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Article

Comparative Study of the Morphology and Chemical Composition of Airborne Brake Particulate Matter from a Light-Duty Automotive and a Rail Sample

by
Andrea Pacino
1,
Antonino La Rocca
1,*,
Harold Ian Brookes
1,
Ephraim Haffner-Staton
1 and
Michael W. Fay
2
1
Department of Mechanical Materials and Manufacturing Engineering, University of Nottingham, Nottingham NG7 2RD, UK
2
Nanoscale and Microscale Research Centre, University of Nottingham, Nottingham NG7 2RD, UK
*
Author to whom correspondence should be addressed.
Atmosphere 2026, 17(1), 34; https://doi.org/10.3390/atmos17010034 (registering DOI)
Submission received: 29 October 2025 / Revised: 17 December 2025 / Accepted: 22 December 2025 / Published: 26 December 2025
(This article belongs to the Special Issue Non-Exhaust Vehicle Emissions: Measurement, Impacts, and Mitigation Strategies)
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Abstract

Brake particulate matter (PM) represents a significant portion of the non-exhaust related soot emissions from all forms of transport, posing significant environmental and health concerns. Euro 7 standards only regulate road automotive emissions, while no regulation covers train transportation. This study compares two brake PM samples from rail and automotive applications. Rail brake PM was generated from composite brake pads subjected to real-world urban rapid transit braking conditions, while automotive brake PM was generated using ECE brake pads and discs under World Harmonized Light-Duty Test Cycle (WLTC) conditions. Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analyses were performed to assess PM morphology and composition. Both samples showed PM in coarse (10–2.5 µm), fine (2.5–0.1 µm), and ultrafine (<0.1 µm) size ranges, with angular flakes in automotive PM and rounded particles in rail PM. The rail PM exhibited a uniform size distribution, with a mean Feret diameter of 1 µm. In contrast, the automotive PM shifted toward larger particles, with ultrafine PM representing only 4% of the population. Excluding carbon and oxygen, automotive PM was dominated by iron (6 at.%) and magnesium (1 at.%). Rail PM showed lower iron (0.6 at.%) and higher aluminium (0.7 at.%) and calcium (0.8 at.%), with a broader non-C/O composition. This study tackles source-specific PM features, thereby supporting safer and more efficient non-exhaust emissions regulations.

