A numerical study of reducing the concentration of O and PM simultaneously in Taiwan.

Since the 24-hr PM (particle aerodynamic diameter less than 2.5 μm) concentration standard was regulated in Taiwan in 2012, the PM concentration has been decreasing year by year, but the ozone (O) concentration remains almost the same. In particular, the daily maximum 8-hr average O (MDA8 O) concentration frequently exceeds the standard. The goal of this study is to find a solution for reducing PM and O simultaneously by numerical modeling. After the Volatile Organic Compounds (VOC)-limited and nitrogen oxides (NO)-limited areas were defined in Taiwan, then, in total, 50 scenarios are simulated in this study. In terms of the average in Taiwan, the effect of VOC emission reduction is better than that of NO on the decrease in PM concentration, when the same reduction proportion (20%, 40%) is implemented. While the effect of further NO emission reduction (60%) will exceed that of VOC. The decrease in PM is proportional to the reduction in precursor emissions such as NO, VOC, sulfur dioxides (SO), and ammonia (NH). The lower reduction of NO emission for whole Taiwan caused O increases on average but higher reduction can ease the increase, which suggests the implement of NO emission reductions must be cautious. When comparing administrative jurisdictions in terms of grids, districts/towns, and cities/counties, it was found that controlling NO and VOC at a finer spatial resolution of control units did not benefit the decrease in PM but did benefit the decrease in O. The enhanced O control strategies obviously cause a higher decrease of O throughout Taiwan due to NO and VOC emission changes when they are implemented in the right places. Finally, three sets of short-term and long-term goals of controlling PM and O simultaneously are drawn from the comprehensive rankings for all simulated scenarios, depending on whether PM or O control is more urgent. In principle, the short-term scenarios could be ordinary or enhanced version of O decrease with lower NO/VOC emissions, while the long-term scenario is enhanced version of O decrease plus high emission reductions for all precursors.