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Bottom-up emission inventories of atmospheric nitrogen oxides (NOx = NO + NO2) are usually limited to annual updates and have large uncertainties. The recent launch of the Geostationary Environmental Monitoring Spectrometer (GEMS) first provides hourly measurements of trace gas pollutants from space, enabling new insights into the diurnal variations in anthropogenic NOx emissions. In this study, we present an improved top-down estimation of NOx emission using GEMS NO2 observations and characterize the hourly NOx emissions over cities in East Asia. We use the Gaussian model and polynomial fitting to calculate the hourly NO2 lifetimes for several “point-source” cities and then derive NOx emissions using the flux divergence method. GEMS observations show significant hourly variations in the NOx emissions. Systematic biases in NOx emission estimates are found between the GEMS-based hourly estimations and previous polar-orbiting satellites with a single daily overpass. Compared to using empirically diurnal emission factors, chemical model simulations using GEMS-based NOx emissions substantially reduce the biases with satellite and surface NO2 measurements (e.g., for Wuhan, the biases decreased by 31%). This study highlights the essential role of geostationary satellite observations in characterizing the emission and chemistry of atmospheric pollutants and informing emission control policies.
Xu et al. (Mon,) studied this question.