1. Introduction

Urban particulate matter (PM) emissions from road traffic and rail transport pose serious environmental and health implications [1]. Several epidemiological studies revealed that short and long-term exposure to PM is directly linked to cardiovascular and pulmonary diseases [2,3,4]. The 2019 estimate of premature mortality associated with fine particles over Europe was 904,000 deaths/year [5], with an associated cost of 158 billion euros/year [6]. The limits recommended by World Health Organization (WHO) guidelines are 5 µm/m3 for PM2.5 and 15 µm/m3 for PM10 [7]. Similarly, the EU has set average annual limits for PM2.5 and PM10 at 20 µm/m3 and 40 µm/m3, respectively [7]. However, over 80% of fine particle (PM2.5) emissions monitoring sites in the UK recorded levels that exceeded the WHO’s recommended limit [8]. Research to reduce PM emissions from all forms of transport is therefore required to meet national and international targets and lower their harmful effects.
Exhaust and non-exhaust emissions are the main sources of PM in urban areas [9]. Exhaust emissions are generated from the incomplete fuel combustion [10], whereas non-exhaust emissions (NNEs) include wear nanoparticles from automotive brakes, tyres, and roads [11]. In urban environments, brake wear can contribute up to 55% by mass of the total non-exhaust traffic-related PM10 emissions and up to 21% of total traffic-related PM10 emissions [12,13]. Exhaust PM emissions have been reduced through technological advancements, improving combustion and after-treatment efficiencies [14]. By contrast, recent research suggests that PM NNEs have increased as a share of total road transport PM emissions [15].
The EU commission has proposed the EURO 7 legislation to mitigate traffic-related pollutant emissions, including, for the first time, non-exhaust emissions from the brakes and tyre–road interactions [16]. This regulation sets brake particle emission limits of 7 mg/km/vehicle for 2025 and 3 mg/km/vehicle for 2035 during the standard driving cycle [17]. However, the legislative actions specifically address urban road PM emissions, while current legislation on rail transportation remains limited. Furthermore, existing standards focus on the particulate mass per unit volume of air, neglecting other PM properties, such as morphology and chemical composition. However, research has indicated a close relationship between PM properties and health issues [18,19,20]. For instance, PM2.5 could penetrate the blood–brain barrier and enter the circulatory system [18], while metals bound to it were found to be positively correlated with ischemic heart disease [19].
Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analysis are commonly employed to study PM properties. PM is composed of small carbon units called primary particles, bonded to form branched carbon aggregates [21]. PM can be generally divided into PM10 (<10 μm), PM2.5 (<2.5 μm), and ultrafine particles (<0.1 µm) [22]. Lyu et al. [23] employed TEM to study PM emissions from high-speed train brakes. They found particle peak sizes between 10 nm and 200 nm. Similarly, Abbasi et al. [24] observed particles in all size regimes, ranging from 10 nm to 32 μm in diameter. Liati used EDX to study the elemental composition of PM emitted from automotive brakes [25]. They observed a diversified composition, including aluminium, tin, titanium, and manganese. By contrast, Namgung found that rail brake-emitted particles were dominantly composed of iron, copper, and chromium [26]. These are commonly attributed to the friction between rail, brakes, and wheel, accounting for 84% of the total traffic PM emissions in rail stations [26]. According to Karlsson, PM particles in subway stations are eight times more genotoxic than road particles, causing mutations and damage to genetic information [27]. Further research on PM composition from rail transport is therefore essential to mitigate its impact on human health.
PM size and composition, however, are highly affected by the test conditions. Studies indicate that wear rate drastically increases with temperature, brake pressure, and speed [23,28], while PM composition is significantly affected by the brake disc-pad materials and humidity [29]. Lyu observed a progressive increase in particle mass and number concentrations with sliding speed during pin-on-disc tribometer experiments on high-speed train brakes [23]. Olofsson et al. [30] found that a higher braking force leads to an increase in the wear rate and particle formation. Namgung reported a reduction in PM size at the contact temperature of 70 °C on rail–wheel interfaces [26]. However, Octau et al. [31] observed a drastic increase in the number concentrations of ultrafine particles only for a transition temperature above 230 °C. The complexity is further exacerbated by different wear mechanisms associated with the disc brakes [32], with the number and size distribution of the suspended particles being influenced by the prevailing type of wear [33]. Lastly, studies vary between laboratory tests and in-field trials, balancing data repeatability with simulating real-world breaking conditions. This further complicate the comparison between individual studies.
The knowledge gained so far suggests a discrepancy in wear results found in the research. Moreover, literature to date is often focused on automotive brake PM, with limited research on rail transport. Primary focus is given to contact tribology with limited emphasis on PM morphology and chemical composition. PM features are often measured using analytical techniques. However, multiple setups and conditions have been shown to affect sample comparability. To the authors’ best knowledge, no single study has yet provided a direct, side-by-side comparison of the chemical and morphological characteristics of brake particle emissions from rail and automotive sources using a common sampling, measurement methodology, and instrumentation framework.
This study aims to address these gaps by undertaking a comprehensive investigation of PM wear particles emitted from automotive and rail brakes. The morphological features of PM were quantitatively evaluated by TEM analysis. EDX was employed to assess the chemical composition. This work interconnects the morphology and chemistry of PM particles. Aspects that are frequently considered when examining the PM impact on human health. These features are presented in an attempt to assess the differences and similarities across PM source emissions observations reported in the literature.

