Acidity of Atmospheric Waters Induces Enhanced H₂O₂ Production through Photosensitized Chemistry of Phenolic Substances

Abstract

Hydrogen peroxide (H2O2) is known to convert SO2 to sulfuric acid and acts as a dominant reservoir of highly reactive hydroxyl radical (OH) in atmospheric waters (cloud, fog, rain, and aerosol liquid water). Here, we conclusively demonstrate that photosensitized oxidation of phenolic substances (catechol, o-cresol, and guaiacol) by the excited triplet state of nonphenolic compound (3,4-dimethoxybenzaldehyde, DMB) represents an unrecognized significant source of H2O2. Intriguingly, the highest H2O2 formation rate, (3.43 +/- 0.14) x 10-9 M s-1, and H2O2 yield (Phi H2O2 ), (7.68 +/- 0.08) x 10-1, were observed by photosensitized chemistry of catechol at low pH values (2.50) typical of cloud and aerosol water. The quantum chemical calculations revealed that the fraction of the protonated triplet state of DMB increases with a pH decrease, resulting in a faster formation of H2O2. A detailed mechanism was proposed describing the formation of H2O2 from the photosensitized reaction