Dynamic spatial analysis of NO2 pollution over China: Satellite observations and spatial convergence models

Owing to rapid urbanization and industrialization over decades, nitrogen dioxide (NO2) has become a serious air pollutant in China. The Chinese government has proposed a series of strict strategies for the reduction of nitrogen oxides (NOx) emissions. This research studies the spatio-temporal characteristics of NO2 pollution over China from October 2004 to June 2020 based on satellite observations. The findings are as follows. Overall, NO2 vertical column densities (VCDs) first increased from 2005 to 2012, and then displayed a decreasing trend after 2013. In addition, the centroid of NO2 values over China during the period of 2006–2010 was located in north Henan and shifted towards the west, suggesting that the NO2 pollution in the eastern region decreased more rapidly than that in the western region. On the other hand, another important aim of this research is to forecast the trajectory of dynamic spatial differences of NO2 pollution. Technically, we investigated σ and β convergence of the NO2 pollution across Chinese cities. The results of the σ convergence test based on standard deviations of annual mean NO2 VCDs showed that, from 2005 to 2011, the differences in NO2 VCDs widened year by year. However, since 2013, we have witnessed a narrowing gap of NO2 pollution among different cities. We found that the σ convergence results based on the kernel density estimates revealed that the convergence of NO2 pollution across cities are heavily dependent on the improvements of the cities with high levels of NO2 concentrations. Besides, the results of the β convergence models confirmed the absolute and conditional β convergence. In addition, we found that the convergence speed in winter was faster than in summer.