Input Parameters for Airborne Brake Wear Emission Simulations: A Comprehensive Review

Non-exhaust emissions, generated by the wear of brake systems, tires, roads, clutches, and road resuspension, are responsible for a large part of airborne pollutants in urban areas. Brake wear accounts for 55% of non-exhaust emissions and significantly contributes to urban health diseases related to air pollution. A major part of the studies reported in the scientific literature are focused on experimental methods to sample and characterize brake wear particles in a reliable, representative, and repeatable way. In this framework, simulation is an important tool, which makes it possible to give interpretations of the experimental results, formulate new testing approaches, and predict the emission produced by brakes. The present comprehensive literature review aims to introduce the state of the art of the research on the different aspects of airborne wear debris resulting from brake systems which can be used as inputs in future simulation models. In this review, previous studies focusing on airborne emissions produced by brake systems are investigated in three main categories: the subsystem level, system level, and environmental level. As well as all the information provided in the literature, the simulation methodologies are also investigated at all levels. It can be concluded from the present review study that various factors, such as the uncertainty and repeatability of the brake wear experiments, distinguish the results of the subsystem and system levels. This gap should be taken into account in the development of future experimental and simulation methods for the investigation of airborne brake wear emissions.

Enclosure Design for Brake Wear Particle Measurement Using Computational Fluid Dynamics

The harmfulness of fine dust generated by automobile brakes to the environment has recently received attention. Therefore, we aimed to analyze and regulate the brake wear particles in dynamometers. To accurately measure the number of particles and particle mass, the sampling system used needs to minimize transportation losses and reduce the residence time in the brake enclosure system. The brake dust measurement system currently used can estimate the main transportation loss but cannot evaluate the complex flow field in the brake enclosure system under different design conditions. We used computational fluid dynamics (CFD) technology to predict the behavior of brake wear particles and analyze the static pressure characteristics, the uniformity of the system flow, and the residence time of the brake dust particles in the system. In addition, we compared the design of the basic structure of the brake enclosure system, combined with the four factors affecting the design of the brake dynamometer, with the enclosure system. As a result, we proposed that the design of the cross section of the brake dynamometer enclosure should be circular, the outlet angle of the enclosure should be 15°, the caliper should be fixed to 150°, and two sets of splitters should be added. This design improves pressure loss and reduces the residence time of brake dust particles in the brake enclosure system.

Overview of Sources and Characteristics of Nanoparticles in Urban Traffic-Influenced Areas

Atmospheric nanoparticles can be formed either via nucleation in atmosphere or be directly emitted to the atmosphere. In urban areas, several combustion sources (engines, biomass burning, power generation plants) are directly emitting nanoparticles to the atmosphere and, in addition, the gaseous emissions from the same sources can participate to atmospheric nanoparticle formation. This article focuses on the sources and formation of nanoparticles in traffic-influenced environments and reviews current knowledge on composition and characteristics of these nanoparticles. In general, elevated number concentrations of nanoparticles are very typically observed in traffic-influenced environments. Traffic related nanoparticles can originate from combustion process or from non-exhaust related sources such as brake wear. Particles originating from combustion process can be divided to three different sources; 1) primary nanoparticles formed in high temperature, 2) delayed primary particles formed as gaseous compounds nucleate during the cooling and dilution process and 3) secondary nanoparticles formed from gaseous precursors via the atmospheric photochemistry. The nanoparticles observed in roadside environment are a complex mixture of particles from several sources affected by atmospheric processing, local co-pollutants and meteorology.

Global emission, atmospheric transport and deposition trends of microplastics originating from road traffic

