Understanding the interaction of pollution control and carbon mitigation is crucial for addressing both air pollution and climate change This paper investigates the carbon-mitigation spillovers of pollution control policies and the underlying firm-level reduction strategies, leveraging prefecture-level variations in policy intensity under China's Eleventh Five-Year Plan. To address endogeneity concerns, the critical sulfur loads—an exogenous-given measure of environmental capacity—are employed as an instrumental variable. The results suggest that stricter SO2 emission constraints generate significant carbon-mitigation spillovers, primarily through firms' source control strategies. These spillovers exhibit a non-linear pattern, intensifying as emission constraints become more stringent. By offering a micro-level and dynamic perspective, the study contributes to the literature on the indirect climate benefits of pollution control and underscores the value of integrating these co-benefits into both local policy design and international climate frameworks.