Drivers and implications of declining fossil fuel CO₂ in Chinese cities revealed by radiocarbon measurements

Abstract. China’s clean air policies have successfully mitigated fossil fuel CO₂ (CO_2ff) emissions in bottom-up inventories since 2013. Yet, evidence from top-down measurements and their underlying drivers remains lacking. Here, we quantify CO_2ff concentrations and fuel-specific contributions using atmospheric Δ(¹⁴CO₂) and δ(¹³CO₂) measurements across representative Chinese cities. We found distinct regional trends: megacities like Guangzhou show significant CO_2ff declines (35 % decrease from 2011 to 2022) along with their source regions, while smaller cities have yet to demonstrate similar reductions. These improvements can be attributed to a 23 % coal consumption reduction, 17 % increased natural gas use (evidenced by stable isotope analysis), and improved combustion efficiency (indicated by 63 % falling R_CO/CO2ff ratios). Notably, the three-decade observational record shows steeper declines in urban R_CO/CO2ff ratios than inventory estimates, suggesting current emission inventories may underestimate combustion efficiency improvements and CO emission reductions relative to CO_2ff mitigations. These findings indicate nationwide progress toward CO_2ff emission peaks, with megacities leading the transition. 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.