Among the historical data of ozone event days, the top five frequent weather types, which contribute 74.43% of ozone event station days, are mild northeast monsoon, high-pressure system with recirculation, Pacific high-pressure system, standard northeast monsoon, and low-pressure zone. Higher monthly average ozone concentrations were observed in spring and autumn. The monthly average ozone concentrations measured in Puli are higher than the ones measured in Nanto and Chushan. Higher monthly average nitrogen oxides and non-methan hydrocarbons concentrations were observed in winter. There were 90 ozone event station days in Nanto County during 2000~2005, weather types with highest frequency are mild northeast monsoon, high-pressure system with recirculation, and Pacific high-pressure system. In this project, the mechanisms of ozone event are simulated by photochemical grid model. In event day 1 (10/27/2003 ~ 10/31/2003), lower wind velocities were simulated inland. The pollutants were transported from the areas near sea to the inland area and react to form ozone. In event day 2 (6/1/2003 ~ 6/5/2003), the circulation around typhoon leads to lower wind velocity and pollutant accumulation behind the montains. The simumlted and measured ozone concentrations exhibit good correlationship. During the event day 1, areas with high ozone concentration originate in the southwest part of Nanto County, and develop along the northeast-southwest direction, then diminish in the night. In the event day 2, areas with high ozone concentration originate in the northwest part of Nanto County, and develop from west to east, then diminish in the night. The dominating precursor of ozone can be determined as nitrogen oxide by using the photochemical indicators H2O2/HNO3, O3/HNO3, H2O2/NOy, O3/NOy, and HCHO/NOy. The reduction of nitrogen oxides emission should have higher priority in developing the ozone control strategy. The major contributing areas of ozone are different for ozone event days with different weather types. Doshe, Chanhwa City, and Nanto City are the frequent contributors. However, the ozone concentration is contributed mainly by emission sources outside the central air quality zone. The strategy to reduce the ozone violence days is suggested to focus on the event hours with only small ozone violence. From the data of 2000~2005, there are 69 days with only one hour ozone concentration exceedence among the 227 ozone violence station days.

ozone, event day, precusor