Black carbon (BC) aerosols exacerbate air pollution and climate warming, but their climatic impacts and sources are poorly constrained by bottom-up emission inventories (EIs). China's Clean Air Action (CAA), which was launched in 2013, provides an excellent opportunity for investigating interannual variations in source contributions and validate the accuracy of EIs. Here, we present an 11-y (2008-2018) record of the BC concentration and its source-diagnostic radiocarbon (14C) and stable carbon isotope (13C) signatures at a receptor site in the Pearl River Delta (PRD) region, South China. The results revealed that the implementation of the CAA (2014-2018) led to a 41% reduction in the BC concentration compared with that in the preaction period (2008-2013). There is a large and systemic discrepancy over the whole period in the contribution of biomass burning to BC in South China between predictions from technology-based EIs (4 to 9%) and these source-diagnostic dual-isotopic fingerprints of actual ambient aerosols (21 to 32%). Observational constraints by source-diagnostic delta 13C/Delta 14C isotope measurements revealed that the reduction in biomass burning contributed 22% to the decrease in BC associated with the CAA, whereas predictions from EIs assigned a much smaller fraction. These results emphasize the need for observation-based source diagnostics of changing BC emission sources. Detailed source apportionment using independent delta 13C/Delta 14C isotope methodology is crucial for refining air pollution control strategies and improving the accuracy of models used for assessing the air quality and climate effects of BC in China and elsewhere.