Quantifying interprovincial NO₂ transport over the Yangtze River Delta: Dynamics and drivers.

Structural transitions in the energy and industrial sectors are reshaping air quality in the Yangtze River Delta (YRD). Interprovincial NO₂ fluxes over the YRD are quantified using a closed-loop integration method that combines TROPOMI, ERA5, and GEOS-CF datasets. Regional NO₂ column concentrations show a marked decline of (-3.16 ± 1.08) %/yr, with the steepest decreases in Anhui and Jiangsu. Flux analysis reveals weakening trends in both overall and interprovincial NO₂ transport in the YRD. The western segment shows a net influx of 3.29 kg/s, while the eastern segment exhibits a dominant net efflux of 15.25 kg/s. Interprovincial exchanges are also notable, with mean fluxes of 0.68 kg/s, 0.37 kg/s, and 0.20 kg/s for Jiangsu-to-Anhui, Anhui-to-Zhejiang, and Jiangsu-to-Zhejiang, respectively; of which, the Jiangsu-to-Anhui flux shows the fastest decline ((-5.87 ± 1.79 ) %/yr), forming a progressively weakening northwest-to-southeast transport chain. Surface NO₂ concentrations in eight major cities show steady declining trends ((-3.54 ± 0.45) %/yr to (-6.1 ± 0.31) %/yr), with Suzhou and Nanjing exhibiting the steepest declines. The synchronous decline of surface and column NO₂ concentrations, along with transport fluxes, highlights the potential of structural emission mitigation to deliver air-quality benefits across administrative boundaries via regional atmospheric circulation. Our findings underscore that achieving sustainable improvements in air quality in megaregions such as the YRD requires coordinated structural transitions, where the synergy between reforms in the industrial and energy sectors ensures that emission reductions translate into tangible decreases in both local burdens and cross-boundary pollutant fluxes.