2. Materials and Methods

2.1. Testing Conditions

A brake assembly from a light-duty passenger vehicle was employed in this investigation. The brake assembly was driven according to 30 min repetitions of the Worldwide Harmonised Light Vehicles Test Cycle for a class 3b vehicle to simulate brake PM emissions under real-world driving conditions [34]. A Tallano’s TAMIC brake particles collection system (Tallano Technologies, Paris, France), specifically designed for grooved pads, was used to trap brake particles directly at the pad–disc interface [35]. The system includes a brake calliper and an aspiration system, powered by a turbine with a high-efficiency filter and pipes connected to it. When the brake is applied, an electric motor drives the Tamic® system. The turbine operates two suction channels within the brake callipers, which carry the trapped brake PM into the Tamic® filter where they are stored. A more detailed description of the filter is provided in [35]. A collection efficiency up to 90% has been previously reported. ECE-brake pads and discs were used in this study.
Rail brake PM was generated from composite brake pads tested under conditions simulating the Réseau Express Régional (RER) Line C train operation. The RER is a rapid transit system comprising a regional express network that services Paris and the surrounding area [36]. During the tested journey cycle, the maximum recorded brake application speed was 120 km/h. The corresponding maximum peak braking temperature was 150 °C. Multiple journey cycles were reiterated for rail brake PM generation to obtain sufficient yield.
The brake blocks and pad materials used in this investigation reflect those employed in modern braking systems and, therefore, are considered representative of current rail and automotive applications, respectively [16,37].

2.2. TEM and EDX Analyses

Following the work of Sinha [38], 0.5 mg of each brake PM sample was suspended in acetone and subjected to a 15 min sonication process. Acetone was used to disperse brake particulate matter for single particle analysis by TEM. Sonication was employed to split any loosely bound aggregates formed during the sampling process. Small amounts of PM suspension in acetone were transferred onto carbon-coated TEM grids. Upon deposition, the acetone evaporated rapidly, leaving the brake PM on the grid. The process was done twice to maximize brake PM deposition on the grid surface. The TEM copper mesh grid consists of a graphene oxide support film and a holey amorphous carbon layer. TEM images were obtained using a JEOL 2100F TEM microscope (JEOL (U.K.) Ltd., Welwyn Garden City, UK) equipped with a Gatan Orius CCD camera (Gatan, Inc., Pleasanton, CA, USA), from the Nanoscale and Microscale Research Centre (nmRC) at the University of Nottingham. An incident electron beam set to a voltage of 200 kV was used to image the PM with magnifications ranging from 50× to 500,000×. Standard bright-field imaging mode (BF-TEM) was used to study the morphology of the brake PM. The annular dark-field scanning transmission electron microscopy (ADF-STEM) mode was instead employed for specific particle characterization. In the BF-TEM mode (Figure 1a,e,f, for example), darker regions of the PM corresponded to the presence of either a thicker area or denser materials. The opposite is true for the regions observed on ADF-STEM mode (Figure 1b–d,g–f, for example). EDX was performed using an Oxford Instruments X-MaxN 80T (Oxford Instruments, Oxfordshire, UK). These settings are consistent with those previously applied by Pfau [39].

2.3. SEM Analysis

Two samples were prepared for imaging, with the first prepared being a section of the automotive TAMIC® filter. The sample was surrounded by conductive tape to prevent filter fibres in the sample from becoming charged during imaging. This sample was used to assess any potential interaction of brake PM with the filter. During electron microscopy, interactions may indeed result from the electrostatic charging of the particulate matter and filter fibres under the electron beam, potentially causing particle movement or agglomeration. To mitigate these effects, conductive tape was applied to provide an effective grounding path and minimize charge accumulation on the filter substrate. The second sample, extracted from a different region of the filter’s surface, was compared with the TEM images to determine the degree to which TEM preparation affected brake PM formation. Environmental factors, including humidity, may also influence particle size distribution. Therefore, before analysis, both samples were exposed to near-vacuum environments to ensure stable and controlled imaging conditions. SEM images were obtained using a FEI 650 Quanta ESEM (Thermo Fisher Scientific, Waltham, MA, USA). An incident electron beam set to a voltage of 10 kV was used to image the PM with magnifications ranging from 100× to 30,000×. As per [38,40], secondary electron (SE) images were used to define the overall structure of PM and its interaction with filter fibres. Back-scattered electron (BSE) images defined the location of different elements in the spectrum investigated.

