Can a Policy Mix Achieve a Collaborative Effect? Exploring the Nested Implementation Process of Urban Carbon Emission Reduction Policies

China's government has adopted several carbon emission reduction policies to strive to achieve the dual carbon goal of "carbon peaking and carbon neutrality". In practice, various multi-objective policy instruments are implemented in a staggered manner, which might lead to the overestimation or underestimation of single-policy effect evaluation. This paper evaluates the combined effect of the Low-carbon City Pilot (LCP) and Comprehensive Demonstration City of Energy Saving and Emission Reduction Fiscal Policy (CCEEFP), investigating whether their carbon emission reduction effects are complementary and achieve collaborative outcomes. The empirical results indicate that the LCP, the CCEEFP, and their nested implementation could all promote carbon emission reduction. Their collaborative policy effects are sustained, being more obvious in higher-grade cities. Furthermore, there is a greater adoption of the source treatment technology for carbon emissions. And the implementing order of these policies could affect governance performance because of the "path dependence" of local government's attention towards multi-objective policy intentions. The empirical evidence demonstrates that policymakers should carefully design policy mix particulars towards a common purpose and carefully co-ordinate their implementation process.