Abstract
Humic-like substances (HULISs) are a major fraction of cloud-water organic matter that modulate aqueous reactivity and light absorption. We sampled Mt. Tai cloud water (North China Plain) in summer 2021 under land-influenced (LI) and marine-influenced (MI) air-mass trajectories, isolated HULIS by solid-phase extraction, and characterized them using ultrahigh-resolution FT-ICR MS. HULIS averaged 3.1 ± 2.1 mgC L-1 (39 ± 16% of WSOC). LI concentrations were ∼2× MI, whereas the HULIS/WSOC fraction was higher in MI. We assigned 7,104 molecular formulas with substantial overlap but distinct unique pools (1,879 LI-unique; 787 MI-unique). LI-unique formulas were enriched in mid-high-mass CHOS and reduced carbon oxidation state signatures, while MI-unique formulas featured a low-mass, highly oxygenated diacid-like pool and aromatic/unsaturated character. These results show that air-mass provenance leaves persistent molecular fingerprints in cloud-water HULIS, informing source-to-cloud processing links and cloud-phase chemistry.