Volatile organic compounds emitted from the brakes of heavy and light duty vehicles

Contributions of non-exhaust sources to urban particulate matter (PM) pollution now often exceed those from vehicle exhaust in many high-income countries. Brake wear is one major source of such non-exhaust emissions. Particulate brake wear emissions have therefore come under increasing scrutiny, and are now for the first time being regulated within the EU through the recently established Euro 7 emission regulations. In contrast, there is only limited information regarding any potential gaseous emissions from the braking process. However, several recent studies indicate that these gaseous emissions should not be neglected.

In this study, we employed a proton transfer reaction time-of-flight mass spectrometer (PTR-MS) in combination with a chemical ionization mass spectrometer (CIMS) with iodide as the reagent ion to characterize the emission of volatile and semi-volatile organic compounds from brake wear. In total, four different brake materials were studied: two for heavy duty vehicles (bus and truck), and two for light duty vehicles. All brake wear emissions were generated in the laboratory using a pin-on-disc tribometer under different user case scenarios.

The PTR-MS results show that all four brake materials emitted a variety of organic compounds, including nitrogen- and sulphur-containing organics, oxygenated hydrocarbons, siloxanes as well as pure hydrocarbons. Out of these different groups, the oxygenated hydrocarbons contributed most to the overall concentrations. The emitted concentrations varied with the harshness of braking and type of brake pad. As expected, total emissions increased with increasing harshness of braking. As a novel result, we found that light duty brake pads emitted higher concentrations than heavy duty brake pads under the same braking conditions. Determination of emission ratios and evaluation of the CIMS data are currently ongoing.