Abstract Taiwan saw clear COVID-19 societal mobility-related changes in air quality, but most assessments have relied on national/regional aggregates or long averaging windows, leaving a gap for Taipei and for short-run, activity-linked effects. To address this, we conduct a Taipei-specific event study of the May–June 2021 Level-3 alert, integrating four monitoring sites with mobility indicators to isolate activity-driven changes and extract practical guidance for traffic management. The results indicate that the lockdown policy contributed to a more than 25% decrease in NO 2 and CO and a slight decline in PM 2.5 . We further compared a month-long lockdown period with the same period in the preceding two years, revealing NO 2 and CO declined up to 40% during the lockdown. On the other hand, PM 2.5 concentrations either decreased or increased depending on the comparison year. In contrast, O 3 concentrations consistently increased during the lockdown, likely driven by the decrease in NOx, which is in correlation with the decrease in transportation and fuel consumption. This study finds that COVID-19 restrictions yielded cleaner air for NO 2 , CO, and PM 2.5 while their characteristic diurnal patterns remain unchanged. By contrast, the O 3 levels climbed as an unanticipated consequence of lower NOx emissions. According to Taiwan's new regulation, electric vehicles should reach 35% of new vehicle sales by 2030, which would be analogous to the COVID-19 period, and may reduce NOx concentrations up to 15% in the area, a significant reduction. Although PM 2.5 concentrations may not dramatically decrease with the adoption of electric vehicles, ozone increases could be curtailed by careful and simultaneous reduction of hydrocarbon emissions. Graphical Abstract
Tran et al. (Sat,) studied this question.