Measurement report: Effects of transition metal ions on the optical properties of humic-like substances (HULIS) reveal a structural preference – a case study of PM2.5 in Beijing, China

Abstract. Humic-like substances (HULIS) are complex macromolecules in water-soluble organic compounds (WSOCs) containing multiple functional groups, and transition metal ions (TMs) are ubiquitous in atmospheric particles. In this study, potential physical and chemical interactions between HULIS and four TM species, including Cu2+, Mn2+, Ni2+, and Zn2+, were analyzed by optical method under acidic, weakly acidic, and neutral conditions. The results showed that Cu2+, Mn2+, and Zn2+ only slightly enhanced mass absorption efficiency (MAE365) of HULIS in winter and had indiscernible effects on the absorption Ångström exponent (AAE) of HULIS in both seasons under all acidity conditions. All four TMs had fluorescence quenching effects on winter HULIS, and only Cu2+ had similar effects on summer HULIS, with the highest quenching coefficients found under weakly acidic conditions in both seasons. The 1H-nuclear magnetic resonance (1H-NMR) and Fourier-transform infrared (FTIR) spectra revealed that Cu2+ mainly bound with aromatic species and tightened the molecule structures of HULIS. The parallel factor analysis (PARAFAC) results extracted four components of HULIS, including low-oxidized humic-like substances (C1), N-containing compounds (C2), highly oxidized humic-like substances (C3), and the mixing residuals (C4), from the fluorescence spectra in both winter and summer. The spectral characteristic of HULIS with Cu2+ additions under three acidity conditions indicated that electron-donating groups of HULIS mainly corresponded to C1 and C3, with Cu2+ binding with HULIS by replacing protons, while electron-withdrawing groups of HULIS could correspond to C2, with its connection with Cu2+ through electrostatic adsorption or colliding-induced energy transfer.