Brake-derived PN1 emissions from NAO pads under continuous high-temperature operations

While it is well established that high temperatures can significantly increase brake wear particle emissions, the detailed emission patterns have seldom been reported. To address this gap, this study investigated brake particle number (PN) emissions under high-temperature conditions, focusing on detailed PN patterns and fundamental reasons. Particles smaller than 1 μm (PN1), which are the dominant source of total PN, were analyzed under various brake temperature conditions. The main findings include: delayed and reduced PN1 spikes were observed due to the reduce of the materials with low decomposition temperature and the reduce of organic material availability. The protective layer, which is composed of pyrolyzed residues and secondary plateaus, could also reduce the PN1 concentration. Within one brake event, alternating PN1 increase and decrease were noted, which was caused by the alternating formation and breakdown of protective layer. The break of the protective layer is hard to predict, which leads to occasional PN1 spikes and significant PN1 concentration variations among tests. Both increase and decrease in PN1 concentration were observed upon brake release due to the coupled effects from temperature, surface condition, and organic material availability. Notably, PN1 emissions occur even without disc rotation, indicating that PN1 is caused by thermal processes rather than the brake drag. These findings highlight a potential approach to PN1 control: rapidly establishing and stabilizing the protective layer while utilizing thermally stable organic materials.