China's high-speed rail (HSR) construction provides an ideal ‘quasi-natural experiment’ in which regional issues are investigated. This study explores how HSR affects innovation efficiency and explores the mediating effect of urban form. Based on panel data comprising 280 cities in China from 2006 to 2016, this study uses a time-varying difference-in-differences method to analyze whether HSR affects regional innovation, and it explores the mediating effect of urban form. The results show the following. (1) HSR significantly promotes regional innovation. The parallel trend test shows that the treatment group and the control group have the same trend before HSR construction. (2) Knowledge spillovers are important manifestations through which HSR promotes regional innovation. HSR can significantly improve the level of total factor productivity and human capital. (3) Urban form mediates the impact of HSR on regional innovation significantly, which varies across the different indexes of different urban forms.

Based on the panel data of 283 cities in China from 2009 to 2018, this paper analyzes the effect of urban green scientific and technological innovation enhancement on hazardous air pollutants using the GS2SLS method, which simultaneously controls for model endogeneity and spatial spillover effects and reveals the transmission mechanism of urban green scientific and technological innovation level. It was found that (1) There is a significant spatial spillover effect of hazardous air pollutants between regions, both in China as a whole and in the eastern, central, and western parts of the country, and the spatial spillover effect of hazardous air pollutants is significantly greater in the eastern and central parts of China than in the western parts. (2) Green technological innovation has a significant inhibitory effect on hazardous air pollutants in cities in eastern and central China. An extended study found that the improvement in green technology levels in innovative cities has a better effect on controlling hazardous air pollutants than in non-innovative cities. (3) The energy- saving and green economy effects have a mediating influence on the effect of green technological innovation on hazardous air pollutants in cities, and the simultaneous occurrence of these two effects in green technological innovation serves to enhance the transmission of hazardous air pollutants in order to facilitate the long-term management of haze.