The article by M. O. Andreae and A. Gelencsér discusses the nature of light-absorbing carbonaceous aerosols, focusing on the recent discovery of brown carbon (C_brown) in the atmosphere. They argue that the presence of C_brown can significantly bias measurements of black carbon (BC) and elemental carbon (EC), particularly in heavily polluted regions. The authors highlight several issues with current measurement techniques, including the lack of a unique conversion factor between light absorption and carbon concentration, the complex matrix interferences, and the spectral properties of different types of light-absorbing carbon. They also suggest that single-wavelength light absorption measurements may not be sufficient for assessing solar radiation absorption in the troposphere due to the increasing absorption of C_brown in the UV range. The article reviews the background on the study of BC, the evolution of terminology, and the challenges in accurately measuring BC and EC. It emphasizes the need for a comprehensive understanding of the physical and chemical properties of C_brown to improve the accuracy of atmospheric light absorption measurements and their implications for climate models.The article by M. O. Andreae and A. Gelencsér discusses the nature of light-absorbing carbonaceous aerosols, focusing on the recent discovery of brown carbon (C_brown) in the atmosphere. They argue that the presence of C_brown can significantly bias measurements of black carbon (BC) and elemental carbon (EC), particularly in heavily polluted regions. The authors highlight several issues with current measurement techniques, including the lack of a unique conversion factor between light absorption and carbon concentration, the complex matrix interferences, and the spectral properties of different types of light-absorbing carbon. They also suggest that single-wavelength light absorption measurements may not be sufficient for assessing solar radiation absorption in the troposphere due to the increasing absorption of C_brown in the UV range. The article reviews the background on the study of BC, the evolution of terminology, and the challenges in accurately measuring BC and EC. It emphasizes the need for a comprehensive understanding of the physical and chemical properties of C_brown to improve the accuracy of atmospheric light absorption measurements and their implications for climate models.