Estimating high-resolution top-down nitrogen oxides emissions for improving air quality and public health in China

Nitrogen oxides (NO_x) are key air pollutants that directly affect health and are precursors of health-harming fine particulate matter (PM_2.5) and ozone. China has implemented stringent emission control measures over the past decades, leading to significant declines in air pollution. To further improve air quality and protect public health, more precise and timely tools are needed for pinpointing emission sources and assessing changes in emissions. While bottom-up emission inventories are essential, they often suffer from reporting lags, coarse resolutions, and potential systematic biases. Here we use satellite NO₂ observations from the TROPOspheric Monitoring Instrument (TROPOMI), ERA5 meteorological reanalysis products and a divergence-flux method to derive high-resolution (0.025° × 0.025°) NO_x emissions across China in 2019-2024. We independently evaluate bottom-up emission inventories, identify emission sources, and report on emission trends in China. Our top-down estimates show good spatial correlations (r ≥ 0.7) with widely-used national and global bottom-up inventories, but there is a systematic underestimation in bottom-up emissions (~60% at the city level and ~70% at the provincial level). Top-down emission estimates effectively pinpoint large point sources (e.g., industrial clusters and ports) that are either missing or underrepresented in bottom-up emission invntories. The 2019-2024 trend analysis shows a significant decline in NO_x emissions across many of China's populated and industrialized regions, despite increases in some rapidly developing areas. Currently underway is the use of our top-down emission estimates to improve air pollution modeling and enable more accurate health burden assessments in China. Our results will also enhance the understanding of regional air quality and atmospheric chemistry.