Study finds total PM10 emissions from EVs equal to those of modern ICEVs; role of weight and non-exhaust PM
A new study by a team from the University of Edinburgh and independent engineering company INNAS BV has found that, when factoring in the additional weight and non-exhaust PM factors, total PM10 emissions from electric vehicles (EVs) are equal to those of modern internal combustion engine vehicles (ICEVs). Non-exhaust PM factors include tire wear, brake wear, road surface wear and resuspension of road dust.
For PM2.5 emissions, EVs deliver only a negligible reduction in emissions, the team found. Compared to an average gasoline ICEV, the EV emits 3% less PM2.5; compared to an average diesel ICEV, the EV emits 1% less PM2.5. Therefore, Victor Timmers and Peter A.J. Achten conclude, the increased popularity of electric vehicles will likely not have a great effect on PM levels. Their paper is published in the journal Atmospheric Environment.
|Comparison of the expected total emissions of PM10 for EVs, gasoline ICEVs and diesel ICEVs. Data from Timmers and Achten (2016). Click to enlarge.|
Non-exhaust emissions tend to contain mostly PM10, but a significant proportion of the emissions contains fine PM2.5 as well. The chemical characteristics of non-exhaust PM emissions vary per source, but are mainly made up of heavy metals such as zinc (Zn), copper (Cu), iron (Fe) and lead (Pb), among others. There are several toxicological studies that have found links between non-exhaust emissions and adverse health effects, such as lung-inflammation and DNA damage, and a review of epidemiological studies concluded that PM10 indeed has an effect on mortality.
… It can be hypothesized that each of the sources of non-exhaust PM emissions should be influenced by vehicle weight. We know that road abrasion and tire wear are caused by the friction between the tire thread and road surface. Friction is a function of the friction coefficient between the tyres and the road, as well as a function of the normal force of the road. This force is directly proportional to the weight of the car. This means that increasing vehicle weight would increase the frictional force and therefore the rate of wear on both the tire and road surface. Brake wear is caused by the friction between the brake pads and the wheels. The energy needed to reduce the momentum of a vehicle is proportional to the vehicle’s speed and mass. Therefore, as the mass of the vehicle increases, more frictional energy is needed to slow it down, leading to greater brake wear.
Resuspension is caused by the wake of a vehicle, which in turn is determined by the size, weight and aerodynamics of the vehicle. Furthermore, heavier vehicles are able to grind down larger particles into smaller, more easily suspended PM. In addition, many heavier vehicles will also be larger, resulting in a larger wake. These factors together should cause increased resuspension.—Timmers and Achten (2016)
Timmers and Achten analyzed the existing literature on non-exhaust emissions of different vehicle categories, and found that there is a positive relationship between weight and non-exhaust PM emission factors.
Further, they found that EVs are on average 24% heavier than equivalent ICEVs. For example, the Ford Focus Electric and gasoline-powered Ford Focus hatchback have almost exactly the same specifications; the EV, however is 219 kg heavier. Likewise, the Honda Fit EV is 335 kg heavier than the conventional version; the Kia Soul EV is 311 kg heavier than the regular Kia Soul, etc.
A 2013 study by a team at Paul Scherrer Institute found that an increase in weight of 280 kg will result in a PM10 increase of 1.1 mg per vehicle-kilometer (mg/vkm) for tire wear, 1.1 mg/vkm for brake wear and 1.4 mg/vkm for road wear. For PM2.5, these values are 0.8 mg/vkm, 0.5 mg/vkm and 0.7 mg/vkm for tire, brake and road wear, respectively.
However, a different study found that the brake wear of EVs tends to be lower because of their regenerative brakes. Because there is little research which has investigated the actual reduction in emissions resulting from EV braking, Timmers and Achten assumed a conservative estimate of zero brake wear emissions for EVs.
Based on a different study, they assumed a linear relationship between weight and resuspension, and used a 24% increase in resuspension for EVs (due to the on average 24% increase in weight).
On the combustion side, the advent of PM emission standards and new particulate filter technology has greatly reduced exhaust particle emissions from new ICEVs.
Averaging the emission factors from US and European emission inventories, Timmers and Achten obtained a PM10 emission factor of 3.1 mg/vkm for gasoline cars and 2.4 mg/vkm for diesel cars. In terms of PM2.5, these values were 3.0 mg/vkm and 2.3 mg/vkm for gasoline and diesel cars, respectively.
|Timmers and Achten (2016). Click to enlarge.|
… EVs are not likely to have a large impact on PM emissions from traffic. Non-exhaust sources account for more than 90% of PM10 and 85% of PM2.5 emissions from passenger cars, and this proportion is likely to increase in the future as vehicles become heavier. Policy so far has only focused on reducing PM from exhaust emissions. Therefore, future European legislation should set non-exhaust emission standards for all vehicles and introduce standardized measurement methods. In addition, it is recommended that EV technology such as lightweight car bodies and regenerative brakes be applied to ICEVs, and incentives provided for consumers and car manufacturers to switch to less heavy vehicles.—Timmers and Achten (2016)
Victor R.J.H. Timmers, Peter A.J. Achten (2016) “Non-exhaust PM emissions from electric vehicles,” Atmospheric Environment, Volume 134, Pages 10-17, doi: 10.1016/j.atmosenv.2016.03.017