Advanced Oxidation of Catechol in Atmospheric Waters

Phenols such as catechol emitted during biomass burning and combustion contribute to aqueous secondary organic aerosol (aqSOA) formation, influencing air quality and climate. Here, catechol (QH₂) is used as a surface-active, water miscible model phenol to investigate advanced oxidation in atmospheric waters. We demonstrate that exposure to ozone and Fenton's reagent (FR; Fe²⁺ + H₂O₂) accelerates catechol oxidation and promotes o-quinone formation. Ozone contributes through direct oxidation, iron redox cycling, and reactive oxygen species generation in the coupled O₃/Fenton system. Experiments were conducted with 200 μM catechol under variable conditions: (i) pH 1.0-9.0; (ii) ozone mixing ratios of 0.4-3.6 ppmv; (iii) presence/absence of FR (1 μM Fe²⁺, 50 μM H₂O₂); and (iv) dimethyl sulfoxide (DMSO) as a hydroxyl radical (HO^•) scavenger. Catechol loss was monitored by UV-visible spectroscopy at λ_max = 276 nm. Product formation was tracked by UV-visible at λ_max = 389 nm (o-quinone) and λ_max = 490 nm (catechol-Fe complex). Kinetics were quantified within an early oxidative window (0-2 min), when H₂O₂ is rapidly consumed and radical production is most active. Under representative conditions (pH 3.0; O₃(g) = 0.4 ppmv; FR present), catechol decayed with a pseudo first order rate constant k' = 1.69 × 10^-2 min^-1, substantially faster than in the absence of FR. Within the same window, o-quinone reached ∼7.5 μM. These results highlight how coupled ozone-Fenton chemistry can accelerate phenolic oxidation in atmospheric aqueous phases, affecting aqSOA aging and optical properties.