(42) For instance, T crit of 180 and 240 ☌ has been reported for brake pads with organic and inorganic binder contents, respectively. (23,40−45) T crit increases during multiple runs of laboratory analysis of BWPs, presumably due to differences in volatilization onset temperatures of brake wear organic materials. (23) Moreover, brake temperature can affect the BWP size distribution above a critical brake temperature (140 ☌ < T crit < 240 ☌) when ultrafine BWPs are generated. Reported number distributions are influenced by factors such as brake lining material (19,39) and maintenance history. (24,34−38) Figure 1 illustrates greater variability, with a mode diameter number distribution from nanoscale to coarse size range. While mass-based BWP size distribution in the fine and ultrafine size ranges have been reported, (33) the majority of studies have reported a unimodal mass-based BWP size distribution with mode diameters in the range of 1–10 μm. (12)ĭuring a braking event, mechanical interaction between the brake pad and rotor produces brake wear particles (BWPs) of different sizes (32) ( Figure 1a,b). It is not surprising, therefore, that calls have been made for NEE from traffic to receive immediate recognition as an important source of ambient PM. A steady growth in this nonexhaust contribution is forecast, owing to phasing-out of older vehicles, increased electrification of road transport and the absence of legislation to limit/reduce nonexhaust particles. (12) In the UK, 2016 emissions data from the National Atmospheric Emissions Inventory (NAEI) showed that nonexhaust particles are the main source of primary PM (by mass) from road transport, for both the PM 2.5 (60%) and PM 10 (73%) size fractions. As a consequence, atmospheric emission inventories indicate that the proportion of NEE has increased, (10) widely exceeding exhaust emissions. This has been effective in progressively driving down gaseous pollutants and PM from the exhaust of new vehicles. In developed countries, tightening emission regulations for gasoline and diesel vehicles has mandated technological upgrades of combustion control and exhaust emission treatment systems. (13) While invariably being associated with coarse-mode PM, a considerable fraction of abrasion-derived particles exist within the fine and ultrafine fractions, (14,15) engendering NEE with a high capacity for harm owing to a larger reactive surface area and the ability to penetrate deeper into the lung and possibly into the blood to impact other organs in the body. (12) These metals, as well as those such as Fe from brake wear, catalyze the formation of reactive oxygen species (ROS) in the respiratory tract lining fluids, challenging antioxidants and metal-binding proteins that protect the epithelial surface of the lung. (11) The UK emissions inventory estimates mass contributions of 47% and 21% to national total airborne emissions of Cu and Zn, respectively. NEE, especially those from brake and tire wear, are an important source of metals in urban atmospheres. In contrast, particulates from NEE have been woefully understudied. (9,10) Most epidemiological and experimental research into traffic-related pollution focuses on particulate and gaseous pollutants emitted from the exhaust, particularly from diesel-fueled vehicles. These comprise of particles from mechanical abrasion of brakes and tires, erosion of road surfaces and resuspension of a mixture of dust that accumulates on road surfaces, and volatile organic compounds from evaporative loss of fuels and release of solvents. Road transport is also a source of nonexhaust emissions (NEE). Traffic-derived air pollution comprises a mixture of gaseous pollutants and PM from fuel combustion and lubricant volatilization in exhaust (tailpipe) emissions.
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