Extensive publications have assessed the effectiveness of individual climate policies in mitigating climate change, yet evidence on their synergies and joint impacts on air pollutants and carbon emissions remains scarce. In China, the staggered implementation of the Low-Carbon Pilot Policy (LCP) and the Smart City Pilot Policy (SCP) provides a quasi-experimental setting to examine such interactions. Drawing on system innovation theory, this study posits that the interaction between the LCP and SCP generates synergy rather than trade-offs, thereby enhancing the effectiveness of each policy in reducing urban air pollution and carbon reduction. Empirical analysis based on a multi-period difference-in-differences model provides robust evidence supporting this policy synergy effect (PSE). Mechanism analysis indicates that green technological innovation and energy structure optimization are the primary pathways through which PSE operates. The results further reveal that PSE is shaped by the sequence of policy implementation, exhibits time-lagged effects, and generates spatial spillovers. Against the backdrop of increasingly complex global environmental governance, these findings provide theoretical and empirical evidence on urban-level policy synergies and offer practical insights for designing more effective policy portfolios.

policy portfolios; synergy effects; air pollution; carbon reduction; system innovation theory; difference-in-differences model