Spatiotemporal characteristics of nitrogen dioxide pollution in mainland China from 2015 to 2018

With the rapid industrial development and urbanisation in China, nitrogen dioxide [Formula: see text] pollution has become a severe environmental problem that threatens public health. Based on hourly concentration monitoring data of the six main air pollutants in mainland China, a space-time Bayesian hierarchy model was employed to analyse the spatiotemporal trends of the absolute and relative [Formula: see text] concentrations (i.e., the proportion of [Formula: see text] in the six main air pollutants: [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]). Both the absolute and relative [Formula: see text] concentrations were higher in the autumn and winter of each year during the study period. Four regions in particular-the North China Plain, the Yangtze River Delta, the Sichuan Basin, and the Pearl River Delta-experience the largest amounts of [Formula: see text] pollution, with a high local magnitude of more than 1.0 relative to the overall absolute and relative [Formula: see text] concentrations; this affects an area with a human population of 571.85 million, which is 42.47% of the total population. Central China (i.e., the Shaanxi-Shanxi-Henan region) and the Tarim Basin (northwest of Xinjiang) were heavily polluted by [Formula: see text] and other pollutants throughout the year, with a high local magnitude of more than 1.0 relative to the overall absolute [Formula: see text] concentration. The [Formula: see text] pollution in most of the cities in western and southern China is less serious, along with cities in the northeast. Local trends reveal that in general, cities with high [Formula: see text] pollution are accompanied by upward trends. Specifically, except for in the summer, there were about 86 cities showing the increasing trend, of which 66 cities are located in areas with higher absolute and relative [Formula: see text] concentrations. Taiyuan, for example, represents the maximal local trend, with an average annual increase of 4.39 (95% CI 1.61-7.43) [Formula: see text] and 0.43 (95% CI 0.16-0.73) %, respectively, which will lead to further increases in the population exposure-risk in heavily polluted areas.