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Air quality: concentration of very fine particles (pm2.5)

The annual average concentrations of PM2.5 show an overall downward trend in the long-term, and comply with the European standard. However they exceed by far the WHO recommendations. Unlike for PM10, there is no daily limit value being fixed for PM2.5. The objective on horizon 2015 for the average exposure of the population did not pose any problems. Just like PM10, PM2.5 is also likely to be transported by air masses over long distances and its spatial distribution is very extended. Only between 16 and 20% of the measured concentrations in the Brussels Region are attributable to local sources.


PM2.5 is an acronym for particulate matter and refers to all particles with an aerodynamic diameter less than 2.5 micrometres, regardless of their chemical composition or physical aspect. Airborne particles in the atmosphere come from various sources, either natural or anthropogenic, and can be primary or secondary (see indicator PM10).

Specific monitoring of the PM2.5 fraction is necessary since secondary mineral particles are mainly formed within the PM2.5 fraction. It has been found that the concentration of PM2.5 can significantly increase during agricultural spraying, if meteorological conditions are favourable to their formation (humidity, temperature, dispersion conditions). Ammonia emitted during spraying could actually transform into ammonium, since it can be transported over long distances, and can in turn react with nitrates and sulphates formed from nitrogen oxide and sulphur oxide present in the atmosphere. This combination causes the formation of ammonium nitrate and ammonium sulphate, particulate compounds belonging to PM2.5. Due to similar spraying periods in Belgium and in neighbouring countries, "secondary particle occurrences" (often in the spring) can be spatially very extended.

European limit values

With a view to protecting public health, the European directive 2008/50/EC requires that the annual average PM2.5 concentration must not exceed 25 µg/m3. Between 1 January 2010 and 31 December 2014, this was a target value. Since 1 January 2015, this objective has become mandatory (limit value).

European legislation has also imposed a common method to calculate and evaluate an average exposure indicator (AEI) for the population of each Member State, to allow the monitoring of urban background concentrations of PM2.5. The Belgian AEI was calculated based on the concentrations measured at the Molenbeek-St-Jean and Uccle measuring stations in the Brussels network, as well as at 4 background stations in Flanders (at Bruges, Ghent, Antwerp and Schoten) and 2 background stations in Wallonia (at Liège and Charleroi). A limit value of 20 µg/m3 was set for the Belgian AEI up to 2015 (the average of 2013, 2014 and 2015). Furthermore, the AEI must fall by 20% in 2020 (in other words as an average over the years 2018-2019-2020) compared with its 2011 value (in other words as an average over the years 2009-2010-2011). By applying a similar reasoning and the same calculation method only at the Molenbeek-St-Jean and Uccle stations, we have obtained an AEI for Brussels for which the target value to be achieved in 2020 corresponds to 16.7 µg/m3 (see methodological sheet for the PM2.5 indicators). This value will serve as the reference value for the evaluation of our regional exposure indicator.

PM2.5 concentration in the air

In the Brussels-Capital Region, five stations of the telemetric network for air quality continuously measure PM2.5. The dataset began in 2006, when TEOM-FDMS monitors were installed in the Brussels-Capital Region. Before 2006, the Region was equipped with TEOM monitors. The data observation since 2006 ensures that it can be compared from one month to another, or from one year to another, given that there is no difference in terms of instrumentation.

To give a general idea of the overall time evolution of PM2.5 concentrations, the monthly PM2.5 concentrations, as an average across the five measuring stations in the Region over the period 2006-2014, are plotted on the chart below.

Evolution of monthly concentrations of PM2.5 as an average across all Brussels measuring stations, over the period 2006-2014
Source : Brussels Environment, Laboratory for Environmental Research (air)

Despite large fluctuations from one month to another, mainly resulting from the quality of the meteorological dispersion (wind, rain, atmospheric stability, etc.), a (linear) downward trend can be observed in the concentrations (cfr. red line). The trend has evolved from the 20-25 µg/m3 bracket, before 2010, to the 15-20 µg/m3 bracket, since 2011.
Je ne mettrais pas cette dernière phrase

The overall decreasing trend in the long-term of fine particle concentrations is explained by measures taken to reduce pollutant emissions (for example the introduction of increasingly effective particle filters in vehicles). Thanks to improvements in these technologies, emissions of NOx are decreasing in Europe from year to year, except at the measuring stations which are influenced by road traffic, where a more stable trend is observed (see the indicator on the emissions of acidifying substances). NOx is a precursor of secondary particles, mainly formed in the PM2.5 fraction. It is therefore possible that the gradual decrease in PM2.5 concentrations is partially explained by the decrease in NOx.

Indicators for PM2.5 in the air

The PM2.5 indicator is based on data from the two following stations in order to be representative for the exposure of the majority of the population in the Brussels-Capital Region:

  • the Molenbeek-Saint-Jean station at Sluice no. 11, which is representative of an urban environment influenced by road traffic,
  • the Uccle station which records urban background concentrations, in other words concentrations in the air which are far from the source.

In order to evaluate the time evolution in the light of European values, the Brussels indicator for PM2.5 also takes into account annual averages (2nd chart) and the moving averages over three consecutive years (3rd chart) of the concentrations measured at these two stations.

