Brown carbon unveiled: Molecular complexity, observational frontiers, and climate forcing

Brown carbon (BrC), a key class of light-absorbing organic aerosols, encompasses a range of organic substances with varying absorption characteristics and molecular structures, and exhibits multiscale complexity in environmental behavior and climate effects. In the past decades, studies have been conducted to obtain multi-scale knowledge of BrC employing diverse research tools. This review synthesizes the current understanding and clarifies the BrC research chains from fundamental molecular identity and characterization identified by field studies to large-scale evolution shown by remote sensing retrieval and climate impacts assessed by model simulation, highlighting the intrinsic connections. Molecular analysis and measurements reveal source-dependent molecular composition and solubility-related absorptivity, source apportionment differentiates primary and secondary sources, and atmospheric aging research indicates a dynamic photochemical equilibrium of bleaching and enhancement. Observational frontiers are being advanced by remote sensing, with algorithms enabling direct ground- and satellite-based retrieval of BrC through optical and microphysical properties. Models assess the positive radiative forcing and highlight climate impacts with the support of parameterization and process mechanisms derived from observational results. Based on the current knowledge and limitations, this review further outlines research gaps and proposes potential future development pathways. Future work necessitates integrating hyperspectral polarimetry with artificial intelligence, establishing a “space-air-ground” closed-loop observation system, and developing dynamic complex refractive index models to precisely quantify BrC’s climate weight under carbon neutrality.