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Article

Emission Characteristics of Tyre Wear Particles from Light-Duty Vehicles

1
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
2
CATARC Automotive Test Center (Tianjin) Co., Ltd., Tianjin 300071, China
*
Author to whom correspondence should be addressed.
Atmosphere 2023, 14(4), 724; https://doi.org/10.3390/atmos14040724
Submission received: 30 March 2023 / Revised: 12 April 2023 / Accepted: 12 April 2023 / Published: 17 April 2023
(This article belongs to the Special Issue Traffic Related Emission)

Abstract

:
Tyre wear particle emissions have gained significant attention due to their harmful effects on the environment and human health. However, studies on tyre wear particles generated under chassis dynamometer conditions are still scarce. This study measures the instantaneous number concentrations and elemental species of tyre wear particles in different light-duty vehicle test cycles. The results show that the particle number (PN) concentrations of the US06 test cycle are much higher than those of the WLTC test cycle due to the larger and more frequent accelerations and decelerations in the former. High PN concentrations are observed during high driving speeds with rapid accelerations, while PN concentrations are much lower during low driving speed with rapid acceleration. Furthermore, tyre tread temperature is found to be related to the formation of tyre wear particles. The PN concentration in the second and third US06 test cycles are similar, indicating that once the tyre temperature exceeds the critical value, the tyres become heated to a steady state, and the PN concentrations will not be affected by the average temperature of the tyre. A low initial tyre temperature can produce high PN concentrations during the cold start phase of test cycles and prolong the time required for tyres to warm up. In addition, the particles contained a high mass fraction of Zn, which can serve as a tracer of tyre wear particles in non-exhaust particle tests of vehicles.