Evolution of the annual average of PM2.5 in µg/m³ at the Molenbeek-Saint-Jean and Uccle stations, from 2006 to 2014
Source : Brussels Environment, Laboratory for Environmental Research (air)

It can be seen from the chart above that since 2006, the annual concentration of PM2.5 at the Molenbeek-Saint-Jean station has systematically remained below the European limit value of 25 µg/m3, except in 2011 when this value slightly exceeded this value (25.1 µg/m3). As a reminder, this limit value was not yet applicable at that time. It was therefore just a target value that was not mandatory in terms of European legislation. Since 2012, none of the stations of the Region monitoring network has exceeded the annual European limit value. It should be noted however that, despite compliance with the European standard and the clear improvement in concentrations of PM2.5 as an annual average in Brussels, it far exceeds the guide value of the WHO which is set at 10 µg/m3.

The Uccle station presents concentrations which are systematically lower than those of Molenbeek-Saint-Jean (except in 2014). This is logical since it is an urban background station which is not influenced by local sources. In 2014, the similar concentrations recorded at Uccle and Molenbeek-Saint-Jean (17 µg/m3 versus 16.7 µg/m3, respectively) can be explained on the one hand by the fact that it is possible, if the dispersion is excellent (as was the case in 2014), that the levels of background concentration and urban concentration become comparable, and on the other hand by the measurements uncertainty.  The fluctuations from one year to another largely results from the quality of the atmospheric dispersion depending on the meteorological conditions.

Evolution of the regional AEI corresponding to the moving average concentration of PM2.5 over three years, as an average across the Molenbeek and Uccle stations.
Source : Brussels Environment, Laboratory for Environmental Research (air)

The evolution of the Brussels AEI is decreasing overall, with a strong reduction between 2013 and 2014 and a fall below the threshold of 20 µg/m3 which corresponds to the objective set for 2015. Among other reasons, this can be explained by the fact that 2014 was an exceptionally good year in terms of air quality.

Although the objective for the average exposure of the population up to 2015 did not pose any problems, it is nonetheless too soon to draw any conclusions with regards to the objectives for 2020. The target value to achieve in 2020 for the Brussels AEI is 16.7 µg/m3. If we extrapolate the decreasing linear trend of AEI concentrations since 2008, this target value will only be achieved after 2025 (it is however probable that the introduction of new measures to reduce emissions will induce a quicker reduction in concentrations). However, if we base our calculation on the decreasing linear trend of concentrations since 2011, the value of 16.7 µg/m3 should be achieved between 2018 and 2020.

Sources of PM2.5

Due to their small size, PM2.5 particles are likely to be transported by air masses over long distances and, consequently, the concentrations measured at Brussels do not only result from local emissions. The concentrations can be explained by:

  • background pollution (as measured in the Ardennes, for example), coming from movements of air masses on a Europe-wide scale,
  • trans-regional contribution, brought into the Brussels-Capital Region via airflows between the Regions,
  • background urban pollution, in other words urban pollution measured far from its source, and resulting from heating and traffic emissions, as it is the case in the stations at Uccle and Berchem-St-Agathe,
  • local urban contributions, mainly due to traffic (as it is the case in a more densely populated environment such as in Molenbeek-St-Jean), and, as the case may be, the additional contribution from traffic that occurs in areas with high traffic density.

In addition to these various contributions, PM2.5 can also be:

  • emitted directly (primary particles) and
  • formed on a large scale on the basis of gaseous pollutants which are present in the air (secondary particles). With regards to mineral particles, these are mainly formed on the basis of nitrogen dioxide, ammoniac and sulphur dioxide.

Contributions to the concentrations of PM2.5 measured in the Brussels Region.
Source : Brussels Environment, Laboratory for Environmental Research (air)

The chart above shows an estimate of various contributions to PM2.5 from different measuring stations which are representative of the environments indicated above for the period 2010-2014 (last 5 years). The background calculation was calculated on the basis of the average from the stations at Vielsalm (43N085) and Habay-la-Neuve (43N132), the trans-regional contribution from the station at Corroy-le-Grand (43N063), the background urban contribution from the stations at Uccle (41R012) and Berchem-Sainte-Agathe (41B011), and finally the urban contribution from the station at Molenbeek-St-Jean (41R001). This estimate was drawn up on the one hand for every day of the period 2010-2014, and on the other hand for days of high PM10 concentration (in particular, days with a daily average in exceedance of the standard of 50 µg/m3), in the absence of a daily standard for PM2.5.

If we take into account all the days of the period 2010-2014, we can see that background pollution contributes 45% of the measured PM2.5 concentrations in Brussels, whereas the trans-regional and background urban contributions together account for 35%, and finally the local urban contribution represents 20%. If we only take into account the days which are characterised by an exceedance of the daily European standard for PM10, there is little variation in these proportions: the background contribution increases to 46%, the trans-regional and background urban contributions increase to 38%, and the local contribution decreases to 16%. We can conclude therefore that a minimum of 80% of the PM2.5 comes from long-haul and medium-haul transport. Just like PM10, PM2.5 particles are a pollutant with a large spatial extent. The remaining proportion (between 16 and 20%) is attributable to local sources.

Finally, it can be observed that the phenomenon of particle resuspension does not concern PM2.5, but mainly larger particles with diameter typically between 2.5 and 10 µm.

The PM2.5 situation at the Belgian level

In general, PM2.5 particles will be higher, as an annual average, in areas where population density is higher, due to the local contribution. In Belgium, concentrations of PM2.5 are higher to the north of the Sambre-Meuse line, as shown on the interpolation map below.

RIO Interpolation of the annual average of PM2.5 in Belgium in 2013
Source: IRCEL-CELINE, Annual report 2013 of the air quality in Belgium

The small black circles represent the measuring stations

Datum van de update: 26/10/2018