2.4. Morphological Analysis

PM morphology was investigated using ImageJ 1.51w Java 1.8.0_322, an open-source image processing software. A total of 70 TEM pictures were analysed for each sample to ensure representative coverage and to account for local variability across the TEM grid. Imaging was conducted at multiple magnifications, ranging from 50× to 2,000,000×. This enabled the assessment of overall PM features as well as detailed observations of aggregates and individual particle morphologies. Multiple grid locations were examined for each sample to further minimize spatial bias. The morphological properties of the PM particles were quantified using manual particle detection in ImageJ. For each sample, 50 individual PM particles were measured, and their projected area, Feret diameter, and circularity were recorded. Basic descriptive statistics (mean and standard deviation) were calculated. To assess whether the observed differences between automotive and rail PM were statistically significant, non-parametric Mann–Whitney tests were applied. This test was selected because it does not require assumptions on data distribution, enabling comparisons between two datasets [41]. The statistical analyses were two-tailed tests. Values were considered significant at p < 0.05.

3. Results

Figure 1 shows the TEM images of PM particles in the coarse fraction from the automotive and rail samples. Both samples exhibited particles ranging from 2.5 μm to 10 μm in diameter, with some exceeding 10 μm (Figure 1a). Flat flakes of about 3.5 μm in diameter, with significant aggregation of small particles (0.5–1.5 μm), were predominantly detected in the automotive sample, resulting in asymmetrical and irregular structures (Figure 1a–d). Singular flake materials contribute to the sharp edges of the particles (Figure 1b,c), whereas the aggregation of individual spherule-like PM particles results in the formation of more rounded PM structures (Figure 1d).
Aggregates containing flat flakes were less frequently identified in the rail PM (Figure 1f). The agglomeration of fine and ultrafine PM dominated the formation of coarse particles in this sample (Figure 1g,h). Coarse PM from both samples exhibited surface aggregation of fine and ultrafine particles attached to the surface of coarse PM (Figure 1b–d,g,h).
A preliminary EDX analysis was conducted on different PM particles, and examples of common elements detected are shown in Figure 2d,g,h. Aggregates in both samples displayed a heterogeneous chemical composition, with elements unevenly distributed within the PM structures. The chemical composition of the automotive samples differed from that of the rail samples in the coarse fraction. Fe was the dominant element in the automotive brake PM, with small amounts of Cu, Si, S, Al, and Sn. Traces of Ca, Cr, Mg, and Mo were also identified. In contrast, the rail PM was primarily composed of a mixture of Al, Ba, Ca, Cu, Mg, and Si, followed by Fe, C, and S.
The fine fraction of the automotive brake PM consists of particles ranging from 0.1 μm to 2.5 μm in diameter. The particles predominantly appear as aggregates of agglomerated formations across the entire size range. PM surfaces are mostly smooth and rounded (Figure 2a–b). Single particles are rare. The rounded morphology suggests that fine particles were produced by thermal processes occurring at elevated braking temperatures. This is supported by the chemical composition analysis, which shows an increased oxygen concentration within the fine fraction, suggesting that oxidation is the dominant formation mechanism [42]. The chemical composition slightly differs between the coarse and fine fractions of the automotive PM. Iron is the most abundant element, followed by Mg, Al, S, Si, Cu, and Sn, while Ca, Cr, and Zn occur in trace amounts.
