The trend of surface ozone in Taipei, Taiwan, and its causes: Implications for ozone control strategies

In recent years, the high levels of surface ozone have become a major air pollutant in Taiwan. Over the western plain of Taiwan, where most of the 23 million inhabitants live, the concentration of ozone increased substantially during the last ten years (1994–2003). This increase is puzzling as the emissions of ozone precursors, oxides of nitrogen (NOx) and volatile organic compounds (VOCs) or non-methane hydrocarbon (NMHC) decreased significantly in the same period. An in-depth analysis focusing on “total oxidant” (estimated by O3+NO2) showed that its concentration remained essentially constant in Taipei, Taiwan, during that same ten year time period. Given that the regional tropospheric ozone is increasing, the constant oxidant level suggests that the oxidant production from photochemical reactions in Taipei should have been reduced to some extent, due to the reduction in emissions of ozone precursors. Consequently, the increasing trend in ozone was a result of the reduced titration by NO. These results are important to the formulation of ozone control strategies, especially for newly developed countries like Taiwan.