Tracking Tire Wear Particles: Overhyped or Understudied?

MOLLY J. HAUGEN, Siriel Saladin, Phillipp Buhler, David O'Loughlin, Stefan Schlafle, Chiara Giorio, Adam M Boies, University of Cambridge

     Abstract Number: 499
     Working Group: Health-Related Aerosols

Abstract
The ubiquity of tires and known macroscopic wear rates has led to a growing concern of tire wear particles as a source of PM10, PM2.5 and ultrafine particles. The wear of tires as a source of airborne pollution is still poorly understood. This study details the dedicated measurements of tire wear nanoparticles from a dedicated rotating drum system at Karlsruhe Institute of Technology.

We examine the generation of nanoparticles generated from tire and road interactions, with a focus on two key aspects: (1) replicating real-world conditions in a controlled environment for particle generation, and (2) analysing the collected particles through both online and offline techniques. In order to generate realistic wear patterns, third body particles were used in a standardized laboratory tire testing facility across dynamic and static speed and load profiles. The findings indicated that milled stone dust as a third body particle significantly disrupted the nanoparticle size range, complicating the differentiation between tire-based and third-body-based nanoparticles. However, using sand as a third body particle, the interference showed comparatively lower background noise within the nanoparticle region. Here, steady-state cycles were employed to discern the relationships between force events and nanoparticle generation which were compared to analyses over an entire dynamic drive cycle. The steady-state cycles revealed that high lateral forces (>2 kN) yielded the highest nanoparticle concentrations, surpassing background levels by over two orders of magnitude. Whereas the drive cycle trials indicated that approximately 70% of emitted nanoparticles throughout the entire drive cycle were semi-volatile emissions, likely originating from vaporization events. ICP-MS results confirmed the presence of tire-related elements in the nanoparticle region, but definitive attribution to tire or road surface remains a challenge for the field. This study underscores the complexities inherent in generating, collecting, and assessing submicron tire wear particles, laying the groundwork for addressing uncertainties and refining non-exhaust tire emission methodologies.

In order to place tire emissions into context, we found a diversity of reported emissions found in secondary literature. Therefore, we analysed all known tire wear emission factors in primary research. Average and median values of 1.7 mg/vkm and 0.2 mg/vkm, respectively, were found for PM10 solely attributed to tire wear from light-duty vehicles (excluding road wear and resuspension). Notably, these emission rates are substantially lower than broadly cited and accepted factors in secondary literature, including USA and EU recommended emissions factor with an average and median of 6.5 and 6.1 mg/vkm, respectively. The discrepancy between primary and secondary literature appears to have arisen from misunderstandings due to different and unclear definitions for tire wear in combination with miscitations and a selection bias. Currently accepted mass-based emission factors for airborne tire wear need revision, including those from the United States Environmental Protection Agency and the European Environment Agency. Directly emitted tire wear PM10 appears to be a lower source of air pollution than widely stated by academia and governments.