1. Introduction

Transportation is widely recognized as a significant contributor to urban particulate matter emissions [1,2,3]. Exposure to vehicular particle emissions has been associated with various human adverse health effects [1,4,5]. As a result, there is a global effort to reduce vehicular particle emissions. Vehicular particles typically originate from exhaust and non-exhaust sources. Exhaust particle emissions are well controlled due to the extremely strict emission regulations in most countries [6,7]. Available data indicate that the exhaust particle emissions of vehicles have been substantially reduced or even eliminated to zero with the wide application of particulate filters and the growth of new energy vehicles [8,9,10]. On the other side, non-exhaust particle emissions have few concerns. Non-exhaust particles are gradually dominating vehicular particle emissions, and their contribution is increasing due to the continuous reduction in exhaust emissions [5,11,12].
Tyre wear particles are a significant component of non-exhaust particle emissions, as supported by various studies [13,14,15]. Numerous methods have been utilized to measure tyre wear particles. One approach involves directly collecting tyre wear particle samples from real road situations [16,17,18]. Kreider et al. designed two on-vehicle tyre wear particle collecting devices, one for a passenger car and the other for a truck [19]. Each collection device consisted of an aspiration system attached to the rear left tire hub close to the tire. Two driving circuits were used to represent typical driving commutes in Europe. All roads used to generate the particles were constructed using asphalt. Pirjola et al. employed two different inlet systems on the test vehicle opening towards the driving direction to collect particle samples [20]. Particle number concentrations were measured by two ELPIs, one of which measured street dust particles behind the left tyre, while the other measured background particles via the chasing inlet in front of the van. Mathissen et al. installed five stainless steel sampling tubes inside the front right wheel house of the test vehicle and strictly defined the conditions of the proving ground, including varying road surfaces, traffic influence of other vehicles, and the length of the straight track [21]. Nevertheless, it is challenging to avoid mixing tyre wear particles with other particles from non-negligible sources under real on-road driving situations.
Some studies employed various tyre durability benches to generate tyre wear particles [22,23,24]. Foitzik et al. designed an inner drum tire test bench to investigate rubber tire particle emissions [25]. The tested tire ran at the lowest point of a circular track with a horizontal rotation axle and an inner diameter of 3.8 m. The test rig was enclosed to ensure thermal stability and separation from the environment. To prepare the track surface, it was manually processed with an angle grinder to smooth the surface and reduce its roughness. Gustafsson et al. used a road simulator to generate wear particles from studded and friction tyres running on two different pavements [26]. Sampling particles in the simulator hall made it possible to sample wear particles with low contamination from surrounding sources with no influence from exhaust emissions. Kim and Lee developed a tyre simulator consisting of a rotating drum, a test tyre, and a control system [27]. The tyre simulator can control lateral load, drum speed, tyre speed, and slip speed. The tyre simulator is operated within an enclosing chamber equipped with a series of sampling ports. It is important to point out that these tyre simulators cannot directly replicate real-world driving conditions since the tyre contact stress, direction of the load transfer, and aerodynamics in the chamber differ from those of real driving conditions.
Driving vehicles on the chassis dynamometer to produce tyre wear particles seems to be a suitable method that can strike a balance between eliminating environmental distractions and matching realistic driving conditions [18,28]. Kwak et al. performed a road simulator study to characterize the properties of tyre wear particles [29]. In the road simulation, the front tyres of test vehicles are secured to the roller of a chassis dynamometer. The surface of the roller has medium-coarse abrasiveness to simulate asphalt pavements. Sampling inlets are positioned close to the tyre/road interface to measure the particles sampled from the front tyre, as well as close to the brake pad to sample particles produced by brake wear during braking conditions. It should not be ignored that this method places high demands on the simulation of parameters such as vehicle driving conditions and environmental conditions.
Moreover, a series of studies has investigated various aspects of tyre wear particle emission characteristics including morphology [26,30,31,32], chemical compositions [17,23,33,34], size distribution [16,35,36], total PN concentrations [21,29], and mass emission factors [11,37,38,39]. Dahl et al. conducted a study using a tyre durability bench to generate tyre wear particles and identified four major particle structures: single near-spherical particles, spotted particles appearing to float on the substrate, particle inclusion of one or a few near-spherical particles, and particles completely floating on the substrate [40]. Sommer et al. conducted a study using single-particle analysis via SEM-EDX to examine coarse-sized ambient aerosols from two highly trafficked motorways and one federal highway passing through an urban area [30]. The study found that super-coarse tyre wear particles were encrusted by road dust, with encrustment ranging from partial under fluent traffic conditions to complete under stop-and-go conditions. The micro-plastics derived from tyres consist not only of the original rubber core with various additives but also of potentially hazardous metals and metalloids contained in attached brake-abrasion particles. Yan et al. also analyzed the morphology of tyre wear particles obtained from different stages in the collision samplers [32]. In the images, the particles showed long strips that are thick at one end and thin at the other end, proving that the particles are stretched and eventually broken due to adhesion during the micro-vibration process.
Aatmeeyata and Sharma measured polyaromatic hydrocarbons, elemental carbon, and organic carbon emissions of tyre wear particles [22]. The study found that all the pollutants increase with mileage. Pyrene is observed to be the highest substance of all PAHs. Zhang et al. quantified the elemental species of non-exhaust particles collected from four tunnel experiments in four Chinese megacities [13]. They showed that Fe, Ba, and Zr were relatively abundant in brake wear particles, while Si and Zn were key tracers for tyre wear. Source apportionment analysis revealed that the non-exhaust sources (road dust, tyre wear, and brake wear) not only dominate the vehicular PM10 emissions, but also significantly contribute to the PM2.5 emissions. The variations of non-exhaust emissions in different tunnels were found to be mainly caused by fleet composition, road gradient, and pavement roughness. Dall’Osto et al. aimed to characterize tyre wear particles from field studies using aerosol time-of-flight mass spectrometry (ATOFMS) [23]. Two large ATOFMS datasets collected from multiple outdoor studies were analyzed. The results indicated that the majority of tyre wear particles found in the road dust and atmospheric samples were internally mixed with metals (Li, Na, Ca, Fe, Ti), as well as phosphate. The study concluded that the interaction of tyres with the road surface creates particles that are internally mixed from two sources: tyre rubber and road surface materials.
Alves et al. examined the size distribution of tyre wear particles [41]. The PN concentration is dominated by particles smaller than 0.5 μm, while most of the total particle mass is found in particles that are larger than 0.5 μm. Park et al. discovered that more than 60% of tyre wear particles were smaller than 50 nm in diameter, and a large number of particles smaller than 100 nm were produced under tyre slip conditions [24]. Chang et al. observed that the size distribution of tyre wear particles followed an approximately normal distribution, except for wet wear conditions [36]. Tyres with good wear resistance generated fewer particles, and the particle size decreased with an increase in road roughness. Under wet lubrication conditions, the number of fine wear particles smaller than 10 μm was significantly higher, and no particles larger than 300 μm were observed. Grigoratos et al. tested the physical properties of tyre wear particles using a road simulator [39]. The results showed that PM10 concentrations displayed a similar pattern in each run, with quickly increasing concentrations followed by a quasi-stable level, while PM2.5 concentrations continuously increased without reaching a stable level. PN concentrations behaved more randomly than the mass concentration with generally low values. Approximately 50% of emitted PM10 fell within the size range of fine particles, while PN size distributions were dominated by ultrafine particles and most often peaked at 20–30 nm. The tread mass loss ranged from 55 to 212 mg/km.
Accurate information about the emissions and chemical properties of tyre wear particles is crucial not only for understanding their potential health impacts, but also for developing targeted mitigation strategies. While many studies have investigated the characteristics of tyre wear particles under on-road driving conditions or using tyre durability benches, there is still a lack of research focused on particles generated under chassis dynamometer driving conditions. This study measures instantaneous number concentrations and elemental species of tyre wear particles generated during different test cycles for light-duty vehicles on the chassis dynamometer. The critical conditions of tyre wear particle generation are also discussed.

