Effects of braking conditions on nanoparticle emissions from passenger car friction brakes

Laboratory measurement and machine learning-based analysis of driving factors for brake wear particle emissions from light-duty electric vehicles and heavy-duty vehicles

This study investigates brake wear particle (BWP) emissions from light-duty electric vehicles (EVs) and heavy-duty vehicles (HDVs) using a self-developed whole-vehicle testing system and a modified brake dynamometer. The results show that regenerative braking significantly reduces emissions: weak and strong regenerative braking modes reduce brake wear PM2.5 by 75 % and 87 %, and brake wear PM10 by 90 % and 95 %, respectively. HDVs with drum brakes produce lower emissions and higher PM2.5/PM10 ratios than those with disc brakes. A machine learning model (XGBoost) was developed to analyze the relationship between BWP emissions and factors (11 for light-duty EVs and 8 for HDVs, based on kinematic, vehicle, and braking parameters). SHapley Additive exPlanations (SHAP) were used for model interpretation. For light-duty EVs, reducing high kinetic energy losses (Ike > 6500 J) and initial speeds (V > 45 km/h) braking events significantly lowers emissions. Additionally, the emission reduction effect of regenerative braking intensity (BI) stabilizes when BI exceeds 900 J. For HDVs, controlling braking temperature (Avg.T < 200°C) and initial speed (V < 50 km/h) effectively reduces emissions. Our findings provide new insights into the emission characteristics and control strategies for BWPs. SYNOPSIS: The construction and interpretation of a machine learning based model of brake wear emissions provides new insights into the refined assessment and control of non-exhaust emissions.

Beyond the tailpipe: Review of non-exhaust airborne nanoparticles from road vehicles

With the electrification of road vehicles leading to a reduction in tailpipe emissions, the relative contribution of non-exhaust emissions (NEEs) has become increasingly prominent. NEEs, particularly nanoparticles smaller than 100 nm in aerodynamic diameter (PM0.1), present significant health and environmental risks. A comprehensive understanding and strategic management of these emissions are urgently required to mitigate their impact. This article reviews existing studies and reveals that nanoparticles in NEEs are generated from brake and tyre wear under critical temperature conditions, while road wear and resuspension do not directly produce nanoparticles but contribute to larger particles. Common methodologies in studying these emissions include laboratory experiments (with brake dynamometers, tyre dynamometers, chassis dynamometers, and simulators), field tests (tunnel and real road emission tests), and source apportionments. The emission rate of PM0.1, calculated based on particle number concentration, ranges from 1.2% to 98.9%, depending on driving conditions. Extreme driving conditions result in high nanoparticle generation. Emission inventories reveal that PM0.1 emission levels have remained stable since 2020, without an observable reduction. Moreover, emissions attributable to brake wear are found to surpass those from tyre wear. Current mitigation strategies focus on material improvements for brake pads and tyres, better road maintenance, and regulatory measures. Mitigating the environmental and health impacts of nanoscale particulate matter requires additional research and regulations to control it at the source.

Effect of "black carbon" on antimony accumulation in traffic-loaded topsoil

Traffic-loaded areas have been of increasing concern due to the potential risk of carcinogenic pollutants, including antimony (Sb), which accumulates mostly in atmospheric particles (PM) and can interact with soil organic matter (Corg). The stability of Sb in topsoils was studied via the adsorption mechanism using standard soils and Corg-reach vehicle-produced particles as the unique source of "traffic" Sb. The mixed adsorbents were prepared from loamy sand and clay standard soils, and braking abrasion dust and diesel engine soot as Sb sources in atmospheric PM. Whereas the black carbon (BC), as part of Corg, disposes of exceptional adsorption properties compared to the other Corg, all adsorption experiments were performed identically on the adsorbents prepared from the original standard soils and Sb source materials and on the adsorbents prepared from the same materials annealed at 375 °C to ensure only BC participation in adsorption processes. The concentration of the Sb model solution corresponded to the average Sb content in rainwater from traffic-loaded localities. In addition to Sb, the Corg and iron (Fe) were monitored. The sorbability of Sb on the loamy sand soil mixtures increased up to 90% compared with the pure soil due to new active surface sites for Sb binding created due to the Corg added with the source material. The clay soil mixture containing 10 times more Corg compared with the loamy sand soil accumulated the Corg from the source material, which resulted in blocking active sites and a decline in Sb sorbability by up to 20%. The processes performed identically with original and annealed materials showed the same trends and confirmed the key role of BC and soil quality in the accumulation and stability of Sb in traffic-loaded topsoils. The participation of Fe in Sb surface interactions was not observed.

