Molecular characteristics of brown carbon aerosols over a forest atmosphere in the Qinling region of central China

Atmospheric brown carbon (BrC) aerosols were investigated at a forest site in the Qinling Mountains region of central China across the four seasons of 2021–2022. The molecular composition and optical properties of organic aerosols were characterized using an ultrahigh performance liquid chromatograph coupled with a diode array detector and an Orbitrap mass spectrometer. The light absorption of organic aerosols was relatively low in this forest environment, as revealed by their lower mass absorption coefficients at 365 nm wavelength (MAC₃₆₅, mean 0.19 ± 0.13 m² g⁻¹) compared with those of worldwide locations (approximately 0.04–2.8 m² g⁻¹). The light absorption of BrC exhibited strong seasonal variabilities, with higher MAC₃₆₅ values in fall and winter than those in spring and summer seasons. They were mostly classified as very weakly absorbing BrC, while the absorption capacities were increased with enhanced anthropogenic pollution. A total of 51 major BrC species were identified and mostly composed of CHO and CHON species (84%), accounting for 14–39% of the total absorbance of BrC at 300–450 nm. The optical properties of BrC largely depended on their chemical composition and molecular structures, with lignin-like compounds and condensed aromatics being the major BrC components. The lipid- and protein-like compounds with lower unsaturation degrees could be the potential BrC species mainly originating from biomass burning and biological emissions. This study advances our knowledge on the connections of optical properties of BrC aerosols with their molecular characteristics in anthropogenic–biogenic intersection atmosphere.