Abstract: By analyzing surface meteorological observations and air quality monitoring data (O₃, NO_x, and PM10) at two stations from 2 April to 7 April 2008, in Nanjing, eastern China, the inverse correlation between O₃ and particulate matter is determined. A photochemical box model is employed to interpret the possible mechanism. The simulation shows that elevated concentration of particulate matter can increase the aerosol optical depth by 20%-40% and reduce averaged photolysis rates of NO₂ and O₃ and concentration of OH and HO₂ by 20%-30% and 20%-50%, respectively, which leads to a 30%-40% decrease in the net photochemical production rate of O₃. The results indicate that the effects of atmospheric particles on photochemistry and the oxidation ability of the atmosphere are responsible for the reduction of the observed surface O₃ concentration.