While prior research has examined the impact of urban agglomerations (UAs) on particulate matter 2.5 (PM2.5) pollution, few have explored the spatial scope and the mechanism behind the effect. Thus, utilizing data collected from the Yangtze River Economic Belt (YREB) spanning the period from 2002 to 2021, and considering the construction of UAs as a quasi-natural experiment, this research applies a time-varying difference-in-differences (DID) approach to examine the effects of UAs on PM2.5 levels in China, focusing on both spatial scope and mechanism. The outcomes indicate that: (a) UAs could remarkably decrease PM2.5 by about 9%; (b) the most effective spatial scope for this mitigation effect is found to be within 50–100 km (covering Middle reaches of the Yangtze River and Cheng-Yu UAs), while for the Yangtze River Delta UA, the effective reach extends to 100–150 km; (c) mechanism analysis identifies green technology innovation partially mediates the connection between UAs and PM2.5, with industrial agglomerations further enhancing this reduction effect; (d) the effectiveness of the reduction varies according to geographic locations and urbanization levels. These findings offer practical insights for policymakers aiming to refine UA strategies and enact spatially coordinated initiatives for sustainable development.
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