2. Apparatus and Methodology

2.1. Experiment System and Sampling Method

The test system, as illustrated in Figure 1, consists of a direct-current chassis dynamometer to run the test cycle, a DEKATI E-filter (Dekati, Ltd., Kangasala, Finland) for tyre wear particle collection and particle number measurement, and a KNF pump (KNF, Freiburg im Breisgau, Germany) to provide a consistent 10 L/min intake flow to the sampling device. To ensure that the exhaust particles do not influence the tyre wear particle measurement, the exhaust system of the test vehicle is connected out of the test room to the CVS system. The particle sampling point is positioned around 15 cm behind the centerline of the right front wheel. The DEKATI E-filter can measure the instantaneous concentration of particles in the form of current intensity, and collect particles using a 47 mm diameter Teflon filter. More details of the E-filter are shown in Table 1.
The US06 and WLTC test cycles are employed for tyre wear particle measurements following the same pre-processing. WLTC takes 30 min and consists of four speed sections ranging from the low-speed section to the ultra-high-speed section. The US06 cycle, which is more aggressive than the WLTC cycle, features more significant and frequent accelerations and decelerations, resulting in greater mechanical and thermal loads on the tyres. The US06 test cycle is repeated three times consecutively to ensure that the test time is the same among all the test conditions. Both test cycles run under cold start and hot start conditions. Hot start tests are conducted half an hour after the cold start tests, enabling a comparison between two different start temperatures of the vehicle components.
The inertia and road resistance data of the test vehicles on the dynamometer roller are set according to their reference mass (RM), and are confirmed by the executing coast-down procedures. The tyre pressure is adjusted to the manufacturer’s default, and auxiliary equipment such as air conditioning, radio, and headlights are turned off during the tests.

2.2. Test Vehicles

A two-wheel front-drive light-duty vehicle is used for the tyre wear particle tests. Detailed information about the test vehicle is illustrated in Table 2.

2.3. ICP-OES Analysis Method

The elemental analysis of tyre wear particles collected on Teflon filters is conducted using the inductively coupled plasma optical emission spectrometry (ICP-OES) method in accordance with HJ 777-2015 (China). The Teflon filters are cut into small pieces, digested in nitric acid and hydrochloric acid solution, and analyzed using ICP-OES to determine the concentrations of 19 elements including Ca, Al, Mg, Fe, Pb, Zn, K, Ti, Sr, P, Cu, Cr, Mn, Sn, Ba, Ni, Sb, Te, and Bi.

