China’s successful implementation of two phases of stringent clean air actions from 2013 to 2020 (Phase I: 2013–2017; Phase II: 2018–2020) has substantially reduced PM2.5 concentration in Beijing–Tianjin–Hebei and its surrounding areas (BTHSA)—one of China’s most polluted regions. However, the specific role of regional transport in this improvement remains unclear. Here, we investigated the drivers of PM2.5 mitigation in the BTHSA and systematically quantified the contribution of regional transport during 2013–2020, by conducting multi-scenario analysis using a combination of a bottom–up emission inventory and a chemical transport model with an embedded source apportionment module. The simulated regional average PM2.5 concentration across the BTHSA declined by 56.1% from 2013 to 2020, primarily driven by anthropogenic emission control (78.1%), while the remaining 21.9% explained by meteorological variability. Within the anthropogenic impacts, reductions in local emissions, intra-regional transport, and extra-regional transport accounted for 49.0%, 32.9%, and 18.1%, respectively. Nevertheless, the relative importance of these drivers shifted, with local contributions declining while regional transport influences intensified and surpassed local abatement in Phase II, where intra-regional transport remained dominant, but the influence of extra-regional transport rose markedly to 1.4 times its Phase I level. Spatially, emission reductions in Hebei and Shandong contributed the most to the regional PM2.5 decline, representing over 50% of the transport-related improvement. Trends in regional contributions to Beijing, the core city of the BTHSA region, suggest the need for dynamically adjusting joint control policies, expanding coordinated mitigation efforts to extra-regional cities in Southern Shandong, as well as to key regions like the Yangtze River Delta. These findings underscore the growing importance of regional transport in air quality improvement and support more adaptive regional collaboration strategies moving forward.

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