Air quality: pollution peaks in winter

Since 2009, the Brussels Region has an emergency plan for pollution peaks from PM10 and NO2. This plan defines 3 intervention thresholds, and the measures that must be activated for each threshold. Between early November 2009 and late March 2015, only the measures for the first intervention threshold were activated, as the forecasts for PM10 pollution levels never reached the conditions for the activation of the 2nd and 3rd intervention thresholds.

Context

For several years, the European Union has issued directives for the benefit of the air quality, in an effort to limit the impact of pollution by human activities on human health, climate and the environment.
Therefore European framework directive 2008/50/EC regarding the air quality and cleaner air for Europe - replacing directive 1996/62/EC –sets binding limit values for the concentration of nitrogen dioxide (NO2) and fine particles (PM10). When there is a risk for these values to be exceeded, the directive asks the Member States to provide an action plan for the short term, intended to curb this risk of exceedance and limit its duration.

Measures for Brussels

The government of the Brussels-Capital Region drew up an "emergency plan" aimed at informing the population in the event of pollution peaks due to fine particles or nitrogen dioxide in winter, and at taking the appropriate measures.

The provisions of the plan were established in the Brussels ruling (”arrêté”/”besluit”) of 27 November 2008: it provides for 3 intervention levels with measures that become increasingly more restrictive as higher pollution thresholds are reached. The measures are aimed at limiting the local emissions, from traffic on the one hand (speed limits, a system of alternating number plates, complete bans on driving) and from the heating of public buildings on the other hand. The ruling entered into force on 1 January 2009.

The intervention thresholds are attained when the established pollution thresholds are reached during the months November to March in at least two stations of the Brussels' telemetric monitoring network, during at least two consecutive days, and for at least one of the two pollutants envisaged. Adverse situations for the distribution of pollutants are indeed most likely to occur during this coldest time of the year. Weather conditions that cause the most persistent pollution peaks are extremely low wind speeds and the occurrence of temperature inversions. During the winter months, the persistence of these inversions is enhanced particularly by the limited number of sunshine hours.

Incidence of pollution peaks due to PM10 and/or NO2

Between early November 2009 and late March 2015, the first intervention threshold for PM10 was reached nine times and the second one twice. The latter event, however, did not lead to any measures of the 2nd intervention level because the peak was caused by a large-scale formation of secondary aerosols (in particular following incidences of fertiliser spraying on agricultural land), a phenomenon that is entirely unpredictable and which therefore does not fall within the conditions for activation of the plan.

Incidence (in the period November to March) of pollution peaks due to PM10 and/or NO2
Source : Brussels Environment, Laboratory for Environmental Research (air)

If we go further back in time, the concentrations recorded show us that the first intervention threshold is reached on average 2.5 times per year for PM10 and less than once a year for NO2; the second threshold value for PM10 is reached on average once every 3 years. So far, the second threshold value for NO2 has never been reached, nor has the third threshold value (neither for PM10 nor for NO2).