The impacts of pollution sources and temperature on the light absorption of HULIS were revealed by UHPLC-HRMS/MS at the molecular structure level

Abstract. Atmospheric humic-like substances (HULIS), a key component of brown carbon (BrC), significantly promote the light absorption of aerosols. However, their linkages to pollution sources and ambient temperature in cold environments remain unresolved. Here, we analyze wintertime urban aerosol samples in Changchun, Northeast China, using ultrahigh performance liquid chromatography coupled with high-resolution tandem mass spectrometry (UHPLC-HRMS/MS). HULIS show a high light absorption efficiency (MAE₃₆₅ = 1.81 ± 0.24 m² gC^-1) and high mass concentration (2.97 ± 1.54 μgC m^-3), exceeding values reported from other global regions. Through UHPLC-HRMS/MS characterization, we identify 264 compounds at the molecular structure level, accounting for 38.2–78.1 % of the total HULIS mass. Compositional analysis demonstrates biomass burning and coal combustion are the main BrC sources during haze events. We screen out 39 strong BrC chromophores, mainly nitrophenols, that contribute 28.9 ± 10.4 % of the total light absorbance at 365 nm. Low ambient temperatures potentially enhance the accumulation of these strong BrC chromophores in the aerosol particles by suppressing photobleaching processes and altering thermodynamic reaction equilibria. These findings emphasize the potential of BrC to exert a more significant and persistent environmental effect in the cold region atmosphere.