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

Respiratory deposition of secondary organic aerosols (SOA) andiron may lead to the generation of reactive oxygen species and free radicals in lungfluid to cause oxidative stress, but their underlying mechanism and formationkinetics are not well understood. Here we demonstrate substantial formation oforganic radicals in surrogate lungfluid (SLF) by mixtures of Fe2+and SOAgenerated from photooxidation of isoprene,alpha-terpineol, and toluene. The molaryields of organic radicals by SOA are measured to be 0.03-0.5% in SLF, which are5-10 times higher than in water. We observe that Fe2+enhances organic radicalyields dramatically by a factor of 20-80, which can be attributed to Fe2+-facilitateddecomposition of organic peroxides, in consistency with a positive correlationbetween peroxide contents and organic radical yields. Ascorbate mediates redox cycling of iron ions to sustain organic peroxidedecomposition, as supported by kinetic modeling reproducing time- and concentration-dependence of organic radical formation aswell as additional experiments observing the formation of Fe2+and ascorbate radicals in mixtures of ascorbate and Fe3+.center dot OH andsuperoxide are found to be scavenged by antioxidants efficiently. Thesefindings have implications on the role of organic radicals inoxidative damage and lipid peroxidation.