What Do We Emit When We Brake? Chemical Speciation of Representative Vehicle Brake Pads via Thermal Desorption–Pyrolysis Gas Chromatography–Mass Spectrometry

Due to increasingly strict regulations on exhaust emissions from road traffic, their contribution to total emissions has decreased in urban areas. In contrast, non-exhaust emissions such as those from vehicle brake pads are less regulated yet make up a proportion of total road traffic emissions in many countries. To characterize the thermal evolution and breakdown profiles of six representative vehicle brake pads, this research employed evolved gas analysis–mass spectrometry (EGA-MS) and thermal desorption–pyrolysis–gas chromatography–mass spectrometry (TD-Pyr-GC-MS). The EGA-MS results yielded thermal profiles and ionic profiles from mass spectra, allowing for the determination of suitable temperature steps between 200 and 750 °C and the prediction of evolving species. Applying these temperature steps to TD-Pyr-GC-MS analysis, we identified aromatics, nitrogen-containing compounds, polycyclic aromatic hydrocarbons (PAHs), phthalate, sulfur-containing compounds and oxygenates evolving within the TD step. The Pyr step revealed the presence of nitrogen-containing compounds, aromatics, alkenes, alkanes and oxygenates. With braking temperatures usually between 100 and 500 °C, the results suggest that brake pad emissions contribute to global emissions through the release of volatile organic compounds (VOCs) and particulate matter into the atmosphere or the dispersal of contaminating brake dust into soil and water. Although some of these compounds may not be of immediate concern, they can further contribute to the formation of other chemicals such as disinfectant by-products (DBPs) by serving as precursors. More strict non-exhaust emission controls and the development of high-performance brake pads that reduce environmental and health risks are therefore desirable.