Global estimates of ambient NO₂ concentrations and long-term health effects during 2000-2019

High concentrations of ambient NO₂ causes serious air pollution and could also pose great threats to human health. However, the long-term trends (20-year) and potential health effects of ambient NO₂ exposure globally still shows high uncertainties. In this work, the field measurements, satellite dataset, GEOS-Chem output, and multiple geographical covariates were incorporated into the multi-stage model to investigate the global evolutions of ambient NO₂ during 2000-2019. The results indicated that the cross-validation (CV) R² values of ambient NO₂ based on multi-stage model displayed satisfied performance (R² = 0.78), which was superior to the individual model. Besides, the out-of-bag R² was 0.75, which suggested the multi-stage model showed the better transferability. At the spatial scale, the NO₂ concentrations followed the order of China (16.9 ± 9.0 μg/m³) > India (15.5 ± 5.6 μg/m³) > United States (10.7 ± 5.6 μg/m³) > Europe (7.7 ± 4.5 μg/m³), which was in consistent with the anthropogenic NO_x emission. At the temporal scale, the ambient NO₂ levels in China experienced persistent increases (0.29 μg/m³/year) during 2000-2013, whereas they showed slight decreases (-0.23 μg/m³/year) during 2013-2019. The ambient NO₂ levels in the United States experienced continuous decreases during 2000-2019 (-0.20 μg/m³/year), while both of India and Europe remained relatively stable. Long-term NO₂ exposure inevitably increased premature mortalities. The global premature all-cause mortalities associated with the excessive NO₂ exposure increased from 288,169 (95% CI: 43,650, 527,971) to 461,301 (95% CI: 69,973, 843,996) in the past 20 years. This study would provide sufficient policy support for future ambient NO₂ mitigation.