Characterizing optical properties of HULIS versus WSOC in cloud water of Eastern China - Insights into secondary formation in cloud processes

Effects of humic like substances (HULIS) on cloud condensation nuclei, initiation of ice, and global radiative forcing of clouds highlight their significant influence on climate dynamics. However, optical properties of HULIS in cloud water and their indications remain poorly understood. Cloud water samples were gathered from June through August 2021 at Mt. Tai, China, and HULIS and water-soluble organic carbon (WSOC) within these samples were quantitatively analyzed. The optical characteristics of these substances were examined using UV–Vis and fluorescence spectroscopy. The average concentration of HULIS was 3.14 ± 2.14 mgC L⁻¹ contributing 38 ± 15 wt% to WSOC (9.42 ± 7.50 mgC L⁻¹). A mass absorption efficiency value at 365 nm (MAE₃₆₅) of 0.72 m² g⁻¹ and an absorption Ångström exponent (AAE_300–400) value of 3.79 were measured for HULIS compared to 0.54 m² g⁻¹ and 3.58 for WSOC. Three fluorescent components, i.e., less oxygenated HULIS (LO-HULIS), highly oxygenated HULIS (HO-HULIS), and protein-like substances (PRLIS), were identified in both HULIS and WSOC employing EEM and parallel factor analysis. HULIS light absorption was dominated by HO-HULIS, followed by that of LO-HULIS and PRLIS. Compared to WSOC, HULIS exhibited enhanced light absorption and a higher degree of humification, attributed to its elevated levels of HO-HULIS and LO-HULIS alongside reduced PRLIS. Furthermore, the degradation of PRLIS was posited as a potential pathway for LO-HULIS formation based on the increasing trend in the ratio of LO-HULIS:PRLIS with decreasing PRLIS in WSOC. This study explores secondary HULIS formation in cloud water, advancing our understanding of HULIS evolution.