Black carbon (BC) and brown carbon (C_brown) are two types of light-absorbing carbonaceous aerosols in the atmosphere. While BC is typically black and derived from soot, C_brown is a light-absorbing organic matter that is not black, often found in aerosols from biomass burning and other sources. The presence of C_brown challenges traditional definitions of BC and elemental carbon (EC), as it can significantly bias measurements of these components. C_brown has a refractory nature and complex matrix interferences, making it difficult to measure accurately. Optical measurements of BC also face challenges, including instrument bias and the lack of a unique conversion factor between light absorption and BC or EC concentration. The spectral properties of C_brown differ from those of BC, leading to complications in determining their contributions to atmospheric light absorption. Additionally, the absorption of C_brown increases sharply in the UV, making single-wavelength measurements inadequate for assessing solar radiation absorption in the troposphere. The existence of C_brown highlights the need for revised definitions and measurement techniques to better understand atmospheric processes, including their impact on UV irradiance, photochemistry, and cloud radiative transfer. The study emphasizes the importance of distinguishing between BC and C_brown to improve the accuracy of atmospheric measurements and models.Black carbon (BC) and brown carbon (C_brown) are two types of light-absorbing carbonaceous aerosols in the atmosphere. While BC is typically black and derived from soot, C_brown is a light-absorbing organic matter that is not black, often found in aerosols from biomass burning and other sources. The presence of C_brown challenges traditional definitions of BC and elemental carbon (EC), as it can significantly bias measurements of these components. C_brown has a refractory nature and complex matrix interferences, making it difficult to measure accurately. Optical measurements of BC also face challenges, including instrument bias and the lack of a unique conversion factor between light absorption and BC or EC concentration. The spectral properties of C_brown differ from those of BC, leading to complications in determining their contributions to atmospheric light absorption. Additionally, the absorption of C_brown increases sharply in the UV, making single-wavelength measurements inadequate for assessing solar radiation absorption in the troposphere. The existence of C_brown highlights the need for revised definitions and measurement techniques to better understand atmospheric processes, including their impact on UV irradiance, photochemistry, and cloud radiative transfer. The study emphasizes the importance of distinguishing between BC and C_brown to improve the accuracy of atmospheric measurements and models.