Rapid oxidation of phenolic compounds by O₃ and HO^⚫: effects of the air–water interface and mineral dust in tropospheric chemical processes

Abstract

Environmental media affect the atmospheric oxidation processes of phenolic compounds (PhCs) released from biomass burning in the troposphere. To address the gaps in experimental research, 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 mineral aerosols, in the gas phase, and in bulk water using a combination of molecular dynamics simulation and quantum chemical calculations. Of the compounds, 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 A–WPh > TiO2 VL > A–WVL > A–W4-HBA > TiO2 4-HBA > TiO2 Ph and TiO2 VL > A–WPh > A–WVL > TiO2 4-HBA > TiO2 Ph > A–W4-HBA, respectively. Some byproducts are more harmful than their parent compounds, so they should be given special attention. This work provides key evidence for the rapid oxidation observed in the O3/HO⚫ + PhC experiments at the A–W interface. More importantly, differences in the oxidation of PhCs by different environmental media due to the impact of substituent groups were also identified.