Aqueous OH Radical Production by Brake Wear Particles

Particulate matter (PM) emitted from road traffic causes adverse health effects upon inhalation and respiratory deposition. Non-exhaust emissions will eventually become the dominant source of traffic PM upon transition to electric vehicles; however, non-tailpipe PM is currently unregulated as its health impacts are still unclear. In this study, we generated brake wear particles (BWPs) with non-asbestos organic, ceramic, and semimetallic brake pads using custom dynamometers and measured aqueous-phase formation of reactive oxygen species (ROS). We found that BWPs do not contain environmentally persistent free radicals (EPFRs), and all types of BWPs generate exclusively <middle dot>OH radicals in water. BWPs generated by ceramic and semimetallic brakes during heavier braking lead to higher <middle dot>OH yields compared to gentle braking conditions, suggesting higher <middle dot>OH formation potential from ultrafine BWPs. Chemical characterization reveals that organic and elemental carbon correlated positively with <middle dot>OH formation while exhibiting negative correlations with abundant metals including Fe and Mn. We suggest that the source of <middle dot>OH is thermal decomposition of organic hydroperoxides derived from phenolic resin. PM oxidative potential quantified with the dithiothreitol (DTT) assay exhibited a positive correlation with the <middle dot>OH yield. These results provide critical insights into the toxicity and adverse health effects of BWPs.