The rail brake PM exhibits fine particles in the same size range (0.1–2.5 μm) but with a distinct morphology. Individual flakes and agglomerates of 0.5 μm in diameter are frequently observed (Figure 2h). Aggregation of both single particles and agglomerates occurs throughout the size range. Individual particles incorporated within larger aggregates are also found (Figure 2e–f). Similar to the automotive sample, the particles exhibit rounded shapes, indicative of oxidation-related formation at high temperature. The chemical composition contains Al, Ca, Cu, Mg, Na, and Si, followed by S, K, Ba, Cl, and Fe, with trace amounts of Zn, C, and Sn. The elemental composition agrees with that expected from composite brake pad materials.
Both the automotive and rail samples contained ultrafine particles. In both cases, the ultrafine PM formed larger single agglomerates composed of smaller ultrafine constituents (Figure 3a,b,f). In addition to these, the rail PM also showed the presence of individual particles (Figure 3e) and the aggregation of single particles into small clusters (Figure 3g). Crystalline patterns embedded in the PM nanostructure can also be observed in the automotive sample (Figure 3d). The elemental composition of the ultrafine PM in both samples did not differ significantly from that observed in the other size ranges.
SEM imaging revealed that the morphology of brake PM is unaffected by the TEM sample preparation procedure (Figure 4a–c). Particles tend to form aggregates in the tens of microns in size (Figure 4f). SEM-EDX analysis identified the same elements in the coarse fraction as those detected by TEM-EDX (Figure 4e). However, a high carbon signal was recorded in the SEM-EDX spectra due to the predominantly carbon-based filter fibres, introducing uncertainty in the actual carbon content of the sample.
Figure 4d–f show the interaction of brake PM with the TAMIC® filter. PM adheres to the filter fibre surfaces through electrostatic attraction. Fine and ultrafine particles adhered more readily to the fibre surfaces, whereas coarse particles were only occasionally fully attached to it (Figure 4f).
The particle size distributions for both samples are presented in Figure 5. In the rail sample, particle sizes range from 50 nm (ultrafine mode) to over 2500 nm (coarse mode). The distribution is relatively uniform across the entire size spectrum, with a mean Feret diameter of 1 µm. In contrast, the automotive sample exhibits a clear shift toward larger particle sizes. Particles within the 0.5–1 µm range are the most frequent, whereas ultrafine particles account for only 4% of the total population. A statistically significant difference in Feret diameter was observed between the two samples (Mann–Whitney test, p < 0.05), indicating a greater tendency for particle agglomeration in the automotive-derived PM.
The average elemental composition of the two samples is presented in Figure 6. Because the TEM grid introduces additional carbon and oxygen signals from the supporting substrate, only elements other than C and O were considered in the analysis. In the automotive sample, EDX measurements revealed iron (6 at.%) and magnesium (1 at.%) as the predominant elements. This is consistent with the materials used in the corresponding brake pads and discs. Minor quantities of aluminium, silicon, sulphur, calcium, zinc, barium, and sodium were also detected, together accounting for 2 at.% of the total PM composition. In contrast, the rail sample exhibited a more heterogeneous elemental signature. The iron content was an order of magnitude lower (0.6 at.%), while aluminium (0.7 at.%) and calcium (0.8 at.%) were present at higher levels compared with the automotive sample. Overall, elements other than carbon and oxygen represented approximately 7 at.% of the total PM composition in this sample. Detailed elemental compositions for particles across the coarse, fine, and ultrafine size fractions for both samples are provided in Supplementary Materials File. Table 1 summarises the chemical composition of the analysed particles from the three samples.

