The synergistic reduction effect of PM_2.5 and CO₂: evidence from national key ecological functional areas in China

China faces the dual pressure of haze pollution control and carbon emission reduction. The goals of national key ecological functional areas (NKEFAs) are to improve ecological quality and enhance ecological supply. In this paper, a time-varying difference-in-differences model is used to assess the impact of NKEFAs on PM_2.5 and CO₂ by the panel data of prefecture-level cities of China and then investigate the synergistic reduction effect. This quasi-natural experiment reveals that NKEFAs can effectively reduce both PM_2.5 and CO₂ and then achieve the synergistic emission reduction effect. Land use pattern optimization and productivity enhancement are identified as key drivers for promoting this synergistic effect. This effect is observed in NKEFAs of water conservation and soil conservation types, as well as in the northern region, middle and lower reaches of the Yangtze River, and southeast coastal areas of the southern region. This study provides valuable theoretical references and empirical insights for realizing a synergistic status of environmental improvement and low-carbon transformation.