Although pollutant sources are usually in urban and industrialized areas, poor air quality is frequently encountered in downwind rural areas. This study utilized a tethered balloon to measure the pollutant concentrations in a downwind side (Caotun) of the Taichung Basin during the summer of 2002. Furthermore, an annular denuder system ( ADS) was used to measure PM2.5 and gaseous pollutant concentrations. Concentrations of 55 VOCs species were measured continuously by a network of photochemical assessment monitoring stations in central Taiwan. This study discusses the correlation between the ozone concentration and the routes of an air mass during ozone episodes. Backward trajectory, ratio of particulate matter and photochemical indices were used to examine the aging degree of an air mass. During ozone episodes, the trajectory distribution had a clear sea-land breeze mechanism with a relatively slow wind speed, and the air mass primarily entered through the western coastal industrial area. Analytical results demonstrate that daytime ozone concentrations accompany HNO3 concentrations by photochemical reactions and depend on the previous night's HNO2 concentration. The sulfur and nitrogen conversion ratios, and the photochemical indices (ethylbenzene/ m,p-xylene, n-pentane/t-2-pentene and NO2/NO) increased with the ozone concentration. The downwind photochemical indices were strongly correlated with ozone concentration and were higher than those upwind. These phenomena indicate that the air mass during ozone episodes belongs to the "more aged" category.