New study details non-emission particulates

By Michael Brenneis

Greenhouse gas emissions from vehicles deserve a lot of attention, but particulates from vehicles are also a significant health concern. Tiny soot particles can be inhaled deeply into the lungs. Larger-sized particles can contaminate nearby fields and groundwater, and deliver a significant dose of microplastics to surface waters. Of the microplastic particles polluting surface waters, 30 percent originate from tire wear, according to new research by German and U.S. scientists.

The study looks at the source and composition of super-coarse, non-emission particulate matter—particles greater than 10 micrometers, or 1/1000 of a millimeter in size. Of the particles collected in proximity to roads of varying speeds, congestion levels, and fleet makeup, 93 percent came from road wear, tire wear, or brake wear. The remainder consisted of wind-blown soil, road salt, concrete weathering products, or plant material.

Road dust adheres to tire particles, encrusting the outer surface of the natural and synthetic rubber bits. Stop-and-go traffic reworks particles leading to more encrustation than occurs on free flowing, high speed roads. But the turbulent airflow of high speed roads effectively removes particles from the road surface, and deposits them in nearby areas. In addition to microplastics, encrustation delivers metal tire and brake residue such as aluminum, titanium, iron, zinc, antimony, barium, and lead to the environment.

Brake-wear particles are more abundant on more congested roads than on flowing roads, which stands to reason since brakes are used more in stop-and-go conditions. The presence of heavy duty vehicles, which also lose more tire material per mile than cars, may also increase the volume of brake-wear particulates due to the additional stress heavy vehicles place on brake parts. The toxic heavy metal component of brake-wear particles appears to be diminishing over time, perhaps due to regulatory changes.

Vehicle traffic movement lofts particles into the air. Larger particles remain suspended for less time than their smaller counterparts and are deposited nearby, raising concerns of contamination. In rural areas, farm fields and groundwater are of paramount concern as they are convenient avenues for pollutants to enter the food web. Urban highways pass in proximity to neighborhoods and can deliver particulates directly to residents. The well-documented pairing of urban highways and low-income neighborhoods exacerbates the problem of particulate exposure in these communities.

The adoption of electric vehicles may reduce greenhouse gas emissions in proximity to roadways by centralizing energy production at power plants, but the generation of particulates will continue, or even be exacerbated by predicted climate change scenarios. Abrasion increases when tires and roads are hot. Even electric vehicles with regenerative braking have the potential to increase tire wear due to increased torque and elevated vehicle weight in some models with heavy batteries.

Michael Brenneis is an Associate Researcher at SSTI.