Biological effects of brake wear particles in mammalian models: A systematic review

Road traffic is a major contributor to air pollution through aerosols both from exhaust emissions (EE) and non-exhaust emissions (NEE). NEE result from mechanical abrasion of brakes and tires, erosion of road surfaces and resuspension of road dust into the atmosphere by passing traffic. EE have been thoroughly studied and have decreased over time due to a stricter control. On the other hand, NEE have not received such attention and there is currently no legislation to specifically reduce NEE particles. Consequently, NEE relative part has become prevalent, potentially making of these emissions a major human health concern. The aim of this systematic review was to provide an overview of the current state of knowledge on the biological effects of brake wear particles, a type of NEE. To this end, we conducted a bibliographic search of two databases (PubMed and Web of Science) on June 1, 2023, focusing on the toxicological effects of brake wear particles induced in vitro and in vivo. We excluded reviews (no original experimental data), papers not written in English, studies performed in non-mammalian models and papers where no toxicity data were reported. Of the 291 papers, 19 were found to be relevant and included in our analysis, confirming that the assessment of the brake wear particles toxicity in mammalian models is still limited. This review also reports that brake wear particles can induce oxidative stress, proinflammatory response and DNA damage. Finally, some perspectives for further research and measures to mitigate the risk of brake wear emissions are discussed.

Airway contraction and cytokine release in isolated rat lungs induced by wear particles from the road and tire interface and road vehicle brakes

The dominant road traffic particle sources are wear particles from the road and tire interface, and from vehicle brake pads. The aim of this work was to investigate the effect of road and brake wear particles on pulmonary function and biomarkers in isolated perfused rat lungs. Particles were sampled from the studded tire wear of three road pavements containing different rock materials in a road simulator; and from the wear of two brake pad materials using a pin-on-disk machine. Isolated rat lungs inhaled the coarse and fine fractions of the sampled particles resulting in an estimated total particle lung dose of 50 μg. The tidal volume (TV) was measured during the particle exposure and the following 50 min. Perfusate and BALF were analyzed for the cytokines TNF, CXCL1 and CCL3. The TV of lungs exposed to rock materials was significantly reduced after 25 min of exposure compared to the controls, for quartzite already after 4 min. The particles of the heavy-duty brake pads had no effect on the TV. Brake particles resulted in a significant elevation of CXCL1 in the perfusate. Brake particles showed significant elevations of all three measured cytokines, and quartzite showed a significant elevation of TNF in BALF. The study shows that the toxic effect on lungs exposed to airborne particles can be investigated using measurements of tidal volume. Furthermore, the study shows that the choice of rock material in road pavements has the potential to affect the toxicity of road wear PM10.

Laboratory and on-road testing for brake wear particle emissions: a review

Brake wear emission is a significant contributor to vehicle-related particulate matter, especially in areas with high traffic density and braking frequency. Only recently, non-exhaust emissions from car brake wear have been regulated under Euro 7 regulation, which introduces emission limits for both brake and tires. It also introduces a standard brake particle assessment procedure which includes sampling procedure and measurement techniques defined in the Global Technical Regulation on brakes from light-duty vehicles up to 3.5 t. Over the years, various experimental setups have been tried leading to non-comparable results. The brake wear particle emissions, expressed as emission factors, are mostly estimated as particle mass or particle number and described using different units (e.g., mg/stop brake, mg/km brake; particle number/cm3) making the comparison between studies very difficult. The aim of the present literature review is to present the state-of-the-art of different experimental methods tuned for assessing brake wear emissions, including electric vehicles. The experiments are carried in close, semi-closed, and open systems, and depending on the experimental design, different sampling methods are applied to reduce particle transport loss and guarantee the efficiency of the particle sampling. Driving condition (e.g., speed and applied pressure), formulation of brake materials, and friction temperature have been found to strongly affect the emission characteristics of brake particles, and this needs to be considered when designing study procedures. The findings reported in this review can be beneficial to policy makers and researchers.

