Administrative hierarchy, city characteristics and impacts of the Clean Air Act: A quasi-experimental study on PM2.5 pollution in China 2000–2020

Most existing research has assessed and confirmed the impacts of environmental regulations on pollution emissions, while few has investigated the role of city characteristics (e.g., political, economic, temporal, or geographic factors) in the relationship between environmental policies and air pollution. To fill the gap, this study used a quasi-experimental design (difference-in-difference, DID) with high-spatiotemporal-resolution PM2.5 data to explore the role of administrative hierarchy and city characteristics in the impacts of the Clean Air Act (CAA) on PM2.5 pollution in China. The empirical results for 2811 counties from 297 cities during the period from 2000 to 2020 show that PM2.5 pollution has a significant reduction after implementing the CAA (−14.36, 95% Confidence Interval (CI): −14.67, −14.06), and the impacts of the CAA on PM2.5 reduction are much stronger in non-capital cities (low hierarchy) than capital cities (high hierarchy). Our further analyses indicate that among various city-characteristics indicators (e.g., geographical locations, seasonality, population size, and green space) can significantly influence the impacts of the CAA on PM2.5 reduction. The diverse impacts of the CAA also suggest that the Chinese political system and urbanisation might compromise the uniform impacts of environmental policies. Future research is necessary to clarify the heterogeneous effects of the CAA on PM2.5 pollution by collecting more information on city characteristics and environmental policies.