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

Abstract. 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 of the superposition column model in combination with TROPOMI tropospheric NO₂ column data to estimate city-scale NO_x emissions and lifetimes and their variabilities. Using the recently improved TROPOMI tropospheric NO₂ column product (v2.4–2.6), we derive daily NO_x emissions and lifetimes over the city of Wuhan for 335 clear sky days between May 2018 and December 2023. We find a slight weekend reduction in NO_x emission with a weekend-to-weekday ratio of 0.95 and a small seasonal variation of NO_x emissions over Wuhan with a summer-to-winter emission ratio of 0.87. We calculate a steady decline of NO_x emissions from 2019 to 2023, and the emission in 2023 is ~15 % below the 2019 level, indicating the success of the emission control strategy. The estimated NO_x lifetimes range from 0.8 h (summer) to 5.3 h (winter), with an average of 2.6 h. Meanwhile, our method shows ~30 % lower NO_x lifetimes for fast wind (> 7 m s⁻¹) speed. The superposition model method results in ~10 % 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 results of this work nevertheless confirm the strength of the superposition column model in estimating urban NO_x emissions with reasonable accuracy.