Critical supply chains of NOx emissions in the Beijing-Tianjin-Hebei urban agglomeration

China's air pollution policies prioritize city heterogeneity, and launch the one-city-one-policy framework. Production fragmentation extends air pollution policies beyond the local scale. Therefore, air pollution needs to be controlled in coordination between cities rather than individually, considering the pollution embodied in supply chains. We illustrate the embodied NOx in supply chains and its transfer patterns in the highly polluted Beijing-Tianjin-Hebei (BTH) urban agglomeration in China, with a city-level multiregional input-output (MRIO) model in 2012. The results indicate that Tangshan and Beijing are BTH cities with the highest production- and consumption-based NOx emissions, respectively. The electricity and heat, metal, and nonmetal sectors are the main suppliers in the NOx supply chains. The critical supply chain paths largely terminate at the final demand of the construction sector in Beijing, Tianjin, Baoding and Shijiazhuang and the equipment manufacturing sector in southeastern Chinese provinces. Despite industry heterogeneity, the NOx transfer pattern between BTH cities is generally efficient, extending from low-to high-emission intensity cities, while the pattern between BTH cities and other provinces is the opposite. The results reveal the city-level emission reduction potential. The emission intensities of BTH production-oriented cities should be reduced because the transfer of polluting enterprises to southeastern coastal areas is unrealistic. Financial incentives can be offered to enterprises purchasing products from low-emission intensity enterprises. It is an effective measure to establish a capital and technical cooperation system between key supply chains node cities, such as Tangshan and its major steel product consumption regions