Drivers and implications of declining fossil fuel CO 2 concentrations in Chinese cities revealed by radiocarbon measurements

China's clean air policies have successfully mitigated fossil fuel CO 2 (CO 2ff or C ff ) emissions in bottom-up inventories since 2013. Yet, evidence from top-down measurements and their underlying drivers remains limited. Here, we quantify C ff concentrations and fuel-specific contributions using atmospheric Δ( 14 CO 2 ) and δ ( 13 CO 2 ) measurements across representative Chinese cities. We found regional differences in C ff and co-emission characteristics: megacities like Guangzhou show an indicative inter-period decrease in wintertime C ff concentrations, of roughly 56 %–64 % lower in 2022 than in 2010 in afternoon-equivalent terms, while smaller cities have yet to demonstrate comparable decreases. These changes are consistent with a 23 % reduction in coal use, a 17 % increase in the natural-gas contribution (evidenced by stable isotope analysis), and improved combustion efficiency (indicated by a 63 % decline in RCO/CO2ff ratios). Notably, the 24 years observational record (1998–2022) shows steeper declines in urban RCO/CO2ff ratios than inventory estimates, suggesting current emission inventories may underestimate combustion efficiency improvements and CO emission reductions relative to C ff mitigations. These findings are consistent with progress toward mitigating C ff and co-emitted CO in major Chinese cities. They also underscore how coal-to-gas transitions and technological upgrades simultaneously advance air quality and climate goals. Importantly, our results highlight the critical need to integrate top-down observational frameworks (e.g. radiocarbon measurements) with traditional inventories to better capture rapid, policy-driven emission changes and inform future co-benefit optimization strategies.