3. Results and Discussion

3.1. Comparison of Different Test Cycles

Compared to other experimental methods, sampling the wear particles behind the wheel with a suitable tube length and intake flow rate enables the direct measurement of instantaneous number concentrations of tyre wear particles, allowing for a direct connection between tyre wear particle characteristics and driving conditions. The PN concentration of the WLTC test cycle is illustrated in Figure 2. The PN concentration increases sharply at the beginning phase and the end phase of the whole test cycle. Multiple peaks of PN concentration occur during the WLTC cycle, but the current values are much smaller than those at the beginning and end phases. Additionally, the overall PN concentration increases with vehicle speed (from the low-speed section to the ultra-high-speed section), indicating that the increased mechanical load and tyre temperature caused by vehicle speed may have a perceptible effect on tyre wear particle production.
The results also show that tyre wear particles are mainly generated during typical driving conditions with significant acceleration changes, as shown in Figure 2. During the same driving cycle phase, the higher absolute values of acceleration changes lead to higher PN concentrations. Most PN concentration peaks occur under driving conditions with sharp decelerations, while only a few PN concentration peaks occur during rapid acceleration. The results suggest that tyre wear particles are more easily generated under deceleration driving conditions in WLTC tests, as the intense sliding friction of the tyre surface would lead to increased particle production.
The PN concentrations of US06 test cycle are presented in Figure 3, together with the vehicle speed and acceleration profiles. Compared to WLTC, US06 is a more aggressive test cycle characterized by frequent and large accelerations and decelerations, which subject the tyres to higher mechanical and thermal loads. Relative studies [35,42,43] have shown that the generation of tyre wear particles is initiated above the tyre onset temperature and that the PN concentration depends more on the heating rate than the reaction temperature. Thus, the PN concentrations of US06 test cycle are much higher than those obtained during the WLTC, especially in the second half of the tests, indicating that the mechanical and thermal loads are the two key factors influencing tyre PN concentrations.
At the beginning phase of each US06 test cycle, there is a sharp increase in PN concentrations, with the highest PN concentration occurring during the cold start phase. Different from the results of WLTC tests, there are small PN concentration peaks at the beginning phase of the second and third US06 test cycle because of the intense accelerations of these driving conditions as shown in Figure 3. Additionally, the overall PN concentrations of US06 tests increase with driving time, exhibiting a similar increase trend to the WLTC tests. However, the overall PN concentrations of the third US06 test cycle do not significantly exceed the results of the second US06 test cycle, indicating that the tyre wear particle generation reaches a steady state. This phenomenon may be attributed to the thermal load. Once the tyre temperature exceeds the critical value, the tyres reach a warmed-up steady state, and the PN concentrations are no longer affected by the overall tyre temperature. Other research also found that the generation of particles dominantly depends on the heating rate after the tyre temperature is above the critical temperature.
Moreover, different from the results of WLTC tests, numerous PN concentration peaks occur with both high driving speed and rapid acceleration in US06 tests. Thus, a necessary condition for the large generation of tyre wear particles under accelerated driving conditions may be that the vehicles have enough vehicle speed and mechanical load. This condition also applies to deceleration driving conditions. As shown in Figure 3, there are five consecutive sharp acceleration and deceleration driving conditions at the end of every US06 test cycle, but there are no very high PN concentration peaks corresponding to the velocity profile. Only the last driving condition with the higher vehicle speed produces an obvious PN concentration peak. In contrast, extremely high PN concentrations can still be produced at some high-speed conditions, even with smaller deceleration. Thus, there should be a speed limitation for PN production, and high PN concentrations are produced only after the speed limit is exceeded. It is also evident from Figure 2 that the highest PN concentration of WLTC tests occurs at the end of the ultra-high-speed section, which has the highest vehicle speed of the WLTC test cycle.

