Spatio-temporal distribution of Chinese cities’ air quality and the impact of high-speed rail

Chinese government has set a goal to reach carbon peak and achieve carbon neutrality, endeavoring to gradually realize net-zero carbon dioxide (CO 2 ) emission. Moreover, the new transport technology has also been fast developed, represented by the quick expansion of the high-speed rail (HSR) system in China. However, there has been very little investigation on how the HSR can affect city-level emission and urban air quality. Especially, the potential spatial spillover effect and the mechanism of such potential impact have not been examined. Based on the urban air quality index (AQI) of 286 Chinese cities over the 2016–2019 period, this study first adopts the spatial auto-correlation analysis to quantify the spatio-temporal characteristics of Chinese cities' AQI. Then, a spatial difference-in-differences (SDID) model and the spatial Dubin model (SDM) are estimated to shed light on how Chinese cities' air quality can be affected by HSR. This study identifies apparent spatio-temporal distribution patterns in Chinese cities' air quality. We found that the opening of HSR in Chinese cities significantly improves the urban air quality by reducing AQI by an average of 4%. Moreover, the spatial auto-correlation coefficient is 0.0897 and statistically significant. HSR opening in the adjacent city can also improve one city's AQI by an average of 1.3% (i.e., the neighboring effect). We also highlighted and verified the mechanism of such a positive HSR impact on the urban air quality. First, as a cleaner transport mode, HSR helps divert traffic from other more polluting modes. HSR also helps promote the city's tertiary industry, leading to fewer emissions. The heterogeneous analyses further demonstrated that HSR is more effective to improve the urban air quality in eastern region (with the AQI reduction of 5.88%) and western regions (with AQI reduction of 7.25%). • There are apparent spatio-temporal distribution patterns in Chinese urban air quality. • High-speed rail helps reduce the emissions and improve the urban air quality. • High-speed rail reduces more pollutant air travels and upgrades industrial structure. • High-speed rail opening in adjacent city can also improve one city's urban air quality. • High-speed rail impact exhibits significant spatial and heterogeneous effects.