Photochemical Aging of Guaiacol by Fe(III)-Oxalate Complexes in Atmospheric Aqueous Phase

Fe(III)-oxalate complexes are likely abundant in clouds, fogs and aerosol water. They are photoreactive and can act as an important source of reactive oxygen species (^·OH, H₂O₂ and HO₂·) in tropospheric aqueous phases. Although the mechanisms involved in ferrioxalate photolysis have been investigated extensively, few kinetic and mechanistic information is available on the aging of dissolved organic compounds by this photochemical system. In this work, the Fe(III)-oxalate mediated photooxidation of guaiacol (GUA), a model for phenolic compounds emitted from biomass burning, was investigated under typical pH conditions of the atmospheric water. The effect of Fe(III) concentration, oxalate concentration and pH on the photooxidation of GUA was studied in detail. Our results revealed that oxalate can inhibit the oxidation of GUA by Fe(III) under the dark condition. However, the iron-catalyzed photooxidation of GUA can be strongly promoted in the presence of oxalate due to the formation of photoactive Fe(III)-oxalate complexes. GUA was rapidly oxidized to form a number of polymeric, functionalized and open-ring products with low volatility. Detailed reaction pathways for the photooxidation of GUA by Fe(III)-oxalate complexes were proposed based on the results of high-resolution mass spectrometry. This work suggests that ferrioxalate photochemistry can play an important role in the transformation of dissolved organics in atmospheric aqueous phases.