Towards coordinated sustainability: A systematic study of cross-regional synergy in pollution reduction and CO2 mitigation in the Dongjiang River Basin.

With the sustained implementation of pollution control and carbon mitigation policies, cross-regional collaborative governance mechanisms have become increasingly vital to foster synergy between pollution reduction and CO₂ mitigation (PRCM). This study innovatively constructs a cross-regional comprehensive synergy index for PRCM (PRCM-CSI) in the Dongjiang River Basin, which integrates the dimensions of decoupling, coordination, and eco-efficiency. The spatiotemporal dynamics and underlying mechanisms of PRCM synergy are systematically assessed through the application of Seasonal-Trend decomposition using Loess (STL), the spatial Durbin model, and an enhanced spatial gravity model. The results reveal that: (1) The PRCM-CSI shows an overall upward trend with noticeable fluctuations (rising from 0.37 to 0.88 on average), and a significant enhancement in synergy has been observed since 2013, with all values exceeding 0.86 by 2022. Although a temporary decline occurred in 2019 due to the COVID-19 pandemic, the long-term trend remains favorable. (2) STL decomposition indicates that significant periodic fluctuations in PRCM occur, particularly during 2013-2019. Meanwhile, stochastic disturbances reflect the sensitivity of PRCM to short-term disruptions driven by exogenous shocks. (3) A "strong-core, wide-radiation" spatial synergy network has emerged, with Guangzhou, Shenzhen, and Dongguan forming the gravitational core (with gravitational intensities of 1.06, 1.39, and 1.91, respectively) and exhibiting pronounced spatial spillover effects. Meanwhile, Huizhou and Heyuan exhibit growing gravitational intensity, which contributes to a "multi-nodal support" pattern. Furthermore, the driving mechanisms of PRCM exhibit significant spatial differentiation. This study offers theoretical guidance and empirical evidence to support coordinated environmental governance across regions.