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

Abstract. China's clean air policies have successfully mitigated fossil fuel CO2 (CO2ff or Cff) emissions in bottom-up inventories since 2013. Yet, evidence from top-down measurements and their underlying drivers remains limited. Here, we quantify Cff concentrations and fuel-specific contributions using atmospheric Δ(14CO2) and δ(13CO2) measurements across representative Chinese cities. We found regional differences in Cff and co-emission characteristics: megacities like Guangzhou show an indicative inter-period decrease in wintertime Cff 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 Cff mitigations. These findings are consistent with progress toward mitigating Cff 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.