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Nitrogen oxides and its sources

Nitrogen oxide (NOx) is the generic name for a mixture consisting mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen oxides are primarily emitted by human activities during high-temperature combustion processes in which dinitrogen (N2) is oxidized. The major sources of NOx are (road) transport, energy production and industry (including refineries) and building heating. In Belgium, almost half of NOx emissions come from road transport. NO2 is therefore a good proxy for the complex mixture of transport-related air pollution. NOx is mainly emitted as NO and to a lesser extent as NO2 (except in the case of diesel vehicles, where the NO2/NOx ratio can total 60%) (Grice et al. 2009). However, NO has a very short lifetime (a few minutes). NO undergoes photochemical reactions with other substances, including ozone and volatile organic compounds (VOCs), to form NO2 which has a longer atmospheric life time (a few hours to days). Apart from anthropogenic emissions, NOx is also emitted into the atmosphere via biochemical processes in the soil, by lightning and forest fires.

Chemical processes

Nitrogen oxides also play an important role in the formation of ozone and aerosols. On hot summer days with strong solar radiation, the UV light of the sun causes NO2 to dissociate into NO and a free oxygen radical (O-). The latter will subsequently react with an oxygen atom to form ozone (O3). Ozone is a very reactive gas with harmful effects for the population and ecosystems. Nitrogen oxides also play a role in the formation of aerosols. Via chemical reactions in the atmosphere, NOx is responsible for the formation of nitrate ions (NO3-), a secondary component of particulate matter. The longer lifetime of NO2, allows this pollutant to be transported over greater distances, so that damage is also caused in more remote areas where fewer or no sources of air pollution are present.

Nitrogen oxides also cause acidification and eutrophication of the environment (MIRA, 2011; MIRA, 2006). NO2 is converted in the atmosphere to nitric acid (HNO3). Dry or wet deposition of nitric acid causes acidification of soil and water, thereby leading to the degradation of ecosystems. Acidification is defined as the combined effects of air pollutants that are imported via the atmosphere and from which acids (such as HNO3) can be formed (MIRA, 2006). Eutrophication denotes the accumulation or enrichment of soil or groundwater with nutrients (including N). High nutrient concentrations have a disruptive effect on ecosystems (MIRA, 2011).


Exposure to very high NO2 concentrations can cause immediate adverse health effects due to the toxicity of the gas. The effect of long-term exposure to current NO2 concentrations is difficult to isolate in epidemiological studies. It is, however, clear that adverse health effects are associated with transport emissions and NO2 is strongly correlated with the mixture of transport-related air pollution. For that reason, and also because NO2 is indirectly harmful to people and the environment, limit values have been set by the European Commission and the World Health Organisation.

EU and WHO standards

standardsprotection objectiveaveraging periodvaluemax number of exceedancesdate by which value is to be met
EU alert threshold* human health 1 hour 400 µg/m³
EU limit value human health 1 hour 200 µg/m³ 18 1 january 2010
EU limit value human health 1 year 40 µg/m³ 1 january 2010
EU critical level vegetation (NO+NO2) 1 year 30 µg/m³
WHO guideline human health 1 hour 200 µg/m³ 0
WHO guideline human health 1 year 40 µg/m³

*to be measured over three consecutive hours at locations representative of air qualtiy over at least 100 km² or an entire zone or agglomeration, whichever is the smaller.