Iron-Facilitated Organic Radical Formation from Secondary Organic Aerosols in Surrogate Lung Fluid

Abstract

Respiratory deposition of secondary organic aerosols (SOA) and iron may lead to the generation of reactive oxygen species and free radicals in lung fluid to cause oxidative stress, but their underlying mechanism and formation kinetics are not well understood. Here we demonstrate 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, in consistency 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. These findings have implications on the role of organic radicals in oxidative damage and lipid peroxidation.

Effects of Acidity on Reactive Oxygen Species Formation from Secondary Organic Aerosols

“…Detailed procedures of SOA formation can be found in our recent studies. 23 , 26 While the PAM reactor applies high levels of oxidants (i.e., OH • concentration of ∼10 10 cm –3 ) 28 with a short reaction time, the PAM-generated SOA have been found to have similar characteristic with ambient and chamber-generated SOA in terms of mass yield, oxidation state, hygroscopicity, and chemical composition with similar mass spectra measured by an Aerodyne ToF AMS. 29 − 31 The relative humidity in the PAM reactor was 40–50%.…”

“…Under neutral conditions, organic hydroperoxides can preferably undergo unimolecular decomposition to generate highly reactive • OH radicals, which can initiate a cascade of reactions to propagate further radical formation 74 as well as directly attack biological components to induce pathological processes such as lipid peroxidation. 77 The formed organic radicals can be persistent even in the presence of antioxidants, 26 although their capacity to cause oxidative potential still warrants further studies. While this study used the PAM reactor to generate SOA, further experiments are necessary with SOA generated in an environmental chamber that applies lower VOC and oxidant concentrations as well as with organic particles collected from the ambient atmosphere to consolidate the atmospheric and health relevance of acidity effects on ROS formation by SOA.…”

“…Our recent study demonstrated substantial formation of organic radicals from iron-facilitated decomposition of organic peroxides contained in SOA in surrogate epithelial lining fluid with a pH of 7.4. 26 As there have been few systematic investigations of pH effects on ROS formation, it is still highly uncertain how different pH values would affect ROS formation from SOA, and the underlying chemical mechanism is poorly understood.…”

Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols

“…Afterward, 20 mg of potassium iodide (KI, Sigma-Aldrich, ≥99%) was added to the solution, and it was left to sit for 1 h. The absorbance was then measured at 405 nm using the GloMax Discover Microplate Reader. The total peroxide measurement method is based on the iodometric–spectrophotometric method used by Docherty et al , …”

“…Afterward, 20 mg of potassium iodide (KI, Sigma-Aldrich, ≥99%) was added to the solution, and it was left to sit for 1 h. The absorbance was then measured at 405 nm using the GloMax Discover Microplate Reader. The total peroxide measurement method is based on the iodometric−spectrophotometric method used by Docherty et al 40,41 DTT Assay. Dithiothreitol (DTT) analysis was performed to measure the total oxidative potential and redox activity, 32,42 which is often assumed to correspond to ROS formation and to represent indicator of PM toxicity.…”