Particulate matter emissions from brake pads: A comparative study of low-steel and non-asbestos organic materials

This study investigates the particulate matter (PM) emissions generated from the wear of four different commercial brake pads (two Low-Steel and two Non-Asbestos Organic). Brake materials were tested by means of the AK Master test procedure, and emitted brake wear PM was collected by means of a Dekati Low-Pressure Impactor. For each tested pad, the 13 samples collected at each impactor stage during three sections of the test were weighted and then subjected to acidic digestion and ICP-OES and ICP-MS elemental analysis; a suite of 56 elements was analysed. The analysis revealed distinct differences in emission profiles based on both pad composition and braking intensity. Low-Steel pads emitted more PM10 and PM2.5 under mild braking conditions, while Non-Asbestos Organic pads emitted higher levels of PM0.1 during intense braking. The size distribution of PM emissions shifted from unimodal to bimodal as severe braking events increased, with fine and ultrafine particles becoming more prominent. Principal Component Analysis (PCA) was applied to identify chemical composition differences between the brake pads. Low-Steel pads emitted iron-rich particles due to the ferrous materials in their composition, while Non-Asbestos Organic pads emitted particles rich in non-ferrous metals such as barium and titanium. Additionally, driving style had a strong influence on both the quantity and size distribution of the emitted PM. Frequent, intense braking events led to a significant increase in the total quantity of PM and a higher proportion of fine and ultrafine particles. The findings highlight the importance of material composition and braking behaviour in mitigating brake wear emissions, which are projected to become the dominant source of traffic-related air pollution in the future.