Brake Wear Particulate Matter Emissions

Current particulate matter (PM) emission factor models estimate brake wear particulate matter emission rates using data derived from asbestos brakes. However, most brake pads are now produced from nonasbestos materials. Little work has been performed on emissions from brakes using these materials. Therefore, a brake wear study was performed using seven brake pad formulations that were in high volume use in 1998. Included were semi-metallic brakes, brakes using potassium titanate fibers, and brakes using aramid fibers. Brakes were tested on a brake dynamometer under four wear conditions. On average, 35% of the brake pad mass loss was emitted as airborne PM. The observed wear rates correspond to vehicle emission rates of 5.1−14.1 mg/mi. On average, 86 and 63% of the airborne PM was smaller than 10 μm in diameter (PM10) or 2.5 μm in diameter (PM2.5), respectively. The large number of particles observed in some wear tests was attributed to condensation, a process that is highly dependent on dilution condition. Analysis of airborne PM showed very few inhalable fibers. On average, 18% of the airborne PM was carbonaceous material. Elemental analysis indicated that metallic species together with silicon, phosphorus, sulfur, and chlorine accounted for most of the remaining mass. Estimates of brake wear PM10 and PM2.5 emission rates from light-duty vehicles are made from brake dynanometer wear tests.