A new report released by the Air Quality Expert Group (AQEG) in the UK recommends as an immediate priority that non-exhaust emissions (NEE) are recognized as a source of ambient concentrations of airborne PM, even for vehicles with zero exhaust emissions of particles.

Non-exhaust emissions (NEE) from road traffic refers to particles released into the air from brake wear, tyre wear, road surface wear and resuspension of road dust during on-road vehicle usage. These emissions arise regardless of the type of vehicle and its mode of power, and contribute to the total ambient particulate matter burden associated with human ill-heath and premature mortality. No legislation is currently in place specifically to limit or reduce NEE particles, so whilst legislation has been effective at driving down emissions of particles from the exhausts of internal-combustion-engine vehicles, the NEE proportion of road traffic emissions has increased.

Data from the UK National Atmospheric Emissions Inventory indicate that particles from brake wear, tyre wear and road surface wear currently constitute 60% and 73% (by mass), respectively, of primary PM_2.5 and PM₁₀ emissions from road transport, and will become more dominant in the future. Currently they contribute 7.4% and 8.5% of all UK primary PM_2.5 and PM₁₀ emissions. Therefore to achieve further gains in PM_2.5 and PM₁₀ air quality in relation to road transport sources requires attention to reducing non-exhaust emissions, not solely a focus on lowering exhaust emissions.

—“Non-Exhaust Emissions from Road Traffic”

UK emissions of PM₁₀ (top) and PM_2.5 (bottom) from road transport.

NEE particles are also an important source of metals to the atmosphere; the UK national inventory estimates NEE contributions of 47% and 21% for Cu and Zn, primarily associated with brake and tire wear, respectively. The national inventory does not include estimates of road dust resuspension.

The authors note that the magnitudes of NEE are “highly uncertain” as they vary widely according to brake, tire and road-surface material, and with driving style. The NEE emission factors used in inventories have a wide span of uncertainty—greater than a factor of two is typical, the authors said—including uncertainty the PM₁₀ and PM_2.5 fractions.

In the UK, available data indicate that half of NEE occurs on urban roads, owing to the greater braking per km than on non-urban roads. Emissions may also be high in areas such as trunk-road exits. Tire-wear emissions are estimated to be greatest on high-traffic trunk roads and motorways (both urban and rural).

Regenerative braking does not rely on frictional wear of brake materials so vehicles using regenerative braking totally or partially, for example electric vehicles, should have lower brake wear emissions. However, tyre and road wear emissions increase with vehicle mass, which has implications for any vehicle with a powertrain that is heavier (for example due to additional battery and hardware mass) than the equivalent internal-combustion-engine vehicle it replaces.

The net balance between reductions in brake wear emissions and potential increases in tyre and road wear emissions and resuspension for vehicles with regenerative braking remains unquantified, and will depend upon road type and driving mode, as both influence the balance between the different sources of emissions. In locations where brake wear makes a major contribution to overall NEE, it seems likely that there will be a net benefit, but this has yet to be demonstrated. Other as yet unproven technological mitigation methods include trapping brake wear particles prior to emission, and mandating formulation of low-wear/low-emission tyres, brake pads and road surfaces.

—“Non-Exhaust Emissions from Road Traffic”

A further priority, the authors suggest, is to work towards a consistent approach internationally for measurement of NEE and to update and narrow the uncertainties in their emission factors. Such a program could form the basis for subsequently including criteria on brake and tire wear emissions in future type approvals and regulations governing formulation.

AQEG also recommends that further studies be conducted to quantify the efficacy of technical solutions on NEE reductions; in particular, to understand gains from use of regenerative braking versus potential increased tire and road wear due to additional mass of vehicles incorporating such braking—i.e., electric and plug-in hybrid vehicles.

The AQEG is an Expert Committee to the UK’s Department for Environment Food & Rural Affairs (Defra) that provides independent scientific advice on air quality, in particular the air pollutants contained in the Air Quality Strategy (AQS) for England, Scotland, Wales and Northern Ireland and those covered by the EU Directives on Ambient Air Quality. Members of the group are drawn from those with a proven track record in the fields of air pollution research and practice.