Effects of Acidity and Photoirradiation on Reactive Oxygen Species Formation from Secondary Organic Aerosols
Jinlai Wei, Ting Fang, MANABU SHIRAIWA,
University of California, Irvine Abstract Number: 109
Working Group: Health-Related Aerosols
AbstractReactive oxygen species (ROS) play a critical role in the chemical transformation of atmospheric secondary organic aerosols (SOA) and aerosol health effects by causing oxidative stress in vivo. Acidity is an important physicochemical property of atmospheric aerosols affecting numerous environmental processes. Meanwhile, photochemical aging is at the core of atmospheric aging processes, which is essential for the heterogenous chemistry of SOA. Despite the importance of acidity and photochemistry, their effects on the ROS formation from SOA have been poorly characterized. By applying the electron paramagnetic resonance (EPR) spin-trapping technique and the Diogenes chemiluminescence assay, 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. For photochemistry, we combined an in-situ UV-vis irradiation system with EPR spectroscopy to characterize the photolytic generation of free radicals from SOA generated by isoprene, α-terpineol, α-pinene and toluene. We observe substantial formation of organic radicals (yields 0.02 – 1.5%) with enhancement factors up to ~ 100 compared to dark conditions. Total peroxide measurements elucidate that the peroxide fractions decrease 50 – 70% after irradiation, indicating organic peroxides as a potential source of organic radical formation. These findings are critical to bridge the gap in understanding ROS formation mechanisms and kinetics in atmospheric and physiological environments, as well as the photoinduced aqueous phase chemistry of SOA involving radical formation.