Estimating Hourly Nitrogen Oxide Emissions Across Asia Using Data from the GEMS Geostationary Satellite

Conventional bottom-up emission inventories for atmospheric pollutants suffer from infrequent updates and substantial uncertainties. The Geostationary Environment Monitoring Spectrometer (GEMS) now provides columnar measurements for key atmospheric pollutants, including tropospheric O₃, aerosols, and their precursors (NO₂, SO₂, HCHO, and glyoxal), on an hourly basis throughout the sunlit day, with a nominal spatial resolution of a few kilometres. These satellite data represent new constraints to determine top-down estimates of air pollutant emissions, providing complementary information to the bottom-up inventories. Collectively, bottom-up and top-down information provide better actionable information to develop more effective air pollution mitigation strategies. To demonstrate this, we infer emissions of nitrogen oxides (NOx=NO+NO₂) across Asia from GEMS column observations of NO₂ by using the adjoint of GEOS-Chem atmospheric chemical transport model. We explore diurnal variations in NOx emissions across diverse Asian cities, assessing their implications for emission policy formulation. Additionally, we conduct a critical evaluation of our top-down estimates of NOx emissions by comparing model simulations of NO₂, driven by these estimates, with independent observations of NO₂ throughout the region.