Abstract
The emission of volatile organic compounds (VOCs) from brake wear is a relatively underexplored aspect of nonexhaust traffic emissions. We employed a proton-transfer-reaction time-of-flight mass spectrometer to investigate the real-time emissions of VOCs from two commercially prevalent brake materials: low-metallic copper-free (LMCF) and nonasbestos organic (NAO). Experiments were conducted using a pin-on-a-disc tribometer integrated with a fast-mobility particle sizer spectrometer. The results reveal that NAO brake pads emit higher concentrations of VOCs than LMCF pads. Over 200 different sum formulas were identified for both materials, and siloxanes were observed as characteristic of NAO emissions. The ultrafine particle formation was systematically preceded by an increase in gaseous emissions. This observation supports the thesis that ultrafine particle formation emitted by brake wear occurs through VOCs nucleation, addressing a significant gap in the literature.