4. Discussion

This study compared the morphology and chemical composition of brake PM particles from an automotive sample and a rail sample under real-world testing scenarios. A specific brake particle collection system was utilized to trap brake particles directly at the pad–disc interface. PM particles were analysed via TEM and EDX methods.
Based on TEM images, the morphology of brake particles in the coarse size range differs between the two samples. The automotive sample showed more aggregates with irregular and rough shapes (Figure 1a–c) compared to those obtained during rail testing (Figure 1e,g). This might be attributed to the difference in the average disk temperature and the dominance of the mechanical formation process [43]. Average front disc temperatures during WLTC testing are typically reported to be in the 85–180 °C range [44]. Subway and metro transportations are instead characterised by frequent stops and larger vehicle mass, leading to brake disc surface temperatures often exceeding 300 °C [45]. Consequently, for the automotive sample (subjected to lower disc temperatures during testing), the friction materials might not undergo thermal softening or melting. Thus, particle formation is mainly driven by mechanical wear [43]. The materials fracture into big, sharp pieces, which eventually results in particles with more uneven and rougher structures. Conversely, because of the higher disc temperatures, thermal degradation is more noticeable on the rail sample. The heat softens or melts the friction materials, resulting in the formation of smoother and more spherical particles. As a result, these particles are less prone to agglomeration [46], resulting in more evenly distributed individual particles.
These findings are consistent with those reported in previous studies [25,46]. Al Wasif-Ruiz [46] observed that brake particle morphology varied significantly with disk temperature, going from irregular-shaped PM flakes (at 257 °C) to more rounded PM particles (at 307 °C). The study indicated that at the lowest disc temperature, particles typically appear in aggregate formations, with single particles occurring rather infrequently. Similarly, Liati [25], who investigated the morphological properties of automotive brake PM at disk temperatures ranging from 150 °C to 300 °C, observed that isolated, non-agglomerated particles are extremely rare.
Collectively, these temperature-controlled investigations offer a comparative framework that enhances the interpretation suggested herein: although disc temperature was not directly measured in the present work, the observed morphological differences align with the trend documented under clearly specified thermal conditions in the existing literature.
Therefore, a similar correlation between disc temperature and PM morphological properties can be anticipated in the present work, which may ultimately explain the observed morphological differences among the samples.
Rounded, nearly spherical particles were also observed in the ultrafine size range (Figure 3c,e). According to Woo [43] and Liati [25], these are due to the volatilization and nucleation occurring under harsh braking conditions. These particles may also combine into larger fractal-like PM structures (Figure 3b).
EDX analyses revealed that the chemical composition is slightly different among the samples. PM particles are mainly composed of oxygen and carbon. However, the most abundant metallic element in the automotive brake PM sample was iron (Fe), followed by small quantities of magnesium (Mg) and the non-metallic silicon (Si). Traces of other elements (<1 wt.%) included sulphur (S), calcium (Ca), chromium (Cr), and zinc (Zn). Iron is the main component of the brake disc materials [16], while Mg and Si commonly originate from the brake pads [47]. Conversely, a more diverse chemical composition was found in the brake rail sample. PM particles are mainly composed of carbonaceous material with traces of elements consistent with the chemical composition of the brake pad and brake disc [23]. Aluminium and iron mainly originate from brake discs, while calcium, zinc, and magnesium come from the brake pad [48]. Silicon was also identified. This element, present as silicon carbide, is often reported in the brake disc and pad composition [23]. These findings are in agreement with Lyu [23] and Cheng [49], who found a similar composition of the emitted train brake particles.
Thus, a clear distinction in particulate shape and composition between the automotive and the rail PM samples has emerged from this study. These properties have important implications for both environmental and health risks. Larger irregular PM particles are inclined to settle in the respiratory system, potentially causing inflammation and irritation [50]. Conversely, small and rounded nanoparticles from the automotive brake may penetrate the blood–brain barrier, triggering significantly more health hazards [51]. Moreover, small and rounded PM particles remain suspended for a longer duration than their irregularly shaped rail counterparts [52]. This is supported by Okamoto [53], who reported similar metallic species in airborne particles to those detected in the current investigation, suggesting that brake PM is a major source of emission in subway environments. As a result, a more pronounced sedimentation tendency is anticipated for the rail brake PM.
Additional health risks arise from the presence of metals and their oxides in the PM particles. For instance, iron and copper oxides were found to be detrimental to cell damage [53]. Metals such as Zn, Al, and Ca have been associated with increased monthly mortality [27,50,54]. Thus, the heterogeneity found in brake PM composition may also further exacerbate the adverse health impacts to humans.
Previous research has also demonstrated that metal oxides can induce cellular damage, with markedly different biological effects depending on their oxidation state [53,55]. As this work focuses on comparing the morphological and elemental characteristics of rail and automotive brake PM, detailed toxicological analysis of chemical speciation lies beyond its scope. Nevertheless, future work should include chemical state analysis to better evaluate the health implications of brake-derived PM.
This study suggests that, when it comes to brake rail PM, a more detailed analysis of PM shape and chemical state should be considered for future brake PM legislation.
Moreover, the results obtained from the SEM analysis suggest that the setup and TEM methodology employed in the present study do not affect the PM morphological properties. This methodology, therefore, allows us to assess their true impact during real-world operation.

