Aqueous-Phase Photooxidation of Vanillin in the Presence of Nitrite: Characteristics, Products, and Mechanism

Abstract

The present study investigates the reaction characteristics and mechanisms involved in the aqueous-phase photooxidation of vanillin (VL) in the presence of nitrite (NO₂^–). The research entails a comprehensive analysis of the decay kinetics of VL, the composition of reaction products, and changes in absorbance under different pH conditions and VL/NO₂^– molar ratios. The results indicate a notably rapid decay rate of VL under acidic conditions, with the VL/NO₂^– molar ratio emerging as a crucial factor in the photooxidation process. Notably, the photoreaction between VL and NO₂^– leads to the formation of aqueous-phase secondary organic aerosols (aqSOA) comprising hydroxylation products, nitration products, and oligomers that exhibit strong absorption in the near-ultraviolet and visible light regions. The research findings, through theoretical calculations, shed light on the reaction pathways involved in the photooxidation process. This investigation contributes valuable insights into the atmospheric aqueous-phase photooxidation of phenolic compounds initiated by NO₂^–. The obtained results are particularly significant for understanding the formation and evolution of aqSOA and brown carbon (BrC).