Resolving the Ultraviolet‐Visible Spectra for Brown Carbon: Insights Into Structure‐Dependent Light Absorption Variations and Its Potential Environmental Impacts

Abstract Brown carbon (BrC) is a type of light‐absorbing organic carbon and its structural characteristics have significant effects on atmospheric radiative forcing and global climate change. In this work, the main absorbing components of BrC were separated into three fractions and each fraction was analyzed individually and systematically. The compositional and structural characteristics of BrC across different wavelengths were studied, and the potential BrC compounds were revealed. Overall, nearly 100 light‐absorbing compounds were identified. The result showed that aromatic and heterocyclic compounds were main contributors to BrC absorption in the wavelength from 220 to 450 nm, which play an important role in radiative forcing. Interestingly, carbonyl groups dominated the BrC absorption in the wavelength from 200 to 220 nm. Although BrC compounds have minimal direct radiative impact due to limited solar radiation at such a wavelength, it can still play important roles in atmospheric photochemistry by participating in light‐induced oxidation process and lead to severe photochemical pollution. Additionally, carboxyl‐rich alicyclic molecules and lignin‐like substances all promoted the absorptivity of BrC, and nitrogenous substituents further enhanced such a process. These results were further extrapolated to various environmental conditions, which indicated that urban BrC probably contained more carbonyl groups. Conversely, more heterocyclic substances were observed in the forest and ocean areas, which illustrated high burden of radiative forcing in these areas. Overall, our work reveals that the BrC absorption is largely dependent on their molecule properties in different wavelengths, and such a correlation can guide and facilitate the BrC absorption analysis.