Long-term ambient SO2 concentration and its exposure risk across China inferred from OMI observations from 2005 to 2018

Long-term trends in the ambient SO2 concentrations across China from 2005 to 2018 were estimated using Ozone Monitoring Instrument (OMI) SO2 columns and an atmospheric SO2 profile simulated by the Model for Ozone And Related Chemical Tracers (MOZART)-4. The estimated ground-level SO2 concentrations were in strong agreement with the ground observations (R = 0.86, root-mean-square error = 10.49 mu g/m(3), relative prediction error = 19%). From 2005 to 2018, the average ground-level SO2 concentration ranged from 0.97 to 106.41 mu g/m(3) across China, with higher values in eastern China than in western China. Considering the uneven distribution of SO2 concentration and population across China, the population-weighted SO2 concentration was estimated to be 20.30 mu g/m(3) on a national scale. The population-weighted SO2 levels showed decreasing trends, and the absolute values of the decreasing trend peaked in winter (1.41 mu g/m(3) per year), followed by those in autumn (0.92 mu g/m(3) per year), spring (0.74 mu g/m(3) per year) and summer (0.75 mu g/m(3) per year) from 2005 to 2018. Although the SO2 exposure risk in 2018 decreased significantly from 2005 on a national scale, 80% and 17% of people still faced a high SO2 exposure risk (higher than 20 mu g/m(3)) in the North China Plain and Northwest China, respectively.