Unveiling the pH-Dependent Yields of HO and OH by Aqueous-Phase Ozonolysis of -Cresol in the Atmosphere.

Hydrogen peroxide (HO) 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 HO 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) with substituted phenols such as -cresol represents an important source of HO and OH exhibiting pH-dependent yields. Intriguingly, the formation of HO 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 with -cresol increases with increasing pH. The reaction products formed during the ozonolysis of -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 HO and OH in the cloud, rain, and liquid water of aerosol particles; thus, it should be considered in future model studies.