Aqueous-Phase Photooxidation of Vanillic Acid: A Potential Source of Humic-Like Substances (HULIS)

Phenolic compounds are emitted in large amounts from biomass burning and can undergo aqueous-phase reactions in the atmosphere to form secondary organic aerosols (aqSOA). In this study, the kinetics and products of the reaction of vanillic acid (VA) with hydroxyl radicals (OH) were characterized, and their formation mechanisms were determined using ultrahigh-performance liquid chromatography ultraviolet/mass spectrometry (HPLC-UV/MS) and UV-vis absorption and fluorescence spectroscopies. The obtained rate constants of the VA + OH reaction are (9.8 ± 1.5) × 109 and (3.8 ± 0.7) × 109 M-1 s-1 at pH 2 and 10, respectively. A yellowish solution is obtained after illumination, and it absorbs in the UV-vis region and has an unusual fluorescence spectrum, which suggests formation of a humic-like substance (HULIS) by the C-C and C-O coupling of phenoxyl radicals. Additionally, hydroxylation of the aromatic ring occurs through OH addition, which increases the degree of oxidation of the products. This study indicates that the aqueous-phase OH oxidation of phenolic acid compounds may contribute to formation of HULIS in the atmosphere, especially in areas with active burning of biomass. The high-molecular-weight products would remain in the particle phase after fog/cloud evaporation and influence the chemical and optical properties of atmospheric particles.