AnalysisTire (& Road) wear particles

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What can we do about them?

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Ilka Gehrke, Head of Department Environment and Resources Fraunhofer UMSICHT introduced a paper on the study they carried out on the state of knowledge related to mitigation of tire and road wear particles. This study has been extended to cover potential mitigation methods.

The study is expected to be complete by the end of 2022, and there is likely to be a published academic paper describing the results of the study.

The report was funded by the tire industry – specifically, the ETRMA, the USTMA and the Carnegie Mellon University.

The second stage – management & mitigation options – is divided into two sub-categories, each of which is further divided in two.

Management methods

  • Before release
  • After release

Mitigation methods

  • Before release
  • After release

After Gehrke introduced the research, her colleague, Stefan Schläfle went into some detail on mitigation measures prior to release, and Gehrke then looked at mitigation measures after release.

Gehrke started her introduction to the research with definitions: She said, “Tire and road-wear particles are agglomerations, that are spontaneously formed when a tire interact with street.”

She characterised them by size, saying that broadly those particles smaller then 20µm in size become airborne, while larger particles tend to remain on the ground. The particles smaller then 20µm make up about 10% by mass of the total.

She said, “there’s nearly no literature source where you can find directly measuring of tire and road-wear particles. This is due to the lack of standardised measurement method.”

She added that most surveys look at the amount of material abraded from tires and then estimate the total suspended solids and airborne particles.

Having defined what these particles are, she went on to describe the factors contribute to their generation. “There are several factors influencing the generation of tire and road-wear particles such as tire characteristics, road surface characteristics such as the pavement of the road topography, vehicle operation; we have heard about eco-driving and of course vehicle characteristics such as the weight of the vehicle.”

She did not mention the compounds used in the tire and how different tires and compounds can affect the amount of wear particles.

She said, “The aim of our study was to assess and to evaluate mitigation measures against tire and road-wear particles production.” She explained that the main criteria were the cost benefits, practical feasibility and the maturity and technical risk associated with each mitigation measure.
Gehrke then described the main methodologies, starting with a comprehensive literature search for Europe and the US.

She said that prior to the search, they thought they might find around 200 publications related to the mitigation of tire and road-wear particles. However, they identified around double that.

The biggest part of these related to management and technological measures before release. She presented a chart showing publication date of these papers, and it was clear that these publications began around 2000, but with a rapid rise in publication rate from around 2014.

Altogether, the research team identified 58 potential mitigation measures.

Mitigation measures prior to release,

Stefan Schläfle, Research Associate at Karlsruhe Institute of Technology, who worked on the project, continued the presentation with a discussion of the most effective mitigation measures prior to the release of these particles.

He presented a series of slides, each arranged in a similar way. On the right-hand side, the various measures that were most often mentioned in the 400 or so scientific papers published since 2000. Schläfle said that while these are the most-often mentioned, they are not necessarily the most effective, or even the most cost-effective.
On the left-hand side of each slide, he showed an overview of the subcategories

Management measures before release.

In five main categories of measures that might reduce emissions prior to release, the top two – representing around 40% – rely on drivers modifying their behaviour by driving more slowly and using brakes less, relying more on coasting to slow down ahead of road hazards, such as bends, traffic lights and junctions.

After that, he said the third most interesting is better road maintenance, followed by improved traffic flow, with less stop-start traffic conditions and especially, fewer vehicles on the road.

This can be encouraged through more use of public transport and cycling. Local authorities might be encouraged to improve public transport systems and add more bike lanes and more rental bikes, for example.

He said that reducing the number of vehicles by 50% will result in a reduction of particles by around 50%, but in fact might be more effective than that, due to improved traffic flow. “In other words,” he said, “less stop-start traffic will lead to decreasing TRWP emissions.”

Schläfle also pointed to a trial carried out in Helsinki, in which transponders on buses helped to control traffic lights to allow the bus through and hold up traffic in other directions. This improves punctuality, which makes public transport more attractive.

Therefore, he said, “smart traffic management is an effective mitigation measure which is simple to implement.”
Another measure is simply to reduce the speed limit further. Tire abrasion increases with higher speeds. If speed limits are reduced, then tire particle emissions will also fall. However, many people break speed limits, so it has to be enforced with radar and displays that tell drivers how fast they are going.

He referred to a trial in the German city of Halle where limits were reduced to 30 km/h from 50km/h. Even though 85% of drivers broke the limit, emissions of PM10 particles fell by 20%. Researchers estimated that if the vast majority of drivers kept to the speed limits, then PM10 emissions would fall by up to 50%.

He said that public acceptance is crucial to achieve the maximum potential of all mitigation measures, smoother and smarter traffic should be enforced using smart traffic lights.

He added that slower speeds and better traffic management bring other benefits including less exhaust emissions, fewer traffic jams, and the reduction of the overall environmental pollution.

Technological measures before release.

Schläfle’s next slides addressed technological mitigation methods prior to release. These, he said, fell into five broad categories:

  • Vehicle design
  • Driving behaviour,
  • Vehicle maintenance,
  • Road surfaces,
  • Tires.

In terms of the number of papers presented, the vehicle is the major opportunity for reduced tire emissions. The top three categories above contribute 26%, 20% and 27% respectively of possible mitigation solutions.

