Derivation of Emissions From Satellite‐Observed Column Amounts and Its Application to TROPOMI NO2 and CO Observations

ABSTRACT

A unified framework that connects emissions with satellite‐observed column amounts is derived from first principles. The emission information originates from the inner product of the horizontal wind and the gradient of column amount, which is more accurate than the horizontal flux divergence as used in previous studies. Additionally, the topographical and chemical effects are accounted for through fitted scale height and chemical lifetime. This framework is applied to derive NOx and CO emissions over the CONUS from TROPOspheric Monitoring Instrument NO2 and CO observations. High‐resolution (0.04°) emission mapping over the CONUS reveals unprecedented details, including CO emissions in major cities and NOx emissions from large cities, power plants, and major roadways. Monthly resolved NOx emissions show decrease and rebound after the COVID‐19 pandemic. This framework is integrated with the physical oversampling algorithm and can be readily applied to other products from the new‐generation satellite instruments.AlternatePlain Language SummarySatellites usually measure the vertically integrated column amount of atmospheric species from space. For short‐lived species like nitrogen oxides, the observed column amount indicates location and strength of emission sources. However, atmospheric dispersion smears the relationship between emission and column amount as the lifetime of species gets longer. This study directly maps emission based on the principle of mass balance. Namely, the spatial gradient of column amount should align with horizontal wind if there is an emission. Additionally, topography and chemical reaction may cause spatial gradients of column amount that are unrelated to emissions and are accounted for. Unprecedented details in the emission of air pollutants are unveiled by applying this approach to the TROPOspheric Monitoring Instrument products.