3.2. Start Temperature Effects

As introduced in the methodology section, hot start tests are conducted half an hour after the cold start tests, enabling a comparison between two different start temperatures of the vehicle components. Figure 4 plots the PN concentrations of hot start WLTC tests. Compared with the cold start WLTC tests, the most notable difference is the absence of a peak at the beginning phase in the hot start tests. The phenomenon may be caused by the different mechanisms of tyre wear particle formation. Previous studies [35,44] have shown that particles generated under low temperatures are likely to form through severe wear processes. During the high-friction wear process, tyre tread can be torn by frictional forces and directly released as tyre wear particles.
Additionally, as shown in Figure 4, the hot start WLTC tests have higher PN concentrations than those of cold start tests at the same time of the first three test phases of WLTC tests, showing that the tyre temperature of the test vehicle has an obvious effect on PN generation under the low load driving conditions. In the ultra-high-speed phase of WLTC tests, the PN concentrations are similar in cold and hot start tests since the test tyres have been warmed up.
Figure 4. Particle number concentration of hot start WLTC test cycle.
Figure 4. Particle number concentration of hot start WLTC test cycle.
Atmosphere 14 00724 g004
In comparison to WLTC tests, Figure 5 shows a more pronounced increase in PN concentration during the first hot start of the US06 cycle. The tyre temperature of the test vehicle in the first cycle of US06 tests rises more rapidly than in the beginning phase of WLTC tests due to the larger and more frequent accelerations and decelerations of the US06 test cycle. As a result, the PN concentrations of hot start US06 tests are much higher than those of cold start tests in the first US06 test cycle. Meanwhile, the PN concentrations are similar in cold and hot start US06 tests just after the first US06 test cycle, which is much quicker than in the WLTC tests. As shown in Figure 4, the PN concentrations become similar at around 1500 s after the WLTC tests begin. The PN concentrations have similar values at the same time of the second and third test cycles in the hot start US06 tests. Additionally, the PN concentration peaks appear at the same location in the second and third US06 test cycles.

3.3. Element Distributions

Trace and major elements mass fractions of tyre wear particles are listed in Table 3. Among the measured elements, Ca and Al have the highest mass fractions. Some studies have shown that pavement is an important source of Ca and Al in tyre wear particles collected from real road situations [14,42,45]. However, the high mass fractions of Ca and Al in the in-house chassis dynamometer tests indicate that the pavement is not the only main source of Ca and Al in the particles. There should be a part of the two elements coming from the tyre materials. Mass fractions of Mg, Fe, Pb, and Zn are at the same level. As reported in other studies [13,17,26], Zn can be present in tyres either in inorganic forms (ZnS and ZnO) or as an organic compound. The high mass fractions of Zn validate this view, and Zn may be used as a tyre wear particle tracer in vehicle non-exhaust particle tests. Additionally, the results show a much higher mass fraction of Pb than other studies, which may be caused by different tyre additives. Fe in the results may be caused by a small amount of brake particles mixed in the collection sample. K, Ti, and Sr have small mass fractions ranging from 40 to 125 μg/g in the two test cycles. The remaining ten elements have much smaller mass fractions in tyre wear particles than the first nine elements. In general, the tyre wear particles contain complex elements, and the possible health effects must be considered.

4. Conclusions

To gain insight into the generation characteristics of tyre wear particles in different driving situations for light-duty vehicles, WLTC and US06 tests are performed at different vehicle temperatures, respectively. Instantaneous tyre wear PN concentrations and kinds of elements of particles are measured using E-Filter and ICP-OES. Conclusions are made as follows:
(1) Test cycles can significantly influence the results of tyre wear PN concentration. Compared with WLTC, the US06 test cycle is more aggressive and results in more significant and frequent accelerations and decelerations, causing the tyres to experience larger mechanical and thermal loads. Thus, the PN concentrations of the US06 tests are considerably higher than those of the WLTC test, especially during the second half of the test.
(2) The tyre wear PN concentrations are highly associated with typical driving conditions in vehicle chassis dynamometer tests. There are numerous high PN concentration peaks occur under high driving speed with rapid acceleration conditions in US06 tests, while much lower PN concentrations occur with only rapid acceleration and low driving speed. A crucial prerequisite for the large generation of tyre wear particles under accelerated driving conditions may be that the vehicles have enough mechanical load.
(3) Tyre tread temperature is a crucial factor that affects tyre wear particle formation. In the US06 × 3 tests, the PN concentration peaks appear at the same location in the second and third US06 test cycles. The results indicate that once the tyre temperature exceeds the critical value, the tyres reach a warmed-up steady state, and the PN concentrations will not be affected by the overall temperature of the tyre.
(4) A low initial tyre temperature can produce high PN concentrations in the cold start phase of test cycles and prolong the time for tyres to warm up. The particles generated during the cold start phase are likely formed through the high-friction wear process. Frictional forces can tear tyre treads, releasing them directly as tyre wear particles.
The work in this article is still in the preliminary stage, and the characteristic driving conditions and boundary experimental conditions of chassis dynamometer tests have been picked out. This will be followed by a more in-depth study of the particle size distribution and morphological characteristics of the particles.

