How Do Digitalization and Greening Promote the Synergy of Pollution and Carbon Emission Reductions? Evidence from the Dual-Policy Pilots of Key Air Pollution Control Zones and Broadband China

Coordinating air pollution control with carbon mitigation is a central challenge for urban sustainability. However, there is currently a lack of sufficient research on whether digital infrastructure can complement environmental regulations to enhance the synergy of pollution and carbon emission reduction (SPCER). Using the overlapping implementation of China’s Key Air Pollution Control Zones (KAPCZs) and Broadband China (BC) policies as an empirical policy setting, this study applies a partially linear Double Machine Learning framework to panel data for 282 Chinese cities from 2006 to 2023. The results show that the dual-policy pilots significantly improve urban SPCER, while a unified interaction test further provides statistical evidence of a positive synergistic effect between the two policies. Mediation analyses provide evidence consistent with Green Technological Innovation, industrial structure upgrading, and green finance development as potential transmission mechanisms. Formal cross-group tests reveal significant heterogeneity across regions and city characteristics, with larger effects in highly urbanized, low industrial development, non-old industrial base, central, and non-resource-based cities. Spatial Durbin Model estimates further indicate positive spillovers to neighboring cities. These findings demonstrate the potential value of coordinating environmental regulation with digital infrastructure and provide evidence for differentiated policy design and cross-regional collaboration in urban digital–green transitions.