The key focus of research to date has been on reducing the forces in the tire-road contact patch. Less severe braking; less severe corning and so on. Other measures mentioned by Schläfle included lighter vehicles and more abrasion-resistant tire compounds.

He said that many studies mentioned improved driving behaviours as a strong option for reducing these emissions, but there was very little numerical data on the value of such measures.

Nevertheless, there is some research on the increase in TRWP particles under extreme driving. Schläfle said, “one of the few quantitative statements describe a 30 to 40 fold increase of emissions of particles for severe braking in comparison to constant driving.”

He said this is one of the most promising opportunities for reduction, but there is no way to legislate against hard cornering or aggressive acceleration and braking. Furthermore, car makers resist including such measures in cars which are sometimes marketed as enjoyable to drive.

Another option is to fit a collection device to every vehicle. If we believe the argument that friction is necessary to drive the vehicle, and these particles are a necessary consequence of that friction, then some particles will be generated as the vehicle is driven, so it makes sense to collect them as soon as they are produced.

That could be done by changing the design of the body panels around the wheel to change the airflow and direct any particles to a collection device, or adding a device behind the wheel that electro-statically attracts those particles and removes them from the airflow. Although the evidence is sketchy, some claim this approach could collect up to 90% of all particles generated.

A study in Stuttgart fitted a small fleet of delivery vehicles with particulate filters that sucked in air, filtered it and released the cleaned air back into the environment as they were driving on their normal delivery routes.
This indicated that fine particulate dust – from multiple sources – could be reduced by 50% or more.

Schläfle summarised this saying that changing driving behaviour is the most important measure, but adding a variety of collection and filtration systems can also mitigate the amount of particles in the air and on the road surface.

Management measures after release

The most-often mentioned management of roads was street cleaning (see below). And the research seems to show that around 50% of all the TRWP released into the environment are carried by road water run-off into water courses, or drainage systems.

In a rather surprising finding from a study carried out in Istanbul, Cypress trees planted along the roadside appeared to reduce by 90% the fine dust in the local atmosphere along a main road with high traffic density and high traffic volume.

These tall, thin trees familiar to anyone visiting Italy and other Mediterranean countries have a shape similar to a gas plume and appear to collect all kinds of dust and airborne pollution when planted densely along the road margins
Gehrke repeated the comments of her colleague that many of these measures have multiple beneficial effects. The trees also provided shade and cooled the local micro-climate through transpiration.

Another approach is to use porous asphalt for the road surface. This improves drainage of surface water, reducing spray and improving safety. It also provides a pathway for the TRWP particles to disappear from the road surface. A study in the Netherlands showed that the porous asphalt retained around 40% of the total TRWP load deposited on the surface. There was no comment on whether the pores eventually become clogged due to the presence of the particles.

However, such porous asphalt tends not to be used where the roads freeze regularly. This is because when the pores are saturated with water and that freezes and expands, the road surface rapidly degrades.

Gehrke said National road authorities should develop action plans to mitigate these particles on different road types and locations. She said, “nationwide action plans are absolutely necessary involving all the stakeholders from industry for a municipal politics and so on, especially for effective management of road run-off systems.”

One aspect of these action plans, she said is to, “accelerate plans to remove tire road-wear particles or to eliminate tire road-wear particles that are in the sewer sludge.”

She said a lot of these particles find their way into drainage systems and from there into the sewer system. Once in that pathway, the TRWP particles tend to settle into the sludge.

If that sludge is then used as a fertiliser or soil conditioner on agricultural fields, the concentrated TRWP material is then deposited into the environment once more. “We have to avoid this,” she said.

An alternative to the sewer system is to create retention ponds that fill up with road run-off when there is heavy rainfall. These ponds then accumulate the TRWP in the mud at the bottom of the pond.

Street cleaning

We are all familiar with the small vehicles that sweep the streets clean of leaves and other debris. Many imagine that this process also gathers up the TRWP. Unfortunately, that is not the case. Simply brushing the roads does not appear to remove the particles. Many are disturbed and fly up in to the air temporarily before settling back down onto the road after the cleaning vehicle has passed.

It seems to require water. She referred to a case study in Berlin where the researchers performed several field tests investigating various kinds of cleaning methods. They figured out that by cleaning three times per week the main streets in the area of Berlin, 42% of tire abrasion can be reduced in this year.

While this is effective, it is also costly. She said that by treating only the hot-spots of TRWP generation in this way, the cleaning is much more cost-effective, collecting 36% of the total arisings, but at much lower cost.
Gehrke also noted that street-cleaning has not been a high priority and many of the vehicles need maintenance or renewal.

General findings of the study

Although the study is not yet complete, many of the main conclusions can be drawn out form the work so far. Gehrke said the overall picture for mitigation are not complex or even very difficult, but they do cover a broad scope of traffic management, road building and maintenance, driver behaviour and tire design. This will require co-ordination among many different interest groups if TRWP are to be effectively mitigated both prior to release and after release.

She said there is, “a very drastic lack of field and laboratory test data.” While may studies say that cleaning streets is a good idea, very few say how effective that might be, or what it might cost, or what other effects it might have.
It was interesting to us, that the design of tires and the compounds used in tire treads were barely mentioned in the discussion of the study.

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