Suppression of OH Generation from the Photo-Fenton Reaction in the Presence of α-Pinene Secondary Organic Aerosol Material

Although Fenton and Photo-Fenton chemistry is thought to be an important source of OH in cloud and fog water, a high dissolved organic content, especially of secondary organic aerosol (SOA) material, may affect the production of OH via this mechanism. The relative production of OH was measured for Fenton and Photo-Fenton reactions with H2O2 and Fenton-like and Photo-Fenton-like reactions with alpha-pinene ozonolysis SOA material, under cloud water relevant conditions (5 mu M iron, 45 mu M H2O2, and 1500 mu M SOA). It is demonstrated that the generation of OH radicals from Photo-Fenton chemistry can be significantly suppressed by addition of a-pinene SOA material, where the OH yield for solutions containing H2O2 and SOA material together was decreased by a factor of 6 compared to that when only H2O2 was present, likely because of complexation by carboxylic acids (such as pinonic acid). When SOA is examined without additional H2O2 present, OH is generated by Photo-Fenton chemistry but at a rate lower than that for Photo-Fenton chemistry with H2O2 alone. Without taking into account the suppression by SOA material, one may overestimate the generation of OH by Photo Fenton chemistry. Furthermore, the suppression of Photo-Fenton chemistry in aqueous organic aerosol may be enhanced by a higher SOA material concentration.