Nontailpipe Particulate Matter from Brake Wear, Tire Tread, and Road Dust: Environmentally Persistent Free Radicals, Reactive Oxygen Species, and Oxidative Potential

Nontailpipe emissions including brake and tire wear and resuspended road dust have become emerging concerns in traffic air pollution. It is thus important to understand the health-related properties of nontailpipe particulate matter (PM) as it becomes a major contributor to traffic air pollution. Here we characterize the chemical composition, reactive oxygen species (ROS) formation, and oxidative potential of ambient PM collected next to two highways in California, as well as laboratory-generated PM from brake wear particles (BWPs), cryogenically milled tire tread (CMTT) from fresh and used tires, used tire tread particles, and road dust. The major metal components are found to be iron, barium and copper in BWPs, zinc in CMTT, and crustal elements such as aluminum, calcium, potassium, and sodium in road dust. CMTT is found to contain environmentally persistent free radicals (EPFRs), while BWPs and road dust do not contain detectable amounts of EPFRs. Ambient traffic PM generates hydroxyl radicals ( • OH) and carbon-centered organic radicals upon extraction in water, while BWPs exclusively generate • OH, consolidating findings by previous studies. CMTT and road dust exhibit relatively low ROS formation and PM oxidative potential. BWPs generated from three different brakes of varying Cu content show that a higher Cu content leads to higher ROS formation and higher oxidative potential as measured by dithiothreitol (DTT) decay and • OH formation in surrogate lung fluid. These results provide useful insights into health-related properties of nontailpipe PM, supporting that regulation of Cu in brake materials would help reduce potential health effects of BWPs.