• ONGOING: volunteer cyclists measure traffic-related pollution during their routes. We received almost 90 applications in ten days! See the ad.
  • 18 September 2015: Article from Le Soir


The air quality assessment generally involves taking measurements at fixed points. Although it is then well known in these locations, it is difficult to draw up a large-scale mapping of pollutants. The original feature of the ExTraCar project, which began on 1 April 2014 and which will last for two years, is the taking of measurements while moving, thus adding a spatial dimension to the measurements. It has numerous objectives:

  • combining measuring and modelling to establish a high-resolution black carbon map in an urban environment;
  • establishing a quantitative link between road traffic and concentrations of this pollutant;
  • evaluating, using measurements and digital simulation, the exposure of the population during transport;
  • creating tools to predict the impact of urban developments such as the extension of the RAVel, the commissioning of the tram, the transformation of a district into a low-emission zone, etc.
Transmission electron microscopy (TEM) images of aerosol particles, including black carbon from Posfai et al. (1999). In panel A, the black carbon particles (denoted by the small arrows) are mixed with inorganic ammonium sulphate particles. In panel B, a typical chain-like black carbon aggregate is shown with the arrows pointing to a film of carbon that connects the individual spherules within the aggregate. In panel C, fly-ash spheres are shown, which are particles that are often associated with black carbon particles. The scales give an idea of the size of these particles – a human hair is around 100µm or 100,000nm.

Unlike the fine particles regulated at European level, black carbon is an excellent traffic marker. In fact, once released, it doesn’t undergo any physico-chemical transformation, which it easy to model. In addition, in terms of its health impact, studies show that it is at least as dangerous as PM2.5. However, given that it comes from incomplete burning, it is also always accompanied by a series of volatile organic compounds or polycyclic hydrocarbons the toxicity of which is significant. Finally, black carbon also has an impact on climatic change, given its significant power to absorb sunlight, and some experts believe that it should be subject to priority reduction measures.




In order to carry out these campaigns on subjects, whether on foot or by bicycle, it is hardly possible to use conventional equipment; weight, clutter, and power supply would quickly discourage the bravest operators. It was therefore necessary to choose the portable version of these instruments and to carry out a range of tests to check their consistency and their equivalence with the instruments of the continuous measurement networks.



The initial experiences, along the RAVel (right bank of the Meuse) up to the Atlas bridge and returning through the heart of the Ardent City, show a contrast in concentrations of black carbon along these two segments. This difference, which is not noticeable for fine particles (PM10 and PM2.5), reassures us in our choice of this pollutant. Since then, other journeys between the outskirts of Liège and our institute have been completed using different modes of transport. Therefore, a cyclist, using the TEC, and a motorist frequently travel between Soumagne and the ISSeP sampling the air they breathe during their journey. Similarly, at the request of the Walloon agency for air and climate, in addition to a measurement campaign using passive tubes, four members of the team crossed the city of Namur on bicycles provided to the public by the SNCB. We would like to take this opportunity to thank all our nice test subjects, both present and future.

In addition to improving our knowledge of pollution by black carbon, the project aims to promote “soft” transport alternatives and help change thinking. May these two years be enough!

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