Substantial reductions in black carbon from both fossil fuels and biomass burning during China’s Clean Air Action

China has implemented stringent Clean Air Action (CAA) policies over the past decade, yet the effectiveness of controlling key black carbon (BC) sources—aerosols that are harmful to both the climate and health—remains unclear. Using a decade-long dual-carbon-isotope (δ13C–Δ14C) approach, this study quantitatively tracks major BC sources in South China and assesses the accuracy of bottom–up emission inventories (EIs). We find substantial reductions in BC from coal, petroleum, and biomass burning, but also reveal that biomass burning contributes 20 to 30% of the total BC, far exceeding EI estimates. These results reveal a systematic underestimation of biomass-derived BC in current inventories, underscoring the need to integrate EI models with atmospheric observations for more effective air pollution control and climate policy.