Unveiling metal-HULIS binding dynamics in cloud water via multispectral 2D-COS: integrating EEM-PARAFAC and FTIR insights

The interactions between metal ions and humic-like substances (HULIS) in cloud water significantly influence atmospheric metal fate, photochemistry, and cloud processes. Using EEM-PARAFAC, 2D-FTIR-COS, and hetero-2DCOS, under environmentally relevant pH conditions (pH = 6.5), this study systematically investigates complexation mechanisms of six metals (Cu(II), Hg(II), Pb(II), Fe(III), Mn(II), Zn(II)) with HULIS extracted from cloud water. EEM-PARAFAC resolves HULIS into three fluorescent fractions: low-oxygen (C1), high-oxygen (C2), and protein-like (C3). Fe(III) exhibits the strongest binding (logK ≈ 4.89–5.01), preferentially quenching C3, while Pb(II)/Hg(II) interact with C1 and Cu(II) with C2. Mn(II)/Zn(II) enhance fluorescence via alternative mechanisms. 2D-FTIR-COS reveals Fe(III)/Pb(II) target carboxyl/carbonyl groups, Cu(II)/Hg(II) aromatic moieties, and Zn(II) hydroxyl/polysaccharide functionalities. Hetero-2DCOS uncovers sequential structural changes: Fe(III), Cu(II), and Pb(II) induce fluorescence responses before C–H/O–H vibrations, whereas Hg(II) modifies C–H vibrations prior to fluorescence quenching. These findings provide novel molecular insights into metal-HULIS binding dynamics, highlighting HULIS as a critical mediator of metal solubility, transport, and photochemical reactivity in cloud systems, with implications for aerosol properties, climate feedbacks, and pollution mitigation strategies.