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Emissions of acidifying substances (NOx, SOx and NH3)
Between 1990 and 2011, the emissions by human activities of acidifying and potentially acidifying substances dropped by 67 % in the Brussels-Capital Region. Since 2006, the Region has observed the ceilings imposed by Europe. Road transport is responsible for 58% of the regional emissions, while 34% is due to the heating of buildings (residential and tertiary).
Acidification is actually a natural phenomenon caused by sulphur emissions from volcanos, or gas that is released due to the activity of certain bacteria in the soil during the degradation of organic material, ... This phenomenon, however, has spread further as a result of human activities (heating, road traffic, industrial incineration processes, ...). The increased human impact has intensified the problem of soil and surface water acidification, as well as the damage to the vegetation and to certain building materials.
Sulphur dioxide SO2, nitrogen oxide NOX and ammonia NH3 are the three primary gases contributing to the acidification phenomenon; with the understanding that NH3 is potentially acidifying depending on environmental conditions (for more details, please refer to the methodological sheet).
Emitted amounts of acidifying substances per source
In 2011, approximately 125 tonnes of acid equivalents were emitted within the Brussels' territory. Road traffic alone accounted for 45% of the emissions of acidifying and potentially acidifying substances. In that same year, the sum of road transport and heating of buildings (residential and tertiary) represented 92% of the emissions.
Sectoral distribution of acidifying or potentially acidifying emissions in the Brussels-Capital Region (2011)
Source: Environnement Bruxelles-Leefmilieu Brussel, Dpt Planning air, energy and climate
In comparison, the major emitters of acidifying or potentially acidifying substances in the Walloon Region in 2010 were agriculture (35.1%), industry (26.8%), and road transport (21.4 %) [Indicateurs clés de l'environnement wallon, 2012]. The emitters in the Flemish Region in 2011 were agriculture (42%), transport (26%), industry (16%) and energy (10%) [MIRA – Kernset Milieudata, Nov. 2013].
In other words, transport and heating are proportionally less significant sources in the other Regions; this difference being explained by the quintessentially urban character of the Brussels-Capital Region.
Evolution of the emitted amounts
Between 1990 and 2011, the emissions of acidifying and potentially acidifying substances dropped by 67 % (126 tonnes of Aeq. in 2011 versus 385 tonnes of Aeq. in 1990).
Hence, SOx experienced a proportionally stronger decrease than NOx.
Evolution of the acidifying or potentially acidifying emissions in the Brussels-Capital Region between 1990 and 2011
Source: Environnement Bruxelles - Leefmilieu Brussel, Dpt Planning air, energy and climate
The explanatory factors for this evolution vary depending on the substance.
With regard to SOx, the following factors contributed to the reduced emissions:
- the reduction of the sulphur content in vehicle fuels (particularly since 1996) and in fuel oil (limited to 0.2% by weight since 1989),
- the growing share of natural gas in the total fuel consumption, at the expense of petroleum products,
- the reduced production followed by the complete closure of the cokes plant of Marly in 1993,
- and the introduction of a fume scrubbing system in the waste incinerator (mid-1999).
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 same waste incinerator of Neder-Over-Heembeek (2006),
- the enhancements of engine performances thanks to the introduction of certain European directives regarding the emission of pollutants from different vehicle categories ("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 in the Brussels Region should be put into perspective, however, as the effect of a catalytic converter 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.
Finally, the recent decrease in NH3 emissions appears to be linked to the dieselisation of the vehicle stock. The use of three-way catalytic converters on petrol cars indeed seems to result in an increase of NH3 and N2O emissions (which explains why NH3 emissions increased in 2000 compared to 1995). On the other hand, the two-way catalytic converter of diesel vehicles does not remove the nitrogen oxides. Since no reaction occurs with the nitrogen oxides, diesel vehicles do not emit any ammonia.
European Directive 2001/81/EC (the so-called "NEC Directive") sets binding emission ceilings for acidifying air pollutants, among others. Because of the effort sharing decision (in 2000) for the 3 Regions and the federal state, the Brussels-Capital Region had to adhere to the following ceilings as of 2010: annual emissions may amount to maximum 43.82 tonnes Aeq. for S02 and 65.1 tonnes Aeq. for NOX (in both cases this only applies to stationary sources, which means that transport is not taken into account; for mobile sources the country-specific ceiling applies). The BCR was not given a specific ceiling for NH3 as this substance is relatively less significant in the emissions.
According to the results of the models illustrated in the above graph, these ceilings have been observed since 2006.