Ozone variations through vehicle emissions reductions based on air quality monitoring data in Taipei City, Taiwan, from 1994 to 2003

The ozone (O3) ambient air quality is evaluated using two indices in Taiwan: the daily maximum O3 concentration (O3, max) greater than 120 ppb and the daily 8-h O3 moving average (O3, 8 h) greater than 60 ppb. Results of air quality monitoring data from 1994 to 2003 in Taipei City indicate that the 99th percentile O3, max was reduced, but those below the 90th percentile O3, max obviously increased and days with O3, 8 h greater than 60 ppb were doubled after the reduction of O3 precursor emissions. Because of Taipei's unique vehicle emissions pattern in having more motorcycles than cars, Sunday, as compared to weekdays, has more reductions of non-methane hydrocarbons (NMHCs) than NOX. This results in Sunday O3, max being lower than weekday O3, max average for those days with O3, max exceeding 80 ppb. On the contrary, if O3, max is lower than 60 ppb, Sunday O3, max is relatively higher than weekday O3, max average, due to the reduced NO titration effect. In addition, our evaluations of O3 levels with respect to the ambient air quality standards show that the number of Sundays with O3, max greater than 120 ppb is fewer than the number average of weekdays with O3, max exceeding 120 ppb for every month. Concerning O3, 8 h, only in June, July, and August, when the meteorological conditions favor O3 production, does the reduction of Sunday vehicle exhaust reduce the number of days with O3,8h greater than 60 ppb. However, in September and October, the reduction of Sunday vehicle exhaust will increase the number of days. The results presented in this paper indicate that the O3 pollution in Taipei City is VOC-limited. Thus, for a greater reduction in NMHCs, like the Sunday vehicle reduction in Taipei City, the upper percentile O3, max is reduced more and those below the moderately high percentile O3, max are increased less.