Taipei metropolitan O3 concentration seasonal variation (linear trend: + 38.7%)

Kaohsiung metropolitan O3 concentration seasonal variation (linear tiend: + 41.4%)

Figure 10. Increasing trends of ozone concentration in Taipei and Kaohsiung metropolitan areas from 1994 to 2004.

levels of nearly all air pollutants in Taiwan during the past decade. The only exception to this improving trend is the increase in ambient ozone concentration. For example, average ozone mixing ratios in the two largest metropolitan areas, Taipei and Kaohsiung, have increased by 39% and 41%, respectively for the period of 1994-2004 (Fig. 10).

With the reduction of emissions of ozone precursors, one would expect the photochemical production to decrease. What factors have contributed to the large increases in Taipei and Kaohsiung? Chou et al. (2006) showed that the reduction in the titration effect of NO due to decreasing emissions of NO played an important role in the increasing trends of ozone concentration. Another likely factor contributing to the trends is an increase in the long range transport of ozone. The impact of long range transport of ozone can be seen in Table 1, which shows greater increases in O3 but smaller increases in O3 + NO2 at urban stations compared to background stations. Since the lifetime of NO2 is less than a day, its concentration is controlled by local processes. The small trends for O3+NO2 at urban stations are consistent with the reduction

Table 1. Trends of O3 and O3 + NO2 for three large metropolitan areas (Taipei, Taichung, Kaohsiung), one small city (Ilan) and three background stations (Yangming, Wanli, Lanyu).


O3 trend

O3 + NO2 trend

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