Stability of environmentally persistent free radicals (EPFR) in atmospheric particulate matter and combustion particles

Abstract Environmentally persistent free radicals (EPFRs) are an emerging area of atmospheric interest due to their implications in adverse health effects. Previous EPFR studies have looked at 24-h averages of EPFR concentration within ambient PM2.5. However, PM2.5 levels vary diurnally. This study demonstrates instrumental sensitivity that is adequate to assess EPFR concentrations at a much higher temporal resolution than previously reported. Ambient PM2.5 samples were collected for 90 min at 96 L min−1 and analyzed quantitatively via electron paramagnetic resonance (EPR) spectroscopy. For environmental samples that had no measurable PM2.5 mass, EPFR concentrations were found in the range of 1012 spins m−3, which is similar to values that have been reported previously in other urban locations. Additionally, since combustion products are likely to contribute to the EPFR portion of PM2.5, hexane generated soot was assessed for radical stability when exposed to water and ambient atmospheric conditions. Radical concentrations decreased by about 5% over the course of 88 days, but there was no difference in degradation rates between wetted and non-wetted soot. This study highlights the potential for sub-hourly EPFR monitoring which would allow for better insight into human exposure.