Tuesday, 12 March 2019

Air Pollution (Part 2) - Particulate Matter PM10 and PM2.5 - Brake and Tyre Wear are Higher Emitters than Car Exhausts

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In Part 1, I had discussed the nature and adverse health effects of outdoor air pollution that is particularly serious in almost all major cities and is responsible for nearly 5 million deaths annually.  In cities, road transport is a major source of air pollutants - particularly particulate matter - the subject of this publication. 
In Central Londonroad transport is responsible for 54% of particulate matter PM10 (all particles less than 10μm diameter) emissions and for 48% of the harmful  oxides of nitrogen.  
The issue of transport in cities is a complex problem - petrol and diesel vehicles are generally blamed for the pollution.  However, under the new low emission legislation, pollution from exhausts has been falling and the main problem in future will be particulate matter generation by brake wear and tyre wear (BWTW), road surface wear and other particulate matter that already exists in the environment and become suspended due to traffic induced turbulence.  Non-exhaust emissions are equally important for electric vehicles (EVs) and it seems that PM emissions will continue to be a serious health hazard in cities until a better way to move people is found


Airborne PM2.5  are capable of bypassing human noses and throats, penetrating deep into the lungs where they deposit harmful chemicals adsorbed on their surfaces. Their presence has been linked to a number of health conditions, including respiratory illnesses (for details, see Part 1).


Experts estimate that by 2030, as the transport sector moves into electric vehicles, 90 per cent of harmful PM2.5 emissions will be caused by microscopic particles eroding from tyres &  brakes and particles airborne from road and surrounding surfaces.

The UK government's clean air strategy plans to develop new standards for tyres and brakes to address toxic non-exhaust emissions. Similar planning is underway in most major economies. At the same time, the size of cities is growing rapidly with 75% of the world population expected to live in urban areas by 2050 with a corresponding increase in road traffic and emission of particulate matter.   

New products and technologies are crucial in achieving clean air in our cities - for example, Bosch has recently launched a new iDisc brake disc with a tungsten carbide coating that emits 90% less brake dust than a conventional cast iron disc. Regenerative braking, most effective in stop/start driving typical of the traffic in cities, in hybrid and electric vehicles (EVs) reduces the need for friction braking greatly, thus cutting down on particulate emissions. More on this later.

In this publication, I look at the non-exhaust particulate emission in more detail and discuss what may be done to improve the quality of outdoor air.  Unlike exhaust emitted particles, there are no standard methods for characterising non-exhaust particulate matter and it is sometimes difficult to compare results of different studies. A good estimate of how much particulate matter (in mg per km of travel) is emitted is provide by the following figure
Concentrating on PM2.5 particles, we note that heavier cars generate greater quantity of particulate matter - that is why large scale adoption of battery electric vehicles (BEVs), which tend to be heavier than internal combustion engine vehicles (ICEVs), will not reduce tyre or road wear.  However, in urban driving, BEVs will almost totally eliminate brake wear due to regenerative braking - for example, BMW i3 does not generate any brake dust for 90% of the time. 
Fuel Cell Electric Vehicles (FCEVs) are much lighter and hold good promise over battery electric vehicles (BEVs) in reducing non-exhaust emissions of PM2.5.

Other measures for reducing PM2.5 emissions could be the use of drones for delivery of parcels in the city and use of Internet to send documents instead of paper.  Both will reduce the number of vehicles on the road.

Greater use of public transport will help too as well as pedestrianisation of central city areas to exclude road traffic.  

Self-driving electrical vehicles (AEVs) will be much lighter than today's BEVs.  As a taxi service, AEVs can be right size for the purpose - for example, for transporting one or two people, a two-seater AEV is all that is required.  Due to their better safety characteristics, AEVs can be made of lighter materials saving on fuel and reducing PM2.5 emissions from tyre and road wear.  Introduction of AEVs will also reduce the number of vehicles in the urban surroundings. 

Most big cities have a subway system where ventilation is not very efficient and the air quality tends to be poorer.  Non-exhaust emissions are the most important source of airborne particulate matter in the subway system, including platforms and trains. Subway PMs are mostly iron and carbon based particles and are generated from rail-wheel-brake interfaces.  Trains and ventilation systems drive the particles along the tunnel and mix them with dust etc. More effective ventilation systems, magnetic filters and platform screen doors will help to reduce the concentration of PMs on the subway platforms.