Characterization of atmospheric humic-like substances (HULIS) at a high elevation in North China: Abundance, molecular composition and optical properties

The optical absorption of large molecular compounds HULIS (humic-like substances) can significantly impact the aerosol light absorption and radiative forcing, influencing cloud condensation nuclei formation and thus the climate and atmospheric environment. This study collected aerosol (PM_2.5) samples from the summit of Mount Tai in North China to investigate the concentration, molecular composition, and optical properties of HULIS. The average concentration of HULIS in the PM_2.5 in this study was 1.26 ± 0.54 µg/m³, comprising for 56 % of the water-soluble organic carbon (WSOC), with levels lower than urban areas but higher than other mountainous regions. Mass spectrometry revealed that CHO and CHON components, with high aromaticity and phenolic groups, are major contributors to absorption and fluorescence. These results indicate that HULIS is mainly composed of lignin and proteins/amino sugars, derived from combustion and secondary formation, and possesses a high light absorption capacity (with MAE₃₆₅ (mass absorption efficiency) and AAE (Ångström exponent) indices of 0.62 m²/g and 4.99, respectively). Parallel factor analysis identified three fluorescence components of HULIS, with proportions of 60.8 % for less oxygen humic-like substances, 21.0 % for high oxygen humic-like substances, and 18.2 % for protein-like substances. Our study highlights the significance of the light-absorbing capacity and secondary formation of HULIS at Mount Tai, laying the groundwork for investigation into the climate effects, formation mechanisms, and sources of HULIS generation.