.5) would somehow stimulate surface ozone production.Traditional air quality models usually use a simplified photochemical reaction mechanism to simulate the ozone concentration and its precursors with fitted kinetics and products based on chamber experiments,which leads to large discrepancies between model predictions and actual observations.To solve this problem,a near-explicit photochemical mechanism,the Master Chemical Mechanism (MCM) was implemented into the Community Multi-scale Air Quality model (CMAQ) to investigate a high ozone episode during August 27th to September 5th of 2015 over the YRD.The model can generally reproduce the temporal variation of ozone and its precursors in six representative cities.Statistical analysis of the maximum daily averaged 8-hour O₃ showed that both the normalized mean bias (NMB) and normalized mean error (NME) met the criteria,with the best performances in Xuzhou (NMB=-0.15 and NME=0.23).In the YRD,residential source contributed most to the total VOCs,accounting for 39.08%,followed by transportation (33.25%) and industry (25.56%).Power plants contributed least to the total VOCs for about 2.11%.Further analysis of reactive oxidized nitrogen (NO_y),which is the reservoirs of NO_x,indicated that its majority was NO_x (80%),followed by the nitric acid (HNO₃< 10%) in the YRD.The spatial distribution of O₃ was very similar to those of NO_y and NO_x.The distributions of other oxidized products,such as HCHO was similar to that of NO_x,perhaps due to the production by VOCs oxidation under high NO_x conditions.Methyl vinyl ketone (MVK) and methacrolein (MACR) were possibly formed by oxidation of biogenic VOCs,estimated from their similar spatial distributions with that of the BVOCs.The air pollutants in the YRD is affected by strong interactions between anthropogenic and biogenic emissions.