Unraveling Mechanisms of Reactive Oxygen Species Formation from Secondary Organic Aerosols
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Unraveling Mechanisms of Reactive Oxygen Species Formation from Secondary Organic Aerosols

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Abstract

Reactive oxygen species (ROS), including the hydroxyl radical (•OH), superoxide (O2•-), hydroperoxyl radical (HO2•) and hydrogen peroxide (H2O2), play a central role in chemical transformation and health effects of atmospheric aerosols. Respiratory deposition of secondary organic aerosols (SOA) and transition metals may lead to the generation of ROS to cause oxidative stress, but the underlying mechanism and formation kinetics of ROS are not well understood. Using electron paramagnetic resonance (EPR) spectroscopy coupled with a spin trapping technique, the ROS formation is characterized from aqueous reactions of SOA involving transition metals, lung antioxidants, reaction media with different pH as well as photoirradiation. First, we demonstrate dominant formation of superoxide (O2•-) with molar yields of 0.01 – 0.03% from aqueous reactions of biogenic SOA generated by •OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of •OH and O2•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, •OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect relative abundance of organic hydroperoxides and alcohols contained in SOA. In addition, we observed substantial formation of organic radicals in surrogate lung fluid (SLF) by mixtures of Fe2+ and SOA generated from photooxidation of isoprene, α-terpineol and toluene. The molar yields of organic radicals by SOA are measured to be 0.03 – 0.5% in SLF, which are 5 – 10 times higher than in water. We observe that Fe2+ enhances organic radical yields dramatically by a factor of 20 – 80, which can be attributed to Fe2+-facilitated decomposition of organic peroxides, consistent with a positive correlation between peroxide contents and organic radical yields. Ascorbate mediates redox cycling of iron ions to sustain organic peroxide decomposition, as supported by kinetic modeling reproducing time- and concentration-dependence of organic radical formation as well as additional experiments observing the formation of Fe2+ and ascorbate radicals in mixtures of ascorbate and Fe3+. •OH and superoxide are found to be scavenged by antioxidants efficiently. Furthermore, we find highly distinct radical yields and composition at different pH in the range of 1 – 7.4 from SOA generated by oxidation of isoprene, α-terpineol, α-pinene, β-pinene, toluene and naphthalene. We observe that isoprene SOA have substantial hydroxyl radical (•OH) and organic radical yields at neutral pH, which are 1.5 – 2 times higher compared to acidic conditions in total radical yields. Superoxide (O2•-) is found to be the dominant species generated by all types of SOA at lower pH. At neutral pH, α-terpineol SOA exhibit a substantial yield of carbon-centered organic radicals, while no radical formation is observed by aromatic SOA. Further experiments with model compounds show that the decomposition of organic peroxide leading to radical formation may be suppressed at lower pH due to acid-catalyzed rearrangement of peroxides. We also observe 1.5 – 3 times higher molar yields of hydrogen peroxide (H2O2) in acidic conditions compared to neutral pH by biogenic and aromatic SOA, likely due to enhanced decomposition of α-hydroxyhydroperoxides and quinone redox cycling, respectively. Finally, we identify that photoirradiation can induce efficient formation of organic radicals from SOA with radical yields up to 1.5%, ~ 100 times higher compared to dark conditions. Further experiments show that total peroxide fractions in SOA decrease 50 – 70% after irradiation, indicating organic peroxides as a probable source of organic radical formation. High resolution mass spectrometry (HR-MS) confirms the substantial formation of organic radicals by identifying BMPO-organic radical adducts, which may provide insights into the chemical structures of these organic radicals and subsequently their formation mechanisms. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

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