Hydrogen peroxide generation from α- and β-pinene and toluene secondary organic aerosols

In-particle reactions and reactivity may play a role in aerosol aging as well as health effects associated with airborne particles. One of the main hypotheses for the species causing the observed health effects are reactive oxygen species including hydrogen peroxide (H2O2). Secondary organic aerosol (SOA), the focus of this study, comprises a major fraction of ambient fine mode particle mass. SOA were generated via dark ozonolysis of α-pinene and β-pinene, as well as photochemical oxidation of α-pinene, β-pinene, and toluene. SOA particles generate a considerable amount of H2O2 in the aqueous phase. For the same particle mass, α-pinene and β-pinene SOA have higher H2O2 generation ability than toluene SOA. H2O2 levels were sensitive to the pH of the particle extraction solutions, decreasing as the pH was increased. SOA samples aged for 20 h in room air lost a significant fraction of the H2O2 compared to fresh samples. The H2O2 is likely from decomposition or hydrolysis of hydroxyhydroperoxides, peroxic acids and related species for α- and β-pinene SOA, and from redox cycling of quinoid compounds for toluene SOA. This study provides the first quantitative measurement of H2O2 for a range of SOA particle types.