Assessment of NO population exposure from 2005 to 2020 in China.

Nitrogen dioxide (NO) is a major air pollutant with serious environmental and human health impacts. A random forest model was developed to estimate ground-level NO concentrations in China at a monthly time scale based on ground-level observed NO concentrations, tropospheric NO column concentration data from the Ozone Monitoring Instrument (OMI), and meteorological covariates (the MAE, RMSE, and R of the model were 4.16 µg/m, 5.79 µg/m, and 0.79, respectively, and the MAE, RMSE, and R of the cross-validation were 4.3 µg/m, 5.82 µg/m, and 0.77, respectively). On this basis, this article analyzed the spatial and temporal variation in NO population exposure in China from 2005 to 2020, which effectively filled the gap in the long-term NO population exposure assessment in China. NO population exposure over China has significant spatial aggregation, with high values mainly distributed in large urban clusters in the north, east, south, and provincial capitals in the west. The NO population exposure in China shows a continuous increasing trend before 2012 and a continuous decreasing trend after 2012. The change in NO population exposure in western and southern cities is more influenced by population density compared to northern cities. NO pollution in China has substantially improved from 2013 to 2020, but Urumqi, Lanzhou, and Chengdu still maintain high NO population exposure. In these cities, the Environmental Protection Agency (EPA) could reduce NO population exposure through more monitoring instruments and limiting factory emissions.