*6.2. Policy Implications*

This paper has the following three policy implications:

First, this paper finds that the city cluster will significantly reduce city carbon emissions. Therefore, the government should adopt a more diversified approach to air control. It can not only reduce air pollution through environmental regulation but also reduce carbon emissions by setting up the city cluster through city spatial layout. Policymakers should actively adhere to the city cluster model. They should not only continue to vigorously promote the development of mature city clusters in the Yangtze River Delta, Pearl River Delta, and Beijing–Tianjin–Hebei region, but also strengthen the concentration of city clusters in the Yangtze River, Chengdu–Chongqing, and Central Plains.

Second, based on the mechanism analysis, the city cluster can reduce city carbon emissions by improving productivity, improving innovation, and optimizing industrial structure. Therefore, the government can take "industrial transfer and innovation drive" as an opportunity to actively promote the transfer of traditional industries from core cities (or big cities) to non-core cities (or small and medium-sized cities). It can improve the efficiency of the utilization of production factors in the city cluster through specialization and industrial upgrading. At the same time, guide the core cities to build innovation systems. The government can achieve the goal of carbon emission reduction by optimizing the industrial structure of the city.

Third, based on the heterogeneity analysis, only the national-level city cluster can achieve the purpose of city emission reduction. Therefore, the government should set up more nation-level city clusters, rather than regional-level or world-level city clusters. It needs to further improve the resource support capacity of the national-level city cluster and promote the transformation and upgrading of enterprises through the introduction of talents and financial subsidies.
