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Evolution of the tropospheric ozone concentrations

Tropospheric ozone is formed in the atmosphere from other substances such as nitrogen dioxide NO2. The photochemical reaction mainly occurs between mid-June and mid-August, potentially causing ozone peaks in the event of a heatwave combined with the presence of certain other catalysts, such as volatile organic compounds. If ozone occurs at abnormally high quantities, it can cause serious health issues. In the Brussels Region, the European target for the protection of the public health has been observed since 2005.


Ozone is a secondary pollutant. This means that ozone is not formed directly in the ambient air by human activities, but by photochemistry. The phenomenon occurs from mid-June to mid-August as a result of the irradiation of primary pollutants (such as NO2) by ultra-violet radiation (UV) in the presence of oxygen. Please refer to the methodological sheet which gives a detailed description of the ozone formation and depletion processes.
Ozone tops the list of air quality indicators, due to its impact on human health (impaired respiratory function) and on the environment. The toxicity of ozone varies according to its concentration.

European target values

In order to prevent harmful long-term effects on human health and/or on the entire environment, European ambient air quality directive 2008/50/EC sets the following target value for ozone, legally binding since 2010:

  • 120 µg/m³ as the maximum daily moving 8-hour mean,
  • maximum 25 exceedance days per year, averaged over 3 consecutive years.

Evolution of O3 concentration in the air

The Region's telemetric network comprises 7 monitoring sites that continuously monitor the tropospheric ozone. Our indicator is based on the data recorded in Uccle. As the monitoring site in Uccle is located in a residential area with only light traffic and at a reasonable distance from the major trunk roads, the ozone formation processes prevail over the depletion processes that occur when NO is present (gas emitted by traffic for instance).

Evolution of the annual average ozone concentration at the monitoring site of Uccle (1986-2012)
Source: Environnement Bruxelles-Leefmilieu Brussel, Laboratory for Environmental Research (air)
Evolution of the annual average ozone concentration at the monitoring site of Uccle (1986-2012)

In 2011 and 2012, the annual average concentration of tropospheric ozone at the monitoring site of Uccle amounted to 44 µg/m³. The increase in the average concentration during the 1990s is much less pronounced in the years 2000. The annual average seems to stabilise.
The increase during the 1990s can be explained by an overall drop in the NO concentrations (ozone-depleting pollutant).
The monitoring locations in Berchem-Sainte-Agathe and at the European Parliament, albeit to a lesser extent for the latter, reveal high annual average concentrations. By contrast, in the city centre and in the vicinity of trunk roads (monitoring sites of Woluwe-Saint Lambert and the Quai aux Briques) primary nitrogen monoxide emissions from traffic contribute directly to ozone depletion, explaining the reduced ozone concentrations.

Compliance with the European target values

Number of exceedance days recorded at the monitoring site of Uccle for the target of 120 µg/m³ applicable to the 8-hour mean ozone concentrations (1986-2012)
Source: Bruxelles Environnement–Leefmilieu Brussel, Laboratory for Environmental Research (air)
Number of exceedance days recorded at the monitoring site of Uccle for the target of 120 µg/m³

In the Brussels Region, the European target value for health protection has been adhered to since the period 2003-2005. During the 3-year period 2010-2012, the average number of exceedance days which depends on the characteristics of the surrounding area, ranged from 5 (monitoring site Avant-Port) to 13 (monitoring site Uccle). This number is well below the allowed average of 25 exceedance days, averaged over 3 years.
It is striking that, invariably, for all the years in which the months July and August were sunny and warm, more than 25 exceedance days were recorded for that particular year. This occurred for instance in the years 2003 and 2006.
Two summer periods with moderately sunny weather, or an occasional outstanding summer over a 3-year period suffice to either reach or exceed the permissible number of days above 120 µg/m³.


Date de mise à jour: 29/05/2020