Assessment of NO₂ population exposure from 2005 to 2020 in China

Nitrogen dioxide (NO₂) is a major air pollutant with serious environmental and human health impacts. A random forest model was developed to estimate ground-level NO₂ concentrations in China at a monthly time scale based on ground-level observed NO₂ concentrations, tropospheric NO₂ column concentration data from the Ozone Monitoring Instrument (OMI), and meteorological covariates (the MAE, RMSE, and R² of the model were 4.16 µg/m³, 5.79 µg/m³, and 0.79, respectively, and the MAE, RMSE, and R² of the cross-validation were 4.3 µg/m³, 5.82 µg/m³, and 0.77, respectively). On this basis, this article analyzed the spatial and temporal variation in NO₂ population exposure in China from 2005 to 2020, which effectively filled the gap in the long-term NO₂ population exposure assessment in China. NO₂ 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 NO₂ population exposure in China shows a continuous increasing trend before 2012 and a continuous decreasing trend after 2012. The change in NO₂ population exposure in western and southern cities is more influenced by population density compared to northern cities. NO₂ pollution in China has substantially improved from 2013 to 2020, but Urumqi, Lanzhou, and Chengdu still maintain high NO₂ population exposure. In these cities, the Environmental Protection Agency (EPA) could reduce NO₂ population exposure through more monitoring instruments and limiting factory emissions.