## 1 Introduction
However, despite progress at the micro level, there remains a lack of macro-level research, especially city-level panel studies, on the overall impact of GPP on urban carbon intensity, its underlying mechanisms, and spatial spillovers. This imbalance limits comprehensive policy assessments and obstructs the design of coherent decarbonization frameworks [17]. Addressing this gap through systematic city-level analyses would yield critical macro-level insights, empirically validate the policy relevance of GPP, and complement the predominantly regulatory approaches in previous research. To bridge the research gap, this study regards GPP as a demand-side environmental policy instrument that exerts multi-scalar impacts on urban carbon emissions intensity. Using panel data from 285 Chinese prefectural cities (2015-2023), we employ an integrated methodological framework that combines panel fixed-effects regression, mediation analysis, and the spatial Durbin model to systematically examine the impact of GPP on urban carbon emissions intensity. Our findings reveal three distinct mechanisms: (1) direct effects through demand-pull market expansion and regulatory-driven optimization of industrial structures and production technologies; (2) indirect effects via policy demonstration that stimulates urban green innovation, enhances corporate sustainability performance, and strengthens public environmental awareness; and (3) spatial spillover effects via inter-regional policy diffusion, knowledge dissemination, and technology transfer that collectively foster carbon reduction networks. These results demonstrate the systemic decarbonization potential of GPP through integrated direct, indirect, and spatial channels. The contribution of this study is fourfold. First, our work substantially extends the analytical framework in carbon reduction from conventional regulatory approaches to market-based mechanisms, with a focus on GPP. The distinctive features of GPP--such as voluntary participation, market incentives, and social demonstration effects--complement conventional regulatory methods, thereby offering a more integrated view of environmental policy systems and their operational dynamics. Second, this study adopts a socio-technical systems perspective to explore how GPP influences urban carbon intensity through both technological and social pathways. Addressing the fragmented and micro-level focus of current studies offers a more integrated framework for understanding the broader impact of the policy. Third, this study identifies and verifies the spatial spillover effects of GPP using spatial Durbin models, thus providing robust empirical evidence for cross-regional collaborative governance strategies in carbon reduction. Fourth, to the best of our knowledge, our study is the first to integrate micro-level GPP contract data with city-level datasets. By matching 167,062 GPP contract records with urban data from 285 Chinese cities, we construct a comprehensive research database that will serve as a valuable resource for future empirical studies in this field. The remainder of this paper is organized as follows: Section 2 introduces the background. Section 3 develops the theoretical framework and proposes the research hypotheses. Section 4 describes the variables, data sources, and methodology. Section 5 presents the results. Finally, Section 6 presents conclusions, put forward policy implications and prospects for future research.