Green growth though national urban agglomeration policy? Evidence from a quasi-natural experiment

China is currently confronting the dilemma of achieving green development goals while ensuring economic growth. Urban agglomerations, as a new form of economic concentration, may serve as an effective solution to this challenge. This paper treats the State Council's approval of national urban agglomeration as a quasi-natural experiment, utilizing panel data from 213 prefecture-level cities spanning from 2004 to 2020. A multi-period Difference-in-Differences model is applied to assess the impact of National Urban Agglomerations Policy (NUAP) on Green Total Factor Productivity (GTFP). The findings indicate that the implementation of NUAP has led to an average increase in local GTFP of 0.0603. This effect is primarily driven by enhanced technological innovation capabilities, reduced misallocation of factor resources, and the "siphoning" and "spillover" effects from central cities. Heterogeneity analysis reveals that the national urban agglomerations in the expansion phase, larger agglomerations, and those located in the north exbibit more significant GTFP growth. This study provides a new perspective on GTFP research and offers theoretical and empirical support for evaluating the effects of NUAP.