A Review on Brown Carbon Aerosol in China: From Molecular Composition to Climate Impact

As an important type of light-absorbing aerosol in the atmosphere, brown carbon (BrC) is an effective driver for climate change. Field observations of BrC typically involve the extraction of filter samples (e.g., using water and methanol), followed by optical and chemical analyses. This review summarizes the concentration measurements, optical properties, and chemical characteristics of BrC in China, based on results from the extraction approach. We started with measurement techniques for the determination of BrC concentration and compared the extraction efficiencies of different solvents. Then we investigated the temporal and spatial variations of BrC’s absorption Ångström exponent (AAE) and mass absorption efficiency at 365 nm (MAE365), two parameters relevant to climate studies. AAE and MAE365 were found to be mainly influenced by the type of solvent used and BrC sources, respectively. Using the observed AAE and MAE365, BrC was demonstrated to be a non-negligible contributor to climate forcing. Finally, BrC chromophores were discussed on a molecular level, with focus on the nitrogen-containing compounds and polycyclic aromatic hydrocarbons. Despite studies on BrC concentrations measurements were widely conducted, standardized methods remain inconclusive. Regarding the optical properties of BrC, MAE365 exhibited significant temporal and spatial patterns, while AAE showed the opposite results. Through chemical characterization, BrC chromophores were identified and their association with optical properties was highlighted. This review contributes to the understanding of BrC properties and has implications for future studies on BrC.