Nitrogen compounds are present in the atmosphere in both oxidised and reduced
forms. The reduced compounds include ammonia (NH3) and ammonium (NH4). The
oxidised compounds include nitrous oxide (N2O), nitrogen oxide (NO), nitrogen
dioxide (NO2), nitrous acid (HNO2), nitric acid (HNO3), peroxyacetylnitrate (PAN)
and particulate nitrate (NO3
-). NOx is defined as NO+NO2.
Nitrous oxide (N2O) is emitted to the atmosphere due to bacterial activity in the soil. It
is emitted also from anthropogenic sources (e. g. catalytic reduction processes) and
acts as a greenhouse gas. However, it is disconnected from the NOx chemistry in the
atmosphere and is not important for assessment of air quality.
Nitrogen oxides are emitted mainly (in most cases >90%) as NO. NO2 is formed
relatively rapidly from NO by reaction with ozone or radicals, such as HO2 or RO2.
Via a number of different atmospheric reactions some nitrogen oxides will finally
-, which, with NO2 are removed from the atmosphere via wet and
dry deposition processes. Although it is not the major nitrogen oxide species in all
areas, NO2 is one of the most important air pollutants in urban areas, as it is the most
significant nitrogen oxide species from a human healthpoint of view.
Nitrogen oxides play a key role in the formation of photochemical oxidants, and this
position paper will deal mainly with nitrogen dioxide and to a lesser extent, NO in air.
The various sources, concentrations in urban and rural air, and the environmental
effects of nitrogen oxides are considered. The environmental effects include human
health effects, material damage and ecosystem (acidification and eutrophication)
effects caused by NO2 in the air, and deposition of nitrogen compounds. Effects on
the ecosystems will mainly be the same, whether oxidised or reduced nitrogen
compounds are deposited. For this reason, reduced nitrogen compounds will impact
the assessment of oxidised nitrogen in the environment. A further factor is the
interaction between nitrogen oxides and ammonia in the atmosphere.
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