5. Conclusions

In this work, the morphological and chemical characteristics of brake PM particles from rail and automotive applications were widely investigated. The aim was to compare particulates coming from different sources, emphasising the main similarities and distinctions among the samples. The main conclusions can be summarised as follows:
-
The morphological analysis revealed that PM particles primarily form aggregates in both samples, with coarse, fine, and ultrafine sizes. However, differences were observed in particle shape, with larger, angular flakes found in automotive PM and smaller, rounded particles in rail PM. In the former, 16% of particles exceeded 2.5 µm, whereas the latter showed particles down to 50 nm in diameter. Past investigations suggested that disc temperature affects the PM shape, resulting in the formation of smaller, smoother, and more spherical particles in the rail PM. Although temperature was not directly measured here, this assumption aligns with trends reported in earlier studies.
-
EDX investigations revealed that particles are mainly composed of oxygen (~65 at.%) and carbon (~25 at.%). The chemical composition of brake PM from each sample is consistent with the elements from their respective brake pads and discs. Iron is the most abundant element in the automotive PM (7 at.%), followed by magnesium (1 at.%). Conversely, the rail PM showed lower iron (0.6 at.%) and higher aluminium (0.7 at.%) and calcium (0.8 at.%), with a broader non-C/O composition.
-
SEM-EDX analysis on automotive PM proved that TEM-EDX preparation did not affect the natural formation of brake PM in the coarse and fine size ranges. Fine and ultrafine PM adhered to the filter fibres better than coarse PM. The strong match observed between TEM observations and SEM results suggests that this methodology can be adopted for reliable characterisation of particle emissions in real-world scenarios.
-
The practical implication of this work is the necessity to account for source-specific morphological and chemical characteristics of particulate matter for the development of future non-exhaust emission regulations.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/atmos17010034/s1.

Author Contributions

Conceptualisation, A.P. and A.L.R.; methodology, M.W.F., H.I.B. and E.H.-S.; validation, A.P. and A.L.R.; formal analysis, H.I.B.; investigation, H.I.B. and E.H.-S.; resources, A.L.R.; data curation, A.L.R. and A.P.; writing—original draft preparation, H.I.B., and A.P.; writing—review and editing, A.L.R. and M.W.F.; visualisation, A.P.; supervision, A.L.R.; project administration, A.L.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Engineering and Physical Sciences Research Council (grant number EP/M506588/1) through the scholarship provided by the EPSRC DTG Centre in Complex Systems and Processes for one of the authors.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Acknowledgments

