Emissions from road transport tend to be thought of as primarily from vehicle exhausts or from power stations producing the electricity they use. However a significant proportion is from PM due to tyre, brake and road surface wear, defined as abrasion processes. These particles are emitted directly or re-suspended by other vehicles.
Whatever vehicle we choose to use, some form of wear particulates will be generated. However generally the heavier the vehicle and the greater force required to move and stop it the greater the volume of matter produced. For instance bicycle brake pads and tyres still wear, but as the surface areas and forces involved are far less, much lower amounts of material are involved. Driving vehicles that are appropriate to the environment we occupy is the solution to reducing these emissions. Road surface wear emissions also link with infrastructure costs.
Tyre tread wear is a complex physio-chemical process which is driven by the frictional energy developed at the interface between the tread and the pavement. Tyre wear particles and road surface wear particles are therefore inextricably linked. However, for the purpose of determining emission factors, tyre wear and road surface wear must, at present, be treated as separate particle sources due to the lack of experimental data on the emission factors associated with different tyre-road surface combinations. High wear rates may also occur as a result of steering system misalignment and incorrect tyre pressure (Ntziachristos and Boulter, 2013).
Brake linings generally consist of four main components — binders, fibres, fillers, and friction modifiers — which are stable at high temperatures. Various modified phenol-formaldehyde resins are used as binders. Fibres can be classified as metallic, mineral, ceramic, or aramide, and include steel, copper, brass, potassium titanate, glass, asbestos, organic material, and Kevlar. Fillers tend to be low-cost materials such as barium and antimony sulphate, kaolinite clays, magnesium and chromium oxides, and metal powders. Friction modifiers can be of inorganic, organic, or metallic composition. Graphite is a major modifier used to influence friction, but other modifiers include cashew dust, ground rubber, and carbon black. In the past, brake pads included asbestos fibres, though these have now been totally removed from the European fleet (Ntziachristos and Boulter, 2013).
Road Surface Wear
Emission factors for road surface wear particles are even more difficult to quantify than those for tyre and brake wear, partly because the chemical composition of bitumen is too complex for quantification with chemical mass balance and receptor modelling, and partly because primary wear particles mix with road dust and re-suspended material. Few studies have provided emission factors for road surface wear according to PM10 or any other metric. It is estimated around 70 % by weight of airborne particles from bitumen range from 0.35 µm to 2.8 µm with a mean below 0.7 µm (Ntziachristos and Boulter, 2013).