+ Keeping the dust down on London's streets

Is pressure washing roads and applying dust suppressants the way forward in tackling air pollution, or is it simply a way of glossing over the problem that there just too many vehicles on the roads?

The quality of the air in London has long been an issue for the city. Well before the great London smogs of the 1950s, parliament was passing laws to improve the quality of the air we breathe. The problems even in the 1860s were severe enough for an act of parliament to be introduced to require removal of acid gases from certain factory emissions. However, the great catalyst for improvement was the great smog in winter 1952, where a period of cold weather trapped the airborne pollutants produced by the combustion of coal to form a thick smog for five days over London.

The public health consequences were enormous and some 12,000 people were believed to have died as an immediate consequence with several thousand more in the weeks following the smog. Action needed to be taken and the Clean Air Act of 1956 established Smoke Control Areas and measures were taken to relocate power stations out of the city. These measures, together with a shift away from the use of coal as a fuel resulted in large improvements in air quality in London and other cities.

More than 50 years on, it has become evident that a new air quality problem exists in our cities. In the 1950s there were only four million motor vehicles in the UK, but now there are some 28m. Despite large efforts being made to clean up the exhausts from the motor vehicle, the impact of emissions from this source remains a significant issue and it's estimated that some 50,000 people in the UK have their life expectancy reduced owing to excessive exposure to air pollutants.

Because of its size and concentration of activity, some of the highest concentrations of the two key vehicle related pollutants (nitrogen dioxide and fine particles (known as PM10) are found in London and there are particular “hot spots” where concentrations are well above normal. This is due to high traffic flows and the areas being surrounded by tall buildings meaning that air can become trapped and pollutants do not disperse well.

The Mayor of London has a statutory duty to work to meet certain air quality objectives which means that the Greater London Assembly must examine air quality in London and introduce measures to work towards meeting the objectives. It is known that the concentrations of pollutants in the hot spots are so high that the current measures being taken to reduce concentrations will be insufficient to meet the air quality objectives for many years.

Therefore the mayor has had to look for more innovative methods to improve air quality, however, a huge problem is that there are very few techniques available for reducing emissions beyond those already applied through national and European legislation. Once emitted into the atmosphere the pollutants are difficult to remove as a huge volume of air would need to be treated so measures to reduce the emissions at source are always likely to be more effective.

Following a review of possible measures to improve air quality in the hot spots London has embarked on a trial of an approach to reduce levels of fine particles using a system trialled in Austria. The method aims not to reduce emissions of particles from the exhaust gases but to reduce the particles picked up from the road surface as the vehicles move over the road – this is known as re-suspended dust as is responsible for significant proportion of the particulate matter found in the atmosphere.

The method adopted involves high pressure washing of the road surface to remove particles deposited on the road and then application of a dust suppressant intended to hold the particles to the road surface. This is the first large trial of this method in the UK and the first air quality reduction measure applied that does not involve changing the characteristics of the traffic or the speeds and flows. Critics of the approach suggest it's simply a way of glossing over the problem, but a successful outcome would be a major boost for the improvement of air quality in cities.

Image courtesy of Transport for London.