Water-soluble matter mediated phototransformation of guaiacol leading to formation of aqueous secondary organic aerosol: Mechanism and kinetics

The mechanisms and kinetics of the formation of aqueous secondary organic aerosol (aqSOA) via photosensitive water-soluble matters (WSM) mediated photo-aging process of dissolved methoxyphenols, using guaiacol (GUA) as a model compound, in sunlit atmospheric aqueous phase were investigated. The occurrences and optical indices of typical photosensitive WSM, including both water-soluble inorganic matter (WSIM) and water-soluble organic matter (WSOM), in aqueous aerosol extracts were characterized and determined by using a variety of instrumental techniques. The results shown that various photosensitive WSIM and WSOM occur in atmospheric aqueous aerosol. The photolysis of WSM can generate abundant reactive oxygen species (ROS) and ³WSOM*, which play equally important roles in entire GUA transformation process leading to aqSOA formation. Of those, the ³WSOM* is the predominant contributor for initial GUA transformation due to its high [³WSOM*]_ss of 5.35 ± 0.12 × 10⁻¹³ mol/L, generation rate of 3.43 × 10⁻⁷ mol/(L·s), quantum yield of 4.86 %, and the rate constant of reaction with GUA over ∼10⁹ L/(mol·s). Meanwhile, the ROS are key reactive species reacting with soluble intermediates of GUA transformation to produce final insoluble substances such as ether and low-volatile phenols, which are major constituents of formed aqSOA in aqueous extracts of aerosols. This work sheds the deeper insights into the mechanisms and kinetics on aqSOA formation via indirect phototransformation of dissolved methoxyphenols in real atmospheric aqueous phase.