Electrical and Chemical Properties of Brake Wear Particles

JAMES SMITH, Adam Thomas, Paulus Bauer, Véronique Perraud, Lisa Wingen, Michelia Dam, Madeline Cooke, Barbara Finlayson-Pitts, University of California, Irvine

     Abstract Number: 214
     Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts

Abstract
As the world transitions away from internal combustion engines, non-tailpipe sources such as tire and brake wear already rival tailpipes as a chief source of road transport emissions of PM2.5. The physicochemical properties of these particles are not well understood. This study focuses on measurements of the charge state of particles generated from disk brake wear. Experiments were performed using a unique brake dynamometer that applies constant torque to a constantly spinning brake rotor. This allowed for studies of gas and particle emissions under repeatable conditions of speed, position and torque, and temperature. We tested two types of brake pads used extensively in the US: ceramic and semi-metallic. A major finding of this project is that brake wear particles can be highly charged: up to 80% of particles emitted from ceramic brake pads possessed one or more charges. Measurements of charge state using tandem differential mobility analysis showed that, on average, charged brake wear particles hold 20-30 elementary charges for both pad formulations. With regards to the composition of ultrafine brake wear particles, organics, and some inorganics such as sulfates and nitrates, were probed in real time during these experiments using a thermal desorption chemical ionization mass spectrometer and off-line using liquid chromatography high-resolution Orbitrap mass spectrometry (LC-HRMS). Those measurements have shown that ultrafine brake wear particles emitted under harsh braking conditions are mostly organic, containing a complex mixture of unsaturated, oxygen and nitrogen-containing organic compounds.