Significant changes in the light absorption ability of BC-containing particles between the cold seasons of 2016 and 2022 in Beijing

Black carbon (BC), the most important absorbing component in ambient aerosols, profoundly influences the Earth's radiation system and climate change. Previous studies have demonstrated a decline in the mass concentrations of BC and BC-containing particles in Beijing over the past decade, primarily attributed to stringent emission control measures. In this study, we found a substantial decrease in the mass absorption cross-section (MAC) of BC-containing particles during the cold seasons in Beijing, declining from 9.27 to 7.34 m²/g over a span of seven years. Three reasons lead to the reduction of MAC. Firstly, we observed that the average mass equivalent diameter of the BC cores in 2022 was larger (239 nm) compared to that in 2016 (195 nm). Secondly, the corresponding coating thickness of the BC-containing particles decreased from 1.5 to 1.3. Lastly, significant changes were noted in the chemical compositions of BC-containing particles, with more nitrate and less organic matter (OM) in the coating materials, which led to a diminished absorption ability of BC-containing particles. Our research sheds light on the crucial role of microphysical-chemical properties in determining the absorption ability of BC-containing particles. The results indicate that it is imperative to resolve the MAC of BC in the climate system for better estimating BC’s climate effects. Besides, characterizing the current pollution levels, properties, and impacts of BC-containing particles may help reassess the importance of BC-containing particles in the ambient atmosphere and clarify future research priorities and mitigation measures.