Urban and Power Plant NO_x Emissions in Sub‐Saharan Africa Inferred From TROPOMI

Nitrogen oxides (NO_x) emissions are increasing rapidly in Sub-Saharan Africa, affecting local air quality. Outside South Africa, most hotspots are relatively small, posing a challenge for traditional top-down inversions. We tailor an existing top-down wind rotation and Gaussian plume fit inversion to suit the relatively small NO_x hotspots for most of Sub-Saharan Africa. We apply the customized inversion to 3 years (July 2018–December 2019 and January 2021–June 2022) of nitrogen dioxide (NO₂) observations from the Tropospheric Monitoring Instrument (TROPOMI) to derive annual NO_x emissions for 24 isolated hotspots (21 urban, three power plants) compared to at most 5 for Sub-Saharan Africa in past global studies. Annual hotspot emissions total 311.5 kt NO. Urban hotspot emissions range from <2 mol s⁻¹ for Antananarivo, Madagascar, to 27.7 ± 12.6 mol s⁻¹ for the megacity Lagos in Nigeria. Coal-fired power plant emissions are 2.7 ± 1.0 mol s⁻¹ for Hwange, Zimbabwe, and similar (∼70 mol s⁻¹) for Lethabo and combined Matimba and Medupi plumes in South Africa. Top-down estimates are 8%–20% less than Continuous Emissions Monitoring Systems emissions. We conduct a quasi-independent evaluation of urban top-down emissions by assessing improved agreement between the GEOS-Chem model and TROPOMI NO₂ after updating modeled emissions to match the top-down estimates. Annual urban inventory hotspot emissions decline by 43 kt NO and the model root mean squared error is more than halves from 1.2 × 10¹⁵ molecules cm⁻² to 0.48 × 10¹⁵ molecules cm⁻². Our top-down emissions exhibit large, up to six-fold, systematic differences with contemporary global and regional inventories.