Ultrafine particle emissions from a fresh brake system: effect of temperature under a customized high intensity braking cycle

Vehicle transportation still represents an important source of PM in the urban areas. Stringent regulations have driven dramatic reduction of exhaust emissions making non-exhaust sources, such as brake abrasion, dominant. Brake wear particle characteristics depend on several factors including braking pattern, operating temperature, material properties as well as the status of brake components. This study experimentally investigates the emissions from a braking system on a dynamometer test bench. Particle size distributions were measured, and attention was paid on the impact of local disc temperature and the evolution of the emissions across different stages of brake usage. Experimental results showed that a fresh disc - pad assembly emitted particle number concentration with peaks around 108 #/cm3 during aggressive decelerations. At temperatures below 150 °C, emissions were dominated by mechanically generated particles, larger than 300 nm, and ultrafine concentrations were near background. Once the disc exceeded 150 °C, ultrafine particles increased by about 4 orders of magnitude and the size distribution became bimodal, with a nucleation mode peaked at 20 nm and an accumulation mode centred at 1 μm. Across four subsequent braking cycles started at elevated temperatures, the activation threshold for ultrafine formation increased to 215 - 290 °C, while peak of ultrafine particles decreased. Results indicate that brake usage progressively depletes volatile precursors and alters the friction surface, raising the thermal threshold for nucleation and attenuating ultrafine number emissions.