Unveiling the pH-Dependent Yields of H 2 O 2 and OH by Aqueous-Phase Ozonolysis of m -Cresol in the Atmosphere.

Hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (OH) are important oxidants in the atmospheric aqueous phase such as cloud droplets and deliquescent aerosol particles, playing a significant role in the chemical transformation of organic and inorganic pollutants in the atmosphere. Atmospheric aqueous-phase chemistry has been considered to be a source of H 2 O 2 and OH. However, our understanding of the mechanisms of their formation in atmospheric waters is still incomplete. Here, we show that the aqueous-phase reaction of dissolved ozone (O 3 ) with substituted phenols such as m -cresol represents an important source of H 2 O 2 and OH exhibiting pH-dependent yields. Intriguingly, the formation of H 2 O 2 through the ring-opening mechanism is strongly promoted under lower pH conditions (pH 2.5-3.5), while higher pH favors the ring-retaining pathways yielding OH. The rate constant of the reaction of O 3 with m -cresol increases with increasing pH. The reaction products formed during the ozonolysis of m -cresol are analyzed by an Orbitrap mass spectrometer, and reaction pathways are suggested based on the identified product compounds. This study indicates that aqueous-phase ozonolysis of phenolic compounds might be an alternative source of H 2 O 2 and OH in the cloud, rain, and liquid water of aerosol particles; thus, it should be considered in future model studies.