Effects of brake wear nanoparticles on the protection and repair functions of the airway epithelium

Long term exposure to particulate air pollution is known to increase respiratory morbidity and mortality. In urban areas with dense traffic most of these particles are generated by vehicles, via engine exhaust or wear processes. Non-exhaust particles come from wear processes such as those concerning brakes and their toxicity is little studied. To improve our understanding of the lung toxicity mechanisms of the nanometric fraction of brake wear nanoparticles (BWNPs), we studied whether these particles affect the barrier properties of the respiratory epithelium considering particle translocation, mucus production and repair efficiency. The Calu-3 cell line grown in two-compartment chambers was used to mimic the bronchial epithelial barrier. BWNPs detected by single-particle ICP-MS were shown to cross the epithelial tissue in small amounts without affecting the barrier integrity properties, because the permeability to Lucifer yellow was not increased and there was no cytotoxicity as assessed by the release of lactate-dehydrogenase. The interaction of BWNPs with the barrier did not induce a pro-inflammatory response, but increased the expression and production of MU5AC, a mucin, by a mechanism involving the epidermal growth factor receptor pathway. During a wound healing assay, BWNP-loaded cells exhibited the same ability to migrate, but those at the edge of the wound showed higher 5-ethynyl-2'-deoxyuridine incorporation, suggesting a higher proliferation rate. Altogether these results showed that BW. NPs do not exert overt cytotoxicity and inflammation but can translocate through the epithelial barrier in small amounts and increase mucus production, a key feature of acute inflammatory and chronic obstructive pulmonary diseases. Their loading in epithelial cells may impair the repair process through increased proliferation.

Determining factors and parameterization of brake wear particle emission

Brake wear emission contributes to an increasingly significant proportion of vehicle-related particulate matter, but knowledge of its emission features and determining factors is still highly insufficient. Here, brake dynamometer experiments were conducted under controlled variables tests and real-world driving conditions to systematically investigate brake wear particle (BWP) emission. Compared to the decelerating process, the separating of pads and disc releases more BWPs, accounting for 47-76% of the total PM_2.5 mass. Particle number and mass distributions exhibit bimodal (< 0.01 µm and 0.8-1.2 µm) and unimodal (2-5 µm) patterns, respectively. Larger speed reduction exponentially amplifies BWP emission, and the significant enhancement of nanoparticles is proved to be related to the evaporation of organic constituents in the pads with threshold ranging from 170 °C to 270 °C. Emissions from front and rear brake assemblies don't agree with braking torque distribution, mainly attributive to the different braking pressures. A parameterization scheme for BWP emission based on kinetic energy loss is further established and proved to sufficiently predict the variation of BWP under real-world driving conditions. Being corrected by 1.8th power of the initial speed, the scheme improves the prediction.

A Review of Road Traffic-Derived Non-Exhaust Particles: Emissions, Physicochemical Characteristics, Health Risks, and Mitigation Measures

Implementation of regulatory standards has reduced exhaust emissions of particulate matter from road traffic substantially in the developed world. However, nonexhaust particle emissions arising from the wear of brakes, tires, and the road surface, together with the resuspension of road dust, are unregulated and exceed exhaust emissions in many jurisdictions. While knowledge of the sources of nonexhaust particles is fairly good, source-specific measurements of airborne concentrations are few, and studies of the toxicology and epidemiology do not give a clear picture of the health risk posed. This paper reviews the current state of knowledge, with a strong focus on health-related research, highlighting areas where further research is an essential prerequisite for developing focused policy responses to nonexhaust particles.

Revisiting Total Particle Number Measurements for Vehicle Exhaust Regulations

Road transport significantly contributes to air pollution in cities. Emission regulations have led to significantly reduced emissions in modern vehicles. Particle emissions are controlled by a particulate matter (PM) mass and a solid particle number (SPN) limit. There are concerns that the SPN limit does not effectively control all relevant particulate species and there are instances of semi-volatile particle emissions that are order of magnitudes higher than the SPN emission levels. This overview discusses whether a new metric (total particles, i.e., solids and volatiles) should be introduced for the effective regulation of vehicle emissions. Initially, it summarizes recent findings on the contribution of road transport to particle number concentration levels in cities. Then, both solid and total particle emission levels from modern vehicles are presented and the adverse health effects of solid and volatile particles are briefly discussed. Finally, the open issues regarding an appropriate methodology (sampling and instrumentation) in order to achieve representative and reproducible results are summarized. The main finding of this overview is that, even though total particle sampling and quantification is feasible, details for its realization in a regulatory context are lacking. It is important to define the methodology details (sampling and dilution, measurement instrumentation, relevant sizes, etc.) and conduct inter-laboratory exercises to determine the reproducibility of a proposed method. It is also necessary to monitor the vehicle emissions according to the new method to understand current and possible future levels. With better understanding of the instances of formation of nucleation mode particles it will be possible to identify its culprits (e.g., fuel, lubricant, combustion, or aftertreatment operation). Then the appropriate solutions can be enforced and the right decisions can be taken on the need for new regulatory initiatives, for example the addition of total particles in the tailpipe, decrease of specific organic precursors, better control of inorganic precursors (e.g., NH3, SOx), or revision of fuel and lubricant specifications.