Aqueous Photosensitization of Syringaldehyde: Reactivity, Effects of Environmental Factors, and Formation of Brown Carbon Products

Phenolic aldehydes as brown carbon (BrC) chromophores may contribute to the mass of aqueous secondary organic aerosol (aqSOA) due to their potential as atmospheric photosensitizers. However, there is still a lack of knowledge about their sensitizing ability and the impact of environmental factors. In this work, we studied the photosensitized behavior of a phenolic aldehyde, syringaldehyde (SyrAld), in aqueous solutions. Under illumination, the influences of environmental factors such as precursor concentration, solution pH, codissolved inorganic constituents (NaCl and Na2SO4), and organic matter (vanillyl alcohol, VAL, a methoxyphenol produced during biomass burning) were investigated. Our results show that increasing pH and salt concentration causes a strong red shift in the absorption peak of SyrAld, and chloride salts and sulfate salts exert different effects on the photochemical reactivity of SyrAld. Interestingly, the opposite effects of SyrAld on VAL oxidation were observed at different wavelengths of light. Under UV-B irradiation, SyrAld inhibited VAL degradation by the light shielding effect, while under UV-A irradiation, photosensitization of SyrAld promoted VAL degradation. The major photooxidation products were identified as hydroxylated products induced by reactive oxygen species (OH radicals) and dimerized products by direct oxidation of triplet excited state SyrAld (3SyrAld*) using UPLC-Q-TOF-MS. This work suggests that environmental factors play significant roles in determining the fate of phenolic aldehydes in atmospheric waters.