pH-dependent chemical transformations of humic-like substances (HULIS) and further cognitions revealed by optical methods

Abstract

Humic-like substances (HULIS) are macromolecular complex groups in water-soluble organic compounds (WSOC). pH is a crucial factor influencing the chemical transformations of HULIS in atmospheric particles, but this has been rarely investigated, especially under varying pH conditions. This study attempted to unveil the chemical transformation mechanisms of HULIS under a range of pH conditions using optical methods. The pH-dependent light-absorption and fluorescence properties of HULIS were comprehensively analyzed, the acidity coefficient (pK_a) of HULIS in relation to chemical structures was determined, and the hypothetical chemical transformation mechanisms of HULIS with increasing pH were analyzed by optical characterizations. The results suggested that pH greatly impacted the light-absorption and fluorescence efficiencies of HULIS in both winter and summer seasons, and pK_a was important inflection points. The pK_a of HULIS ranged from 3.5 to 8.0 in winter, and 6.4 to 10.0 in summer. The acidic/basic groups were identified as -OH or -NH₂ substituted quinolines, carboxylic aromatics, and pyridines. The pH-sensitive species accounted for about 6% and 21% of HULIS-C (carbon concentrations of HULIS) in winter and summer, respectively. The varying optical spectra with increasing pH might be resulted from charge transfer or complex reactions with HULIS deprotonation.