Accelerated Photochemical Generation of Reactive Species in Microdroplets by Humic-Like Substances: Role of the Air–Water Interface and Molecular Mechanism

Atmospheric humic-like substances (HULIS) are crucial components of aerosols that absorb light and produce reactive species, influencing air quality, climate change, and human health. However, most studies on the photochemical processes of HULIS have focused on bulk solutions, neglecting microdroplets, ubiquitous in aerosols, and exhibiting distinct physicochemical properties from bulk phases. Here, we report on the photochemical production of reactive species by three HULIS representatives. The steady-state concentrations of O₂^•-, H₂O₂, ^•OH, and excited triplet states (³HS*) in microdroplets were significantly higher than those in the bulk aqueous phase. The partial solvation effect and high electric field in microdroplets could facilitate charge separation in light-excited HS, with the enhanced production of HS^•- and ³HS* species. HS^•- could increase the generation of O₂^•- and H₂O₂ and subsequent formation of ^•OH. Fluorescence excitation-emission matrix spectroscopy combined with ion cyclotron resonance mass spectrometry showed that condensed aromatics and lignin were the most active HULIS components. These findings underscore the significant contribution of photosensitization by HULIS in microdroplets to the atmospheric oxidation capacity, which can have significant implications for the conversion of aqueous secondary organic aerosols and the overall quality of the atmospheric environment.