Investigation of brake wear particle emissions via single particle mass spectral signatures: Comparison of laboratory and ambient measurements

Non-exhaust traffic emissions have recently attracted attention due to their increasing contribution to ambient particulate matter (PM). Brake wear particles (BWP) are of particular concern among different non-exhaust emissions owing to their high content of heavy metals. Despite ongoing research and emerging regulatory frameworks, detailed insights into the physicochemical properties of BWP at the single-particle level remain limited. A single-particle mass spectrometer (SPMS) was used in combination with a brake dynamometer to characterize pure BWP and confirmed using scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX). Three main particle types were identified: Ba-containing particles, K-rich particles containing organic matter, and carbonaceous particles. The distinct mass spectral signatures reflect the formation pathways including mechanical frictional wear and thermal degradation processes. Ba-containing particle group was confirmed as a suitable chemical tracer, allowing extension of the laboratory findings to ambient environments through measurements at a highway tunnel entrance and an urban background station in Munich, Germany. Approximately 6 % of detected PM was attributed to brake wear emissions. The presence of BWP at the background monitoring station demonstrated their chemical resilience and transport potential over a distance of ∼1 km. Notably, both laboratory and field measurements revealed minor signals of traditional toxic brake pad constituents such as Cu and Sb, suggesting a shift toward alternative materials in modern brake pad formulations. Our results emphasize the need to integrate BWP into air quality monitoring and regulatory frameworks, particularly using chemically specific tracers to better understand their environmental and health impacts.