Behavior of Iron Species and Free Radicals in Ambient PM2.5 and PM Surrogates

Abstract

Air pollution, consisting of ambient particulate matter (PM), has been a rising health concern to the public. PM contains free radicals and have been known to damage human cells; however, their free radical chemistry is not well understood. This study utilized various vacuum and/or heat treatments to study free radical behavior in PM2.5 (particulate matter with an aerodynamic diameter of 2.5 mm or less) and PM surrogates and simulated sunlight effects on PM2.5. To mimic PM, iron-silica catalysts (i.e. PM surrogates) were synthesized and real-world ambient PM2.5 was selected. The free radicals in PM2.5 have been referred to as environmentally persistent free radicals (EPFRs). It has been proposed that EPFR formation happens when phenol vapor is exposed to the iron surfaces of PM surrogates yielding a narrow electron paramagnetic resonance (EPR) signal with a g-factor of 2. However, adsorption of phenol on PM surrogates at room temperature did not form EPFRs, but poly(p-phenylene) material was detected. Free radical formation was observed when the phenol-exposed PM surrogates were heated above 300 °C. Ferric iron associated with superparamagnetic nanoparticles (less than 4 nm in diameter) as well as free radicals were shown to yield a narrow g = 2 EPR signal in PM surrogates and obey the Curie-Weiss Law. Distinguishing between the superparamagnetic nanoparticles and free radicals was accomplished by the experimental steps used on PM surrogates. Superparamagnetic behavior of the nanoparticles was observed by heating PM surrogates to 120 °C under ambient conditions yielding an increased intensity of the narrow g = 2 EPR signal while free radicals did not respond to this heat treatment. This was the first time the heating treatments were used to identify superparamagnetic species in PM2.5. Also, adventitious carbon yielded a small g = 2 EPR signal and was observed in PM surrogates and the polytetrafluoroethylene filters used to collect PM2.5. To further gain an understanding of the changes at the g = 2 EPR signal under environmental conditions, PM2.5 was exposed to simulated sunlight but did not demonstrate any changes to the intensity.