This study presents the first multi-city observational assessment of fossil-fuel CO₂ emissions in major and mid-sized Chinese cities using Δ(¹⁴C) and δ(¹³C). The measurements reveal substantial declines in fossil-fuel CO₂ concentrations, consistent with documented reductions in coal consumption, increased reliance on natural gas, and notable improvements in combustion efficiency. However, the observed decreases are larger than those reported by bottom‑up emission inventories, suggesting that current inventories may systematically underestimate real-world CO₂ mitigation. These findings hold important policy implications for evaluating the effectiveness of China’s clean‑air measures and for advancing accurate urban emissions monitoring.