Measurement report: Rapid oxidation of phenolic compounds by O3 and HO•: effects of air-water interface and mineral dust in tropospheric chemical processes.

Environmental media affect the atmospheric oxidation processes of phenolic compounds (PhCs) released from biomass burning in the troposphere. Phenol (Ph), 4-hydroxybenzaldehyde (4-HBA), and vanillin (VL) are chosen as model compounds to investigate their reaction mechanism and kinetics at the air-water (A-W) interface, on TiO₂ clusters, in the gas phase, and in bulk water using a combination of molecular dynamics simulation and quantum chemical calculations. Of them, Ph was the most reactive one. The occurrence percentages of Ph, 4-HBA, and VL staying at the A-W interface are ~72 %, ~68 %, and ~73 %, respectively. As the size of (TiO₂)_n clusters increases, the adsorption capacity decreases until n > 4, and beyond this, the capacity remains stable. A-W interface and TiO₂ clusters facilitate Ph and VL reactions initiated by the O₃ and HO^•, respectively. However, oxidation reactions of 4-HBA are little affected by environmental media because of its electron-withdrawing group. The O₃- and HO•-initiated reaction rate constant (k) values follow the order of Ph_A-W > VL_TiO2 > VL_A-W > 4-HBA_A-W > 4-HBA_TiO2 > Ph_TiO2 and VL_TiO2 > Ph_A-W > VL_A-W > 4-HBA_TiO2 > Ph_TiO2 > 4-HBA_A-W, respectively. Some byproducts are more harmful than their parent compounds, so should be given special attention. This work provides key evidence for the rapid oxidation observed in the O₃/HO^• + PhCs experiments at the A-W interface. More importantly, differences in oxidation of PhCs by different environmental media due to the impact of substituent groups were also identified. [ABSTRACT FROM AUTHOR]