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Emissions of ozone precursors (NOx, VOCs, CO and CH4)

Ozone is a major air pollutant as it is harmful to humans, fauna and flora. First and foremost, it causes issues in the summer when the meteorological conditions are favourable in terms of light and heat, but also in the presence of ozone precursors, since these will consequently undergo a chemical reaction transforming them into ozone.
Between 1990 and 2010 the emissions of ozone precursors in the Brussels Region dropped by 69%.
Road traffic is the primary emission source of ozone precursors (with more than 45%).


Tropospheric ozone features in the list of air quality indicators due to its impact on health and on the environment. Its toxicity varies depending on the concentration; in abnormally high quantities ozone can cause serious health issues. Moreover, it can cause alterations in crops and woods, and adversely affect a multitude of materials.
Tropospheric ozone is a secondary pollutant, which means that it is not emitted directly into the ambient air, but formed by photochemistry in the atmosphere. This occurs especially during the summer, as a result of the irradiation of primary pollutants (such as nitrogen dioxide NO2) through ultraviolet radiation (UV). The reaction is disrupted by the presence of various substances: volatile organic compounds (VOCs), the radical that results from methane oxidation (CH4); or a reaction between carbon monoxide (CO) and the hydroxyl radical (OH) in the atmosphere (see methodological sheet).
Thus, the substances NOX, VOC, CH4 and CO are considered to be the primary precursors of tropospheric ozone. 

Emitted amounts of ozone precursors per source

In 2010, more than 11,000 tonnes of VOC equivalents were emitted within the Brussels' territory. Road traffic is the primary emission source of tropospheric ozone precursors, accounting for 45% of the emissions. Another important source is domestic solvent use (12%, included in the category “Others”) .

 Emitted amounts of ozone precursors per source

Evolution of the emitted amounts

Between 1990 and 2010, the emissions of ozone precursors dropped by 69 % (11.2 kt VOC equivalents in 2010 versus 36.0 in 1990).
This decrease was proportionally higher for VOCs than for NOx, which together accounted for 92% of the emissions in 2010.

 Evolution of the emitted amounts

The explanation for this evolution is to be found with factors that vary depending on the substance.
As for the VOCs, the following factors contributed to the decline in emissions:

  • ­the reduced production at the cokes plant of Marly, followed by its closure in 1993,
  • ­the introduction of several European directives: enhanced emission performance of engines (the "EURO" standards), enhanced fuel quality, and reduced VOC emissions from petrol stations and the use of organic solvents.

The decline in NOx emissions is linked to:

  • ­ the closure of the cokes plant of Marly in 1993,
  • ­ the installation of a filter on the waste incinerator of Neder-Over-Heembeek (2006),
  • ­ the enhanced performance of engines ("EURO standards")
  • ­ the standardisation of catalytic converters on new vehicles as of 1993 (which subject the exhaust gases to a post-treatment upon leaving the engine, resulting in reduced NOx emissions, particularly for petrol cars). The importance of the catalytic converter with regard to the reduction of NOx emissions, however, only becomes apparent after travelling several kilometres (when the engine is cold, during ignition and acceleration/deceleration, the catalytic converter is completely or partially ineffective).  Hence, this factor only applies to longer distances.

The decline in CO emissions mainly results from the establishment of the EURO standards, made possible by the introduction of the catalytic converter. The increased share of diesel vehicles in the total vehicle stock has also contributed to this decline: diesel vehicles emit little CO thanks to their catalytic converters and because their strongly oxidising exhaust fumes facilitate the conversion of CO into CO2

European standards

European Directive 2001/81/EC (the so-called "NEC directive") sets binding emission ceilings, for instance for air pollutants that are tropospheric ozone precursors. In the context of the effort sharing decision (in 2000) for the 3 Belgian Regions and the federal state, the Brussels-Capital Region has had to respect the following ceilings from 2010 onwards: annual emissions may amount to 3.66 tonnes of VOC equivalents for NOX and 4 tonnes of VOC equivalents for VOCs (in both cases this only applies to stationary sources and not to transport; for mobile sources the country-specific ceiling applies). The BCR did not receive a specific ceiling for CO and CH4 since these substances play only a marginal role in the emissions.
According to the results of the models illustrated in the above graph, these ceilings are being observed. This has been the case since 2006 for NOX and since 2007 for the VOCs.


Date de mise à jour: 19/01/2018