Air quality changes in Taiwan over the past decades and during the COVID-19 crisis

Abstract Over the past decades, Taiwan has achieved remarkable goals in air pollution reduction with the concentrations of several common air pollutants such as CO, NO x , PM 10 , PM 2.5 , and SO 2 going down. In contrast to these achievements, the mitigation of O 3 remains extremely tough due to the complexity of its formation process involving synergistic effects of precursor reductions and meteorological influences. During the local COVID-19 crises in Taiwan and the Level 3 alert in 2021, air pollutants directly emitted from the traffic such as CO and NO x present clear relationships with the drop of the recorded freeway traffic volume due to the alert, while PM 10 and PM 2.5 which are also relevant to the traffic do not show indications of being greatly influenced by the decrease of the traffic flow. Although road traffic is not regarded as a main source of SO 2 by current understanding, the unusual SO 2 variation patterns found in this study suggest a prolonged impact for months from the changes of travel behavior during the epidemic. In contrast, the epidemic did not exert influences on industrial SO 2 concentration which accounts for a large portion of total SO 2 in Taiwan, and a similar scenario is also seen in each type of O 3 monitoring. Although some results discussed in this study are not in line with current consensuses and understandings in terms of the nation of certain air pollutants, these findings may disclose new perspectives which could be a potential benefit to air quality improvement projects in the future.