University of California, Irvine researchers have demonstrated that up to 80% of brake wear particles (BWP) are electrically charged. In an open-access paper in Proceedings of the National Academy of Sciences (PNAS), the team suggests that the findings suggest that control strategies that exploit the unique electrical properties of brake wear particles can be highly effective in mitigating this key emerging source of pollution.

Although the last several decades have seen a dramatic reduction in emissions from vehicular exhaust, nonexhaust emissions (e.g., brake and tire wear) represent an increasingly significant class of traffic-related particulate pollution. Aerosol particles emitted from the wear of automotive brake pads contribute roughly half of the particle mass attributed to nonexhaust sources, while their relative contribution to urban air pollution overall will almost certainly grow coinciding with vehicle fleet electrification and the transition to alternative fuels.

To better understand the implications of this growing prominence, a more thorough understanding of the physicochemical properties of brake wear particles (BWPs) is needed. Here, we investigate the electrical properties of BWPs as emitted from ceramic and semi-metallic brake pads. We show that up to 80% of BWPs emitted are electrically charged and that this fraction is strongly dependent on the specific brake pad material used. A dependence of the number of charges per particle on charge polarity and particle size is also demonstrated.

We find that brake wear produces both positive and negative charged particles that can hold in excess of 30 elementary charges and show evidence that more negative charges are produced than positive. Our results will provide insights into the currently limited understanding of BWPs and their charging mechanisms, which potentially have significant implications on their atmospheric lifetimes and thus their relevance to climate and air quality. In addition, our study will inform future efforts to remove BWP emissions before entering the atmosphere by taking advantage of their electric charge.

—Thomas et al.

To do the work, the team used a large lathe to spin a detached brake rotor and caliper. They then measured the electric charge of the aerosols emitted into the air.

The new study reveals a problem that may grow as electric cars become more and more common over the next several decades. Electric cars, explained corresponding author James Smith, are not truly zero-emission vehicles, so municipalities need to think about strategies to reduce emissions from brake use as well as tailpipes.

The team found that the percentage of charged particles emitted largely depended on the material makeup of brake pads. Because the particles carry an electric charge, this should make it relatively easy to remove from the air.

If they are charged, they can be removed easily from the air before they have a chance to have an impact at all on health. All you would need to do is to collect them with an electrostatic precipitator—a device that exposes the charged particles to an electric field and efficiently sweeps them away.

—James Smith

The public health risk posed by brake emissions is not borne equally by a population; lower-income parts of cities tend to be more traffic-heavy than others, which creates an environmental justice issue wherein certain socioeconomic classes are more exposed to brake emissions than others.

The UCI team is working with local community organizations such as the Madison Park Neighborhood Association in Santa Ana, which is helping disseminate UCI’s scientific findings to the public. Funding for the study came from fees paid by Volkswagen as part of a 2016 settlement reached with the California Department of Justice, which found that the company used devices that contributed to increased air pollution.

Resources

• Adam E. Thomas, Paulus S. Bauer, Michelia Dam, Véronique Perraud, Lisa M. Wingen, James N. Smith (2014) “Automotive braking is a source of highly charged aerosol particles” PNAS doi: 10.1073/pnas.2313897121