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Evolution of the concentration of fine particles (PM10) in the air
The particles that are suspended in the air (PM) are very different in terms of origin as well as chemical and physical characteristics. The PM concentrations and emissions in the air have been regulated by Europe as these particles have a major impact on human health, specifically on respiration and blood circulation. The annual average concentrations of PM10 across all of the Region's air quality monitoring sites comply with the European limit value. The observance of the daily limit value, however, has been problematic for years. This is due partly to the import of PM from outside the Region and partly to the large-scale formation of secondary aerosols under certain meteorological conditions.
All particles smaller than 10 micrometres in diameter are denoted by the acronym "PM10", regardless of their composition or physical nature. Particles which are suspended in the ambient air come from a variety of sources: "primary" particles are emitted directly through a natural process (for instance soil erosion or particles from the Sahara), or caused by human activities (incineration, road surface wear, construction and demolition works, …), while "secondary" particles occur in the atmosphere through chemical reactions between other molecules that are already present (nitrates, sulphates, nucleation of gaseous substances, …).
European limit values
In view of protecting public health, European directive 2008/50/EC provides for the obligation to adhere to two limit values for the PM10 concentration in the ambient air. These have been in force already since 1 January 2005:
- 50 µg/m3 as the daily average, with a maximum of 35 exceedance days per year.
- 40 µg/m3 as the annual average: this limit value has not been exceeded at any site of the Brussels' monitoring network since 2004.
PM10 concentration in the air
In the Brussels Region, PM10 is permanently monitored at 6 monitoring sites of the telemetric air quality network. We base our indicator on the data of monitoring station Écluse 11 in Molenbeek-Saint-Jean (code 41R001) as it is representative of an urban area with a strong influence of road traffic. The Brussels' PM10 indicator relates only to the daily averages.
Evolution at monitoring site Molenbeek-Saint-Jean of the number of days in exceedance of the 50 µg/m3 daily limit value for PM10 (1997-2012)
Source: Environnement Bruxelles - Leefmilieu Brussel, Laboratory for Environmental Research (air)
Up to and including 2009, the monitoring site of Molenbeek-Saint-Jean systematically indicated more exceedance days than the allowable 35. The years 2010 and 2012, however, formed an exception to this observation due to unusual meteorological conditions: during those years there was a high frequency of north to north-westerly winds which are known to suppress the formation of secondary aerosols.
In 2011 and 2012, on the other hand, the limit value was exceeded at monitoring site Avant-Port (Haren) along the Canal. The Region hence committed an infringement, because any exceedance at one of the stations within the Brussels' monitoring network is considered to be non-compliant with the limit value.
Sources of PM10
Air masses can transport PM10 across great distances because the particles are so small. This means that the concentrations recorded in Brussels are not the sole result of local emissions: the PM10 concentrations are also associated with background contamination (as measured in the Ardennes for instance), contributions from outside the region (brought into the BCR via airflows), urban background contamination (resulting from the emission by heating and traffic, as recorded at the monitoring sites of Uccle and Berchem-Sainte-Agathe), local urban contributions primarily related to traffic (as is the case in a more densely populated area such as Molenbeek-Saint-Jean), and additional intermittent contributions of traffic found in areas with a high ratio of vehicles (monitoring site Arts-Loi being one example).
It is estimated that the contributions from traffic (through direct emission) amount to 10% of the PM10 recorded. Furthermore, traffic is responsible for re-suspending particles with a diameter between 2.5 and 10 µm, as a result of the motion of the vehicles: these indirect emissions from traffic represent on average an additional 10% of the concentrations.
The analysis of the readings from the different stations within the monitoring network teaches us that the urban background contamination and/or the transport of PM through the air masses between the regions alone are at the base of a considerable number of exceedances (more than 20 days of the allowable 35 days in the case of the stations of Uccle and Berchem). The surplus exceedances observed at the monitoring sites of Molenbeek-Saint-Jean and Avant-Port is the result of particles between 2 and 10 µm being re-suspended, or of a direct emission through an activity in the immediate surroundings.