Barium can be used to trace brake pollution

3 November 2009 by Sara Coelho

Traffic pollution is not just fumes coming out of exhaust pipes. The wear and tear of car parts also contaminates the environment along busy roads. But to what extent? A team of scientists from Birmingham offered an answer to this question with a new method to trace back brake dust pollution.


One of the biggest causes of pollution in cities is particulate matter - tiny particles of metals or chemicals dispersed into the air by traffic and industry. Exhaust fumes are an important source of emissions and "we have very strict regulations to control the particulate matter that comes out of exhaust pipes," says Professor Roy Harrison who researches environmental health at the University of Birmingham. Despite falling levels of emissions from engine exhaust, concentrations of particles in urban air have not reduced significantly since 2000.

Particulate matter can also be released by the wear and tear of car brakes and tyres, or even by the normal abrasion of road surfaces. Brake dust, especially, is thought to be an "important source of particulate matter pollution," says Harrison. Its emissions are not regulated, although "they need to be if particulate matter pollution levels are to be maximally controlled," he adds.

One of the reasons behind the lack of legislation is that it's difficult to quantify how much particulate matter comes out of the abrasion of the different parts of the car. Harrison and his Birmingham team set out to find an accurate method to trace brake dust emissions.

They decided to look especially at barium because barite (also known as barium sulfate), used by most manufacturers as filler in break pads, is one of the few sources of barium in cities and can be traced back to brakes.

The team set up camp at Marylebone Road in central London and sampled the air along the busy six-lane road daily for three weeks. They also collected samples from an urban background location at Regent's Park, 800 metres north of Marylebone Road, away from local traffic influence.

They used Regent's Park as a comparison: "the difference between Marylebone Road and the urban background is the pollution emitted by the traffic at the road," explains Harrison.

Back in the lab, they analysed the air samples and measured both the amount of particulate matter, its metal content, and the sizes of the particles. The results, published last week in the journal Atmospheric Environment, show that the concentration of barium, copper, iron and antimony increase dramatically near Marylebone Road.

Iron comprises up to 75 per cent of brake dust particles. Barium represents only one per cent, but it makes a better tracer for brake dust pollution as iron particles can also be emitted by tyre and road surface wear.

The findings help to 'define barium as a tracer for brake wear pollution,' says Harrison. An accurate tracing method is essential to separate particulate matter emitted by brakes from particles emitted by other car parts. This is crucial 'to identify brake dust as a major source of pollution,' he argues.

'Identification of brake wear particles and derivation of a quantitative tracer for brake dust at a major road' - Atmospheric Environment JK Gietl, R Lawrence, AJ Thorpe, RM Harrison. Available online 17 October 2009. doi:10.1016/j.atmosenv.2009.10.016