In springtime, northern Taiwan is in the downwind neighboring area of east Asia so that pollutants in this flow will have a direct impact on the ozone mixing ratio in Taiwan. By coupling long-term ozone observations and regional-scale chemical transport model (CTM) simulations, we obtained an understanding of the transport processes and provided some quantified estimates of the impact. We found that when the Asian continental outflow arrived in northern Taiwan, it always happened under the influence of east China high-pressure systems with dominant wind from the northeast to northern Taiwan and accompanying surface wind speeds greater than 5 m/s. Furthermore, no local high-ozone episodes were observed under such conditions. When local high-ozone episodes do occur, it is always with wind speeds less than 3 m/s and the wind direction generally from east and southwest. Using a three-dimensional CTM, we simulated the ozone evolution over northern Taiwan for 1 month in springtime in 2003. By comparing the simulation with and without different pollutant sources, we estimated the impact of pollutants in eastern Asian continental outflow and how and when this occurs. Our simulations indicate outflow mixing ratio of 50-70 ppb reaching the upwind boundary of Taiwan with the plume reaching a height of 2.5 km. For the nonepisode days the influence of Asian continental outflow of ozone may reach over 30%, and for the episode days it is less than 10% in northern Taiwan. Our findings suggest that for the study period high-oxidant formation in northern Taiwan is predominantly due to local pollution.
