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

At low altitude (up to approximately 10 km), ozone is one of the main air pollutants, and is harmful to human health, flora and fauna, if it is present in abnormally high quantities. It requires solar energy to form in the lower atmosphere, which is abundant in summer. This phenomenon of ozone creation is strengthened in the presence of ozone precursors (essentially NOx and VOCs).
In 2012, almost 12,000 equivalent tonnes of VOCs were emitted in the Brussels territory, of which more than 93% were VOCs and NOx. Road transport was the main source of ozone precursors (almost 51% in 2012).
Between 1990 and 2012, emissions of ozone precursors fell by 66% in the Brussels-Capital Region.


Tropospheric ozone is a pollutant that affects health and 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).

The substances, NOx, VOCs, CH4 and CO are thus considered to be the primary precursors of tropospheric ozone.

Emitted amounts of ozone precursors per source

In 2012, almost 12,000 tonnes of ozone precursors (eq. tonnes of VOCs) were emitted in the Brussels territory. Road transport is the main source of emissions of tropospheric ozone precursors: it is responsible for almost 51% of emissions, as well as industrial processes and the use of products (23%).

Distribution per sector of emissions of tropospheric ozone precursors in the Brussels-Capital Region, in 2012
Source: Brussels Environment, Dpt Planning air, energy and climate

2012 is used here since it corresponds to the most recent data from a validated version of the regional energy balance. Data from 2013 have actually been calculated using a provisional version of this balance.

Evolution of the emitted amounts

Between 1990 and 2013, the emissions of ozone precursors dropped by 66 % (11.9 kt VOC equivalents in 2013 versus 35.4 kt VOC equivalents in 1990).

This decrease was proportionally higher for VOCs than for NOx, which together accounted for 93.7% of the emissions in 2013.

Evolution of emissions of tropospheric ozone precursors in the Brussels-Capital Region, between 1990 and 2013
Source: Brussels Environment, Dpt Planning air, energy and climate
(* : provisional data)

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 decline in production at the Marly coking plant in 1990, followed by its closure in 1993,
• the installation of a fume cleaning system ("DeNOx") at the waste incinerator at Neder-Over-Heembeek (2006),
• the enhancement of engine performance due to the implementation of various directives of the European Union relating to polluting emissions standards for different categories of vehicles ("EURO" standards)
• and 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 of their strongly oxidising exhaust fumes that 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 (VOCs and NOx). 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).

Since 2006, the Brussels-Capital Region has complied with the NOx ceiling; and has complied with its emissions ceiling for VOCs since 2007.

With regards to emissions of other precursors (CO and CH4), Belgium is not subject to any restrictions.

Datum van de update: 26/10/2018