Aqueous OH Radical Production by Brake Wear Particles

Abstract

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 ·OH radicals in water. BWPs generated by ceramic and semimetallic brakes during heavier braking lead to higher ·OH yields compared to gentle braking conditions, suggesting higher ·OH formation potential from ultrafine BWPs. Chemical characterization reveals that organic and elemental carbon correlated positively with ·OH formation while exhibiting negative correlations with abundant metals including Fe and Mn. We suggest that the source of ·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 ·OH yield. These results provide critical insights into the toxicity and adverse health effects of BWPs.

Brake Wear and Airborne Particle Mass Emissions from Passenger Car Brakes in Dynamometer Experiments based on Worldwide Harmonized Light-Duty Vehicles Test Procedure Brake Cycle

“…The reduction in metal wear per particle mass suggests that the reduction in the total brake wear factor, PM10, and PM2.5 is predominant (negatively correlated with the ratio of Ba mass per pad mass). However, although outside the scope of this study, a comparison of non-steel pads and low-steel pads (ceramic pads) showed that the formation of OH radicals in the aqueous phase increased with increasing Ba concentration in brake wear particles [73]. We suggest that the prevention of direct adhesion with steel fibers by Ba results in a decrease in metal wear per particle mass [18], an increase in organic mass per particle mass, and a predominant formation of OH radicals derived from organic matter.…”

“…We suggest that the prevention of direct adhesion with steel fibers by Ba results in a decrease in metal wear per particle mass [18], an increase in organic mass per particle mass, and a predominant formation of OH radicals derived from organic matter. Further investigation is needed to evaluate the reduction of brake wear particles and health effects, as it is suggested that Ba concentration may be negatively correlated with oxidative potential and OH radical formation in the aqueous phase [73], which would be assumed to be in vivo, when evaluating health effects based on oxidative potential.…”

Residential Wood Burning and Vehicle Emissions as Major Sources of Environmentally Persistent Free Radicals in Fairbanks, Alaska

“…These correlations suggest an influence of road dust on EPFRs in Fairbanks. Iron and titanium oxides are known to stabilize EPFRs. ,,, Ti is also a major component in brake wear particles . Thus, tight correlations of EPFR v with Fe and Ti suggests that nontailpipe emissions may play a role in stabilization of EPFR, if they are internally mixed.…”