Mapping high resolution national daily NO2 exposure across mainland China using an ensemble algorithm.

Nitrogen dioxide (NO2) is an important air pollutant and highly related to air quality, short- and long-term health effects, and even climate. A national model was developed using the extreme gradient boosting algorithm with high-resolution tropospheric vertical column NO2 densities from the Sentinel-5 Precursor/Tropospheric Monitoring Instrument and general meteorological variables as input to generate daily mean surface NO2 concentrations across mainland China. Model-derived daily NO2 estimates were high accuracy with sample-based cross-validation coefficient of determination of 0.83, a root-mean-square error of 7.58 μg/m3, a mean prediction error of 5.56 μg/m3, and a mean relative prediction error of 18.08%. It has good performance in NO2 estimations at both regional and individual site scale. The model also performed well in terms of estimating monthly, seasonal, and annual mean NO2 concentrations across China. The model performance appears to better than or comparable to most previous related studies. The seasonal and annual spatial distributions of surface NO2 across China and several regional NO2 hotspots in 2019 were derived from the model and analyzed. Also evaluated were the population exposure levels of NO2 for cities in and provinces of China. At the national scale, about 12% of the population experienced annual mean NO2 concentrations exceeding the Chinese national air quality standard. The nationwide model with conventional predictors developed here can derive high-resolution surface NO2 concentrations across China routinely, benefitting air epidemiological and environmental related studies.