In an open-access paper in the journal Scientific Reports, researchers in Sweden demonstrate, for the first time, that the photooxidation of gaseous brake wear emissions can lead to formation of secondary particulate matter, which could amplify the environmental impact of brake wear emissions.

Due to increasingly strict controls of exhaust emissions from road traffic, the contribution to the total emissions from that vector has strongly decreased over time in high-income countries. In contrast, non-exhaust emissions from road vehicles are not yet legislated and now make up the major proportion of road traffic emissions in many countries.

Brake wear, which occurs due to friction between brake linings and their rotating counterpart, is one of the main non-exhaust sources contributing to particle emissions. Since the focus of brake wear emission has largely been on particulate pollutants, little is currently known about gaseous emissions such as volatile organic compounds from braking and their fate in the atmosphere.

The study by researchers from Stockholm University and KTH Royal Institute of Technology, investigated the oxidative ageing of gaseous brake wear emissions generated with a pin-on-disc tribometer, using an oxidation flow reactor.

It has been demonstrated in previous studies that disc brake temperature strongly affects the number of ultrafine particles emitted from braking; the number of ultrafine particles increases dramatically once the transition temperature is exceeded. Typical values of the transition temperature range from 140 to 240 °C. A recent study linked the exceedance of the transition temperature to the formation of semi-volatile emissions by demonstrating that the number concentration of particles < 200 nm decreased by several magnitudes upon removal of the semi-volatile part.

Additional evidence of gaseous brake wear emissions comes from several studies that have shown that brake wear particles have a lower carbon content compared to the original brake lining. This discrepancy tends to increase with the severity of the braking process, which indicates that under more severe braking conditions, a significant portion of the carbonaceous material becomes volatilized.

This formation of VOCs from the braking process is generally attributed to the thermal decomposition of the organic brake pad constituents. Pyrolysis of phenolic resins, which are commonly used as binders in brake pads, has for example been shown to release VOCs such as benzene, toluene and phenol. So far, only very few studies have characterised VOC emissions from brake wear, with a large fraction of the VOCs remaining unidentified.

Not only has the composition of VOC mixtures from brake wear hardly been studied, but their potential to produce secondary organic aerosols by photooxidation has never been investigated before. This is despite the fact that secondary particle formation could lead to an additional impact of brake wear emissions on air quality and climate that is currently not accounted for. This work presents the first study of this kind.

—Patel et al.

Resources

• Patel, A., Aggarwal, S., Bard, L. et al. Gaseous emissions from brake wear can form secondary particulate matter. Sci Rep 14, 23253 (2024). doi: 10.1038/s41598-024-74378-5