Evolution of Black Carbon Aging Degree in China from 2013–2019 and Key Driving Factors

China has implemented stringent emission control measures, but it remains unclear how these measures affect the aging degree of black carbon (BC), a crucial factor influencing its climate and health effects. By introducing a BC aging module into the two-way coupled WRF-CMAQ model, we analyzed the evolution and drivers of BC aging degree from 2013–2019. Results show significant declines in the number fraction of thickly coated BC particles (f_coated) in eastern China, particularly in megacity clusters: the Beijing–Tianjin–Hebei region (−0.17 decade^–1), the Yangtze River Delta (−0.19 decade^–1), and the Pearl River Delta (−0.21 decade^–1). Emission reductions were identified as the predominant factor, accounting for 38–97% of the f_coated reduction. In Beijing and Shanghai, clean air actions have decreased residential contributions to BC emissions, diminishing the initial aging of BC and reducing f_coated by 35–38%. Regionally, rapid declines in precursor gas emissions have reduced BC coating materials, slowing BC aging rate and decreasing f_coated by 31–106%. Our study highlights the critical role of emission controls in mitigating BC aging in China, providing key insights for understanding the effectiveness of air quality policies and their climate and health implications.