Measurement report: Atmospheric aging of combustion-derived particles – impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media

Environmentally persistent free radicals (EPFRs) are a pollutant found on fine atmospheric particulate matter (PM2.5), particularly on PM2.5 formed from combustion processes. EPFRs are organic radicals that can endure in the environment for days to years. Interest in the toxicity of EPFRs has increased significantly in recent years, as it has been shown to have substantial ability to form reactive oxygen species (ROS), but little is known about how its characteristics change as PM2.5 ages in the atmosphere. Here, we exposed newly produced hexane-generated soot to simulated sunlight for 24 h. Changes to the EPFR characteristics of the particles were measured by electron paramagnetic resonance (EPR) spectroscopy. The soot was then added to water, and a second exposure to light was used to measure hydroxyl radical (OH) formation from both photoaged and dark-aged soot. There were no changes to EPFR characteristics (spin concentration, g factor, peak width, or lineshape) due to the exposure to simulated sunlight; however, the soot's ability to form OH was greatly reduced by photoaging. Photoaged soot resulted in an almost 60 % reduction in OH formation over soot which had been aged in the dark for the same amount of time.