Probing the Molecular Composition of Organic Particulate Matter from Automotive Braking and Identification of Brake Tracers in Near-Roadway Ambient Environments

BERENICE ROJAS, Adam Thomas, Lisa Wingen, Madeline Cooke, Véronique Perraud, Samuel La Salle, Sanjeevi Nagalingam, Barbara Finlayson-Pitts, James Smith, University of California, Irvine

     Abstract Number: 590
     Working Group: Chemicals of Emerging Concern in Indoor and Outdoor Aerosol: Sources, Vectors, Reactivity, and Impacts

Abstract
Understanding vehicle emissions is crucial for mitigating poor air quality. The national trend towards vehicle electrification has reduced tailpipe emissions, leaving non-tailpipe emissions (e.g., brake and tire wear) as the remaining sources of vehicular emissions. Most modern passenger vehicles use ceramic or semi-metallic brake pads with phenolic resins as binder agents. These resins thermally degrade under high temperatures, potentially releasing organic compounds and particles into the environment. Although braking is a recognized source of particulate matter (PM), the exact contributions from different brake pad components and in particular, the role of phenolic resin decomposition in particle formation remain unclear. To address this, we conducted laboratory experiments on the controlled thermal degradation of commercial phenolic resin samples and compared our results with actual braking experiments using a custom-built brake dynamometer with ceramic and semi-metallic brake pads. Particles generated in both experiments were collected on filters and analyzed using liquid chromatography coupled with high-resolution Orbitrap mass spectrometry. This analysis revealed a range of organic compounds emitted during both the heating of phenolic resins and emissions of particles during braking simulations, including prominent nitrogen-containing (CHN) and nitrogen-oxygen containing (CHON) compounds. To evaluate the contribution of organic PM from braking to ambient air quality, we also conducted two field campaigns at two near-road freeway sites in Southern California. Ambient organic PM was collected using a Sequential Spot Sampler, allowing time resolved monitoring. Measurements of the PM collected from both field sites showed the presence of organic compounds that matched key constituents from our laboratory studies. Temporal trends in these species matched daily traffic patterns and local wind directions, confirming that they likely originate from roadway sources. This study highlights the environmental relevance of brake wear emissions, especially in communities near major freeways.