The wear and tear on roads and tyres can be 20 per cent higher for electricified models. Photo: © emirhankaramuk / Shutterstock.com

Particles from electric vehicles

Electric cars cause more wear and tear on road and tyres due to the extra weight of the battery. Nevertheless they have lower net particle emissions than normal cars.

In the 20th century PM emissions from vehicles largely originated from exhaust fumes (80–90%). As stricter regulations were applied, exhaust emissions have fallen (at least in high-income countries), making the relative contribution of nonexhaust emissions larger.With the end of the era of Internal Combustion Engine (ICE) cars, as many governments introduce legislation to end their sale, many motorists will replace their ICE vehicles with electric vehicles. This move will naturally eliminate exhaust emissions, including ultrafine particles and the toxic nitrogen dioxide and other exhaust gases. There have nevertheless been conflicting findings on the contribution of EVs to non-exhaust emissions (from brake, tyre and road surface wear).

It has been suggested that due to their increased weight, non-exhaust emissions of particles from EVs may be higher than their combustion counterpart. An often-cited article found that EVs emitted more non-exhaust PM than ICE vehicles. One important factor that will influence results is which cars you compare, and here the choice of comparison was questionable.

An erratum was later written as the authors had claimed to belong to a university but actually came from a company with a conflict of interest.¹ This study was also the basis of a recent PhD thesis that compared the health impacts of vehicle types. ²A more recent modelling study found that the outcome is critically dependent on regenerative braking relative to the use of friction brakes, but overall found only modest changes in total local PM emissions between EVs and ICEVs. ³

Previous studies were modelling studies based on simple assumptions and had not (as far as we know) been confirmed by experimental studies. It was not until recently that a Korean team compared three cars of the same model, but with different drive trains: petrol, diesel and fully electric (EV). The EV was 20% heavier than its combustion counterparts, which is a common average weight gain for the battery.

The on-road vehicle experiment was run 30 times on asphalt and combined with actual driving experiments with mobile sampling on rural roads, urban roads and motorways. The PM emission factors (EFs) from wear and tear of road and tyres were generally 20% higher for the EV than the ICEVs. The emission factors (EF) for the total PM emissions of ICEVs and EV were highly dependent on the inclusion of secondary exhaust particulate matter (PM) from ICEVs, the brake pad type, and the regenerative braking intensity of the EV. When only primary exhaust PM emissions were considered in vehicles equipped with non-asbestos organic (NAO) brake pads, the total PM¹⁰ EF of the EV was 10% higher than those of the ICEVs.

However, in vehicles equipped with lowmetallic (LM) brake pads, PM emissions from brake wear significantly increased but using regenerative braking effectively reduced the brake wear PM, such that the total PM EF of the EV was comparable or lower than those of the ICEVs.When secondary PM emissions were included, the EF was always significantly lower for the EV than ICEVs. The replacement of ICEVs by EVs can therefore improve air quality and reduce the adverse impact of PM on human health.The particles from brake wear are generated by the friction between the brake pads and discs, and the energy is lost to heat. While EVs still have friction brakes for use in an emergency (faster action) they can also use regenerative braking.

Regenerative braking instead uses another technique in which the braking energy generated in a deceleration event can be stored in batteries and increase the driving range. How much of the braking is provided by regenerative braking will depend on the driver and operating conditions, so the EFs will be lower if most braking is done by regenerative braking.

Another factor influencing the EFs is the pad type. Generally, brake pads can be divided into non-asbestos organic (NAO) and low metallic (LM) types. The emissions from LM pads are reportedly five times higher than from NAO pads, due to the greater surface roughness of LM pads. To summarise, the wear and tear of road and tyre PM emissions can be 20% higher in EVs compared to ICEVs of similar models.

PM emissions from brake wear will depend on the intensity of regenerative braking and choice of brake pad types. This will heavily influence how large the non-exhaust emissions from EVs will be and influence whether the total primary PM is slightly worse (10%) or the same or a little better than their ICEV counterparts. The comparison with ICEVs is complex and secondary organic aerosols from ICEVs will also impact the total emissions, which in this study always gave EVs the advantage in lower total emissions.

Ebba Malmqvist

1 Atmospheric Environment, June 2016, Nonexhaust PM emissions from electric vehicles, https://doi.org/10.1016/j.atmosenv.2016.03.017

2 Forskning.se " Fler elbilar kan ge sämre luft i städerna", 6 September 2022, https://www.forskning.se/2022/09/06/fler-elbilar-samre-luft-staderna/

3 Atmospheric Environment, 1 January 2021, PM₁₀ and PM_2.5 emission factors for non-exhaust particles from road vehicles: Dependence upon vehicle mass and implications for battery electric vehicles, https://doi.org/10.1016/j.atmosenv.2020.117886

Science of The Total Environment, 10 October 2022, Comparison of total PM emissions emitted from electric and internal combustion engine vehicles: An experimental analysis, https://doi.org/10.1016/j.scitotenv.2022.156961

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