Insight into Combustion-Like Wear Processes Due to Unintended CO₂ and Nucleation-Mode Particle Emissions from Passenger Cars at Mild Brake Temperatures (>40 °C) under Realistic Driving Conditions

Numerous studies have focused on brake wear particle emissions from passenger cars, but few have investigated gaseous emissions from brakes. The aim of this study was to investigate the emissions of fine particles, coarse particles, and CO2 under realistic driving conditions. Passenger car brake control was reproduced in an indoor experiment using the Los Angeles City Transit (LACT) cycle as the test cycle. The experiments were performed in a laboratory with a brake dynamometer under realistic braking temperature conditions using one front brake of a midsize passenger car. The preliminary results indicated that particle emission factors were higher than CO2 emissions, with a driving distance-based CO2 emission factor per brake axle of 1.7 ± 0.15 mg/km/brake and a particulate matter of 10 μm or less (PM10) emission factor of 10.1 ± 2.3 mg/km/brake. Although CO2 emissions were less than PM10, the results of this study showed that CO2 emissions and nucleation-mode particles (<20 nm in diameter) are formed even at final braking temperatures of 40–90 °C, which are considerably lower than the commonly accepted critical braking temperature. This research highlights the importance of further study to establish an emission inventory and to analyze the toxic effects of brake wear particles with combustion-like brake wear processes that require consideration of the emissions of nucleation-mode particles and gaseous compounds.