The authors would also like to thank the Nanoscale and Microscale Research Centre (nmRC) of the University of Nottingham for providing access to instrumentation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Atmosphere 17 00034 g001
Figure 1. BF-TEM and ADF-TEM images showing common examples of coarse PM observed from the automotive (a–d) and rail sample (e–h). Images were captured at magnifications from 250× to 100,000×. Physical formation characteristics are shown in the figures using either arrows or circles and are referred to in the text.
Figure 1. BF-TEM and ADF-TEM images showing common examples of coarse PM observed from the automotive (a–d) and rail sample (e–h). Images were captured at magnifications from 250× to 100,000×. Physical formation characteristics are shown in the figures using either arrows or circles and are referred to in the text.
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Figure 2. BF-TEM and ADF-TEM images of fine PM observed from the automotive (a–d) and rail samples (e–h). Images were captured at magnifications from 10,000× to 100,000×.
Figure 2. BF-TEM and ADF-TEM images of fine PM observed from the automotive (a–d) and rail samples (e–h). Images were captured at magnifications from 10,000× to 100,000×.
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Atmosphere 17 00034 g003
Figure 3. BF-TEM and ADF-TEM images showing common examples of ultrafine PM observed from the automotive (a–d) and rail samples (e–h). Images were captured at magnifications from 10,000× to 500,000×.
Figure 3. BF-TEM and ADF-TEM images showing common examples of ultrafine PM observed from the automotive (a–d) and rail samples (e–h). Images were captured at magnifications from 10,000× to 500,000×.
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Figure 4. SEM of the aggregates from the automotive filter sample: (a–c) in situ brake PM; (d–f) interaction of the brake PM with the TAMIC® filter; (e) EDX chemical mapping of PM-loaded filter (spectra S1 and S2); (f) interaction of coarse and fine PM on a single filter fibre. Images were captured at magnifications from 1000× to 26,000×.
Figure 4. SEM of the aggregates from the automotive filter sample: (a–c) in situ brake PM; (d–f) interaction of the brake PM with the TAMIC® filter; (e) EDX chemical mapping of PM-loaded filter (spectra S1 and S2); (f) interaction of coarse and fine PM on a single filter fibre. Images were captured at magnifications from 1000× to 26,000×.
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Figure 5. Particle size distribution of brake-derived particulate matter (PM) for the automotive and rail samples. The rail PM displays a broad size. In contrast, the automotive PM exhibits a shift toward larger particle sizes.
Figure 5. Particle size distribution of brake-derived particulate matter (PM) for the automotive and rail samples. The rail PM displays a broad size. In contrast, the automotive PM exhibits a shift toward larger particle sizes.
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Figure 6. Average elemental composition (at.%) of brake particulate matter from the automotive and rail samples, excluding carbon and oxygen contributions, as they are affected by the TEM grid.
Figure 6. Average elemental composition (at.%) of brake particulate matter from the automotive and rail samples, excluding carbon and oxygen contributions, as they are affected by the TEM grid.
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Table 1. Chemical composition of the brake PM particles observed from the automotive, rail, and automotive filter samples in the coarse, fine, and ultrafine size ranges.
Table 1. Chemical composition of the brake PM particles observed from the automotive, rail, and automotive filter samples in the coarse, fine, and ultrafine size ranges.
PM SizeAutomotive SampleRail SampleFilter
Coarse (10–2.5 µm)
Most common

Least common		Atmosphere 17 00034 i001Fe
Cu, Si, S
Al, Mn
Ca, Cr, Mg, Mo		Al, Ba, Ca, Cu, Mg, Si, Fe, S
Zn
Sr		Fe
Cu, Al
S
Fine (2.5–0.1 µm)
Most common

Least common		Atmosphere 17 00034 i002Fe
Mg, Al,
S, Si, Cu, Sn
Ca, Cr, Cu, Zn		Al, Ca, Cu, Mg, Na, Si
K, S
Ba, Cl, Fe, Zn
Sr		Fe
Al, Zn
Ultrafine (<0.1 µm)
Most common

Least common		Atmosphere 17 00034 i003Fe
Mg, S, Si, Sn
Cr, Cu, Zn		Al, Ba, Ca, Mg, Na, Si
Fe, Cu, S
K		-
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MDPI and ACS Style

Pacino, A.; La Rocca, A.; Brookes, H.I.; Haffner-Staton, E.; Fay, M.W. Comparative Study of the Morphology and Chemical Composition of Airborne Brake Particulate Matter from a Light-Duty Automotive and a Rail Sample. Atmosphere 2026, 17, 34. https://doi.org/10.3390/atmos17010034

AMA Style

Pacino A, La Rocca A, Brookes HI, Haffner-Staton E, Fay MW. Comparative Study of the Morphology and Chemical Composition of Airborne Brake Particulate Matter from a Light-Duty Automotive and a Rail Sample. Atmosphere. 2026; 17(1):34. https://doi.org/10.3390/atmos17010034

Chicago/Turabian Style

Pacino, Andrea, Antonino La Rocca, Harold Ian Brookes, Ephraim Haffner-Staton, and Michael W. Fay. 2026. "Comparative Study of the Morphology and Chemical Composition of Airborne Brake Particulate Matter from a Light-Duty Automotive and a Rail Sample" Atmosphere 17, no. 1: 34. https://doi.org/10.3390/atmos17010034

APA Style

Pacino, A., La Rocca, A., Brookes, H. I., Haffner-Staton, E., & Fay, M. W. (2026). Comparative Study of the Morphology and Chemical Composition of Airborne Brake Particulate Matter from a Light-Duty Automotive and a Rail Sample. Atmosphere, 17(1), 34. https://doi.org/10.3390/atmos17010034

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