Beyond Single-Pollutant and City-Bounded Governance: Differentiated PM2.5–O3 Responses, Spatial Spillovers, and Sustainable Regional Air-Quality Governance in China’s “2 + 26” Cities

Sustainable air-quality governance requires not only local emission reduction but also a shift from single-pollutant control to coordinated PM2.5–O3 control, and from city-bounded management to regional governance under spatial spillovers. Based on balanced annual city-level panel data for the “2 + 26” urban agglomeration in the Beijing–Tianjin–Hebei region and surrounding areas from 2013 to 2020, this paper uses the dynamic Spatial Durbin Model (SDM) to analyze the spatial spillover effect of PM2.5 and O3 pollution and the effect of regional governance policies. The results show that both PM2.5 and O3 exhibit significant spatial autocorrelation and cross-city dependence, indicating that isolated local control measures are insufficient for sustainable air pollution prevention and that city-bounded governance cannot fully address regionally connected pollution risks. Economic output and secondary-industry employment remain important structural factors of pollution. The policy-text analysis shows that measures centered on coal-related control and industrial governance were more directly aligned with PM2.5 reduction, whereas O3-related governance lagged, suggesting that single-pollutant-oriented control may generate a sustainability trade-off when PM2.5 reduction is not accompanied by coordinated O3 control. These findings highlight two sustainability challenges in China’s regional air-quality governance: first, single-pollutant control can improve particulate pollution but may not ensure sustainable air-quality improvement when O3 and its precursors are insufficiently addressed; second, isolated city-level governance may be insufficient when pollution outcomes exhibit significant spatial dependence across administrative boundaries. The study provides empirical evidence for sustainable air-quality governance by emphasizing differentiated PM2.5 and O3 responses, coordinated PM2.5–O3 control, regional governance beyond individual city boundaries, and the integration of spatial spillover assessment into regional environmental policy design.