Estimating the variability of NOx emissions from the city of Wuhan with TROPOMI NO2 data during 2018 to 2023

Accurate NO_x emission estimates are required to better understand air pollution, investigate the effectiveness of emission restrictions, and develop effective emission control strategies. This study investigates and demonstrates the ability and uncertainty of the superposition column model in combination with the TROPOspheric Monitoring Instrument (TROPOMI) tropospheric NO₂ column data to estimate city-scale NO_x emissions and chemical lifetimes and their variabilities. Using the recently improved TROPOMI tropospheric NO₂ column product (v2.4–2.6), we derive daily NO_x emissions and chemical lifetimes over the city of Wuhan for 372 full-NO₂-coverage days between May 2018 and December 2023, and validate the results with bottom-up emission inventories. We find insignificant weekly cycle of NO_x emissions for Wuhan. We estimate a summer-to-winter emission ratio of 0.77, which is overestimated to some extent, though it is even higher provided by the bottom-up inventories. We calculate a steady decline of NO_x emissions from 2019 to 2023 (except for the valley in 2020 caused by the COVID-19 lockdown), indicating the success of the emission control strategy. The superposition model method results in ~15% lower estimation of NO_x emissions when the wind direction is from distinct upwind NO₂ hotspots compared to other wind directions, indicating the need to improve the approach for cities that are not relatively isolated pollution hotspots. The method tends to underestimate NO_x emissions and lifetimes when the wind speed is > 5－7 m s^-1, the estimation for Wuhan is ~4% for the emissions and ~8% for the chemical lifetime. The results of this work nevertheless confirm the strength of the superposition column model in estimating urban NO_x emissions with reasonable accuracy.