Photoinduced evolution of optical properties and compositions of methoxyphenols by Fe(III)-carboxylates complexes in atmospheric aqueous phase.

The changes in optical properties and chemical compositions of methoxyphenols, which acted as an important aromatic compound from the biomass burning, were investigated in the presence of Fe(III)-carboxylates under aqueous phase conditions. The light was confirmed to be a key factor for stimulating the reaction of methoxyphenols and Fe(III)-carboxylates. The photoinduced evolution of optical properties of methoxyphenols was dependent on various factors, including irradiation intensity, types of carboxylates, dissolved oxygen and pH. The changes in the mass absorption efficiency at 306 nm (MAE306) positively relied on irradiation intensity and dissolved oxygen. The acceleration effects of carboxylates on the decreases in MAE306 of methoxyphenols followed the order of oxalate > citrate > malonate. The change amplitude of MAE306 decreased with an increasing pH (3.5-9), while that of the mass absorption efficiency at 364 nm (MAE364) increased with pH ranging from 3.5 to 7. The compositional evolutions of methoxyphenols by the photochemical aging were analyzed with the attenuated total reflection infrared spectroscopy (ATR-IR), confirming the decrease of CO groups and the increase of O-H and C-O groups. The photochemical reaction pathways of methoxyphenols with Fe(III)-carboxylates were proposed according to optical properties and compositions measurements.