Author Contributions

Conceptualization, J.L. (Jiachen Li), M.Z. and Y.G.; data curation, J.L. (Jiachen Li), J.L. (Jiaxin Luo) and D.Y.; formal analysis, D.Y., C.W. (Chongyao Wang) and C.W. (Changyu Wang); methodology, M.Z.; project administration, Y.G., Y.W. and J.L. (Jiaxin Luo); supervision, Y.W.; validation, Y.G.; writing—original draft, J.L. (Jiachen Li); writing—review and editing, M.Z. and C.W. (Chongyao Wang). All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding support from the National Natural Science Foundation of China (Grant No. 52272342 and Grant No. 52172337) and Special Open Foundation for Collaborative Research of Combustion Engine (Grant No. CSICE-K202206).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the experimental setup.
Figure 1. Schematic diagram of the experimental setup.
Atmosphere 14 00724 g001
Figure 2. Particle number concentration of cold start WLTC test cycle.
Figure 2. Particle number concentration of cold start WLTC test cycle.
Atmosphere 14 00724 g002aAtmosphere 14 00724 g002b
Figure 3. Particle number concentration of cold start US06 × 3 test cycle.
Figure 3. Particle number concentration of cold start US06 × 3 test cycle.
Atmosphere 14 00724 g003aAtmosphere 14 00724 g003b
Figure 5. Particle number concentration of hot start US06 × 3 test cycle.
Figure 5. Particle number concentration of hot start US06 × 3 test cycle.
Atmosphere 14 00724 g005
Table 1. Parameter information of the E-filter.
Table 1. Parameter information of the E-filter.
ParameterValue
Dimensions (mm)90 × 85 × 225
Sample temperature (°C)10–50
Sensitivity (fA)3
Maximum concentration (fA)250,000
Minimum particle size (nm)5
Saving interval (s)1
Table 2. Vehicle information.
Table 2. Vehicle information.
ParameterValue
Engine displacement (mL)1300
Rotational Mass (kg)1389
Resistance coefficient A (N)116.10
Resistance coefficient B (N/(km/h))1.0670
Resistance coefficient C (N/(km/h)2)0.0248
Tyre size205/55 R17
Tyre max load (kg)615
Plies tread2 polyester + 2 steel + 1 polyamide
Sidewall2 polyester
Tyre pressure (kPa)260
Table 3. Mass fractions of elements in different test cycles.
Table 3. Mass fractions of elements in different test cycles.
ElementsUS06 × 3 (μg/g)WLTC (μg/g)
Ca3756.692683.60
Al2243.481496.02
Mg695.80481.76
Fe852.10362.48
Pb662.25215.42
Zn429.15149.96
K124.7051.47
Ti86.7757.53
Sr60.3139.71
P5.493.93
Cu0.950.58
Cr3.242.14
Mn2.951.79
Sn3.271.61
Ba0.850.61
Ni0.460.34
Sb0.200.11
Te0.040.04
BiN.D.N.D.
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MDPI and ACS Style

Li, J.; Zhang, M.; Ge, Y.; Wen, Y.; Luo, J.; Yin, D.; Wang, C.; Wang, C. Emission Characteristics of Tyre Wear Particles from Light-Duty Vehicles. Atmosphere 2023, 14, 724. https://doi.org/10.3390/atmos14040724

AMA Style

Li J, Zhang M, Ge Y, Wen Y, Luo J, Yin D, Wang C, Wang C. Emission Characteristics of Tyre Wear Particles from Light-Duty Vehicles. Atmosphere. 2023; 14(4):724. https://doi.org/10.3390/atmos14040724

Chicago/Turabian Style

Li, Jiachen, Mengzhu Zhang, Yunshan Ge, Yi Wen, Jiaxin Luo, Dailin Yin, Chongyao Wang, and Changyu Wang. 2023. "Emission Characteristics of Tyre Wear Particles from Light-Duty Vehicles" Atmosphere 14, no. 4: 724. https://doi.org/10.3390/atmos14040724

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