Abstract: Social capital, functioning as an informal institution, wields an invisible force to influence the efficacy of the formal environmental governance systems, thereby exerting a profound impact on the process of carbon reduction and pollution mitigation. This paper conducts an empirical analysis of 92 sample cities within key urban agglomerations. The findings reveal that the emissions trading, whether implemented solely on carbon emissions trading or in conjunction with pollution reduction trading, can exert significant impact on the synergy between carbon reduction and pollution mitigation in key urban agglomerations. The moderation effect test results demonstrate that the regulation of social capital on the synergy of carbon reduction and pollution reduction through emissions trading exhibits non-linear characteristics, and the regulatory effect is more pronounced in the sample city group with higher economic development level. Further in-depth analysis indicates that the adoption of the “mutual benefit-mutual interoperability-mutual trust”strategy by local governments can promote the regulatory role of social capital in enhancing carbon reduction and pollution reduction synergy through emissions trading, from weak to strong. Based on this, it is necessary to strengthen the supervision and evaluation of the implementation of the joint policies on carbon emission rights and pollution discharge rights trading in key urban agglomerations, and further improve the basic institutional framework for carbon emission trading and the system for paid use of pollution discharge rights; accurately identify the economic and social development stages and the social capital evolution stages of different regions, and design differentiated carbon reduction and pollution reduction collaborative governance plans according to local conditions; while accelerating the cultivation and accumulation of social capital in underdeveloped cities, continuously optimize the strategic choices of local governments to promote collaborative governance of carbon reduction and pollution reduction.

Key words: social capital, carbon and pollution reduction, regulating function, key urban agglomerations