<p>Since the first reports on the presence of plastic debris in the marine environment in the early 70s (1), plastics have been steadily accumulating in the environment. The global production of plastics in 2019 reached 368 Mt (from 311 Mt in 2014 and 225 Mt in 2004), with the largest portion produced in Asia (51%) (2), whereas 10% is believed to end into the sea every year (3). As a result, plastics have been confirmed today in several freshwater (4), and terrestrial (5) ecosystems; they fragment into microplastics (MPs, 1 µm to 5 mm) (6) and nanoplastics (<1µm) (7) via physical processes (8). MP present has been now confirmed from the Alps (9) and the Pyrenees (10), as far as Antarctica (11) and the high Arctic (9). Consequently, MPs have been found to<br>affect coral reefs (12), marine (13) and terrestrial animals (14). Schwabl et al. (15) detected them in human stool, while a recent study by Ragusa et al. (16) reported that MPs were even found in all placental portions.<br>A smaller fraction of MPs originates from road traffic emissions (17). Kole et al. (18) reported global average emissions of tire wear particles (TWPs) of 0.81 kg year-1 per capita, about 6.1 million tonnes (~1.8% of total plastic production). Emissions of brake wear particles (BWPs) add another 0.5 million tonnes. TWPs and BWPs are produced via mechanical abrasion and corrosion (19). Here, we present global trends in emissions, transport and deposition of road MPs.</p><p>References:<br>1. Colton, J. B., et al. Science (80). 185, 491–497 (1974).<br>2. PlasticsEurope. https://www.plasticseurope.org/en/resources/market-data (2019).<br>3. Mattsson, K., et al. Impacts 17, 1712–1721 (2015).<br>4. Blettler, M. C. M., et al.Water Res. 143, 416–424 (2018).<br>5. Chae, Y. & An, Y. J. Environ. Pollut. 240, 387–395 (2018).<br>6. Peeken, I. et al. Nat. Commun. 9, (2018).<br>7. Wagner, S. & Reemtsma, T. Nat. Nanotechnol. 14, 300–301 (2019).<br>8. Gewert, B., et al. Environ. Sci. Process. Impacts 17, 1513–1521 (2015).<br>9. Bergmann, M. et al. Sci. Adv. 5, 1–11 (2019).<br>10. Allen, S. et al. Nat. Geosci. 12, 339–344 (2019).<br>11. González-Pleiter, M. et al. Mar. Pollut. Bull. 161, 1–6 (2020).<br>12. Lamb, J. B. et al. P Science (80-. ). 359, 460–462 (2018).<br>13. Wilcox, C., et al. Sci. Rep. 8, 1–11 (2018).<br>14. Harne, R. J. Anim. Res. 383–386 (2019) doi:10.30954/2277-940x.02.2019.25.<br>15. Schwabl, P. et al. Ann. Intern. Med. 171, 453–457 (2019).<br>16. Ragusa, A. et al. Environ. Int. 146, 106274 (2021).<br>17. Schwarz, A. E., et al. Mar. Pollut. Bull. 143, 92–100 (2019).<br>18. Jan Kole, P., et al. Int. J. Environ. Res. Public Health 14, 1–4 (2017).<br>19. Penkała, M., et al. Environments 5, 9 (2018).</p><p> </p>

Experimental Characterization Protocols for Wear Products from Disc Brake Materials

The increasing interest in the emission from the disc brake system poses new challenges for the characterization approaches used to investigate the particles emitted from the wearing out of the relevant tribological systems. This interest stems from different factors. In the first place, a thorough characterization of brake wear particles is important for a complete understanding of the active tribological mechanisms, under different testing and servicing conditions. This information is an important prerequisite not only for the general improvement of brake systems, but also to guide the development of new materials for discs and brake pads, responding better to the specific requirements, including not only performance, but also the emission behavior. In this review paper, the main material characterization protocols used for the analyses of the brake wear products, with particular regard for the airborne fraction, are presented. Reliable results require investigating the fine and ultrafine particles as concerns their composition together with their structural and microstructural aspects. For this reason, in general, multi-analytical protocols are very much recommended.

Atmospheric Transport, a Major Pathway of Microplastics to Remote Regions

In recent years, marine, freshwater and terrestrial pollution with microplastics has been discussed extensively, whereas atmospheric microplastic transport has been largely overlooked. Here, we present the first global simulation of atmospheric transport of microplastic particles produced by road traffic (TWPs – tire wear particles and BWPs – brake wear particles), a major source that can be quantified relatively well. We find a high transport efficiency of these particles to remote regions, such as the Arctic Ocean (14%). About 34% of the emitted coarse TWPs and 30% of the emitted coarse BWPs (100 kt yr-1 and 40 kt yr-1 respectively) were deposited in the World Ocean. These amounts are of similar magnitude as the total estimated terrestrial and riverine transport of TWPs and fibres to the ocean (64 kt yr-1). Atmospheric transport of microplastics is thus an underestimated threat to global terrestrial and marine ecosystems and affects air quality on a global scale, especially considering that other large but highly uncertain emissions of microplastics to the atmosphere exist. High latitudes and the Arctic are highlighted as an important receptor of mid-latitude emissions of road microplastics, which may imply a future climatic risk, considering their affinity to absorb solar radiation and accelerate melting.

Physical Characterization of Brake-Wear Particles in a PM10 Dilution Tunnel

A dilution tunnel was designed for the characterization of brake-wear particle emissions up to 10 μm on a brake dyno. The particulate matter emission levels from a single front brake were found to be 4.5 mg/km (1.5 mg/km being smaller than 2.5 μm) over a novel real-world brake cycle, for a commercial Economic Commission for Europe (ECE) pad. Particle Number (PN) emissions as defined in exhaust regulations were in the order of 1.5 to 6 × 109 particles per km per brake (#/km/brake). Concentration levels could exceed the linearity range of full-flow Condensation Particle Counters (CPCs) over specific braking events, but remained at background levels for 60% of the cycle. Similar concentrations measured with condensation and optical counters suggesting that the majority of emitted particles were larger the 300 nm. Application of higher braking pressures resulted in elevated PN emissions and the systematic formation of nano-sized particles that were thermally stable at 350 °C. Volatile particles were observed only during successive harsh braking events leading to elevated temperatures. The onset depended on the type of brakes and their prehistory, but always at relatively high disc temperatures (280 to 490 °C).