Dependence of Reactive Oxygen Species Formation on the Oxidation State of Biogenic Secondary Organic Aerosols

Abstract

Reactive oxygen species (ROS) play a central role in the chemical aging of organic aerosols and adverse aerosol health effects upon respiratory deposition. Previous research has shown that biogenic secondary organic aerosols (SOA) form ROS, including hydroxyl radicals and superoxide, via reactions of reactive compounds, including organic hydroperoxides and alcohols in the aqueous phase. However, the influence of oxidative aging and the SOA oxidation state on the ROS yield has not been systematically investigated. In this study, we quantify ROS yields in d-limonene SOA and β-caryophyllene SOA generated via ^•OH and ^•Cl oxidation in an oxidation flow reactor at equivalent atmospheric aging times ranging from 4 h to 22 days. We quantify radical formation using electron paramagnetic resonance spectroscopy combined with a spin-trapping technique and characterize the molecular composition of the SOA samples with high-resolution mass spectrometry. We observe maximum radical formation at an oxygen-to-carbon ratio (O/C) of ∼0.5. Thereafter, we observe a >90% decrease in radical yield as the O/C increases to 1.2 for both d-limonene SOA and β-caryophyllene SOA. Similarly, the radical yield in d-limonene and β-caryophyllene SOA is reduced by >80% after on-filter photoirradiation. Peroxide yields are found to decrease with increasing O/C values and irradiation, suggesting that the aging-induced fragmentation and/or photolysis of hydroperoxides contribute to a decrease of radical formation in aged SOA.