Clean air - Danish efforts
Nitrogen and eutrophication
Air pollution with nitrogen compounds is more complicated than sulphur pollution. Like sulphur dioxide (SO2), nitrogen dioxide (NO2) immediately irritates the respiratory system and harms vegetation. Furthermore, NO2 can be oxidised and contribute to acidification. However, NO2 can under the influence of sunlight react with hydrocarbons and form photochemical oxidants, of which ozone (described in the following section) is the most important.
Finally, nitrogen has a fertilising effect. Air pollution with nitrogen compounds therefore contributes to the eutrophication that puts pressure on sensitive nature. Also the deposition of nitrogen to inner Danish waters is a contributing cause of oxygen deficiency.
Through chemical processes in the soil ammonia can liberate hydrogen ions and, thus, contribute to acidification.
Formation and emissions of nitrogen oxides
As was the case with sulphur dioxide, pollution with nitrogen dioxide is mainly due to combustion - partly because the fuels contain nitrogen compounds. But also nitrogen in the combustion air is oxidised - first into nitrogen monoxide (NO) and later in the atmosphere into NO2, which is the actual harmful compound.
Danish emissions of nitrogen oxide expressed as NO2 increased to about 300,000 tons per year in the middle of the 1980's. It then remained nearly constant until the middle of the 1990's, but is now reduced to about 200,000 tons per year. The reason has mainly been installation of low-NOx burners and de-NOx equipment in power plants and district heating plants. Increasing use of catalytic converters in cars has at the same time more than counteracted the increase in traffic. Emissions today, however, are still larger than the EU and ECE targets for 2010 of 127,000 tons a year.
Air pollution with nitrogen compounds contributes to eutrophication of the aquatic environment. This can result in extensive oxygen deficiency and fish deaths. For the total inner waters around Denmark about 1/4 of the bio-available nitrogen comes via the air. Also for natural ecosystems, where the species are adjusted to a nutrient-poor environment, pollution with nitrogen compounds may change the composition of the species and reduce their number.
Nitrogen dioxide in Danish cities
In the last decade, the yearly average of nitrogen dioxide in Danish cities has also fallen, although not as fast as the emissions. In Copenhagen, thus, from about 50 to 40 µg/m3. This corresponds to the present EU limit value, whereas the level envisaged is 30 µg/m3. The reason why the decrease has not yet been larger is, partly, that the NO2 level depends upon the occurrence of ozone in the air.
Ammonia
Nitrogen is also present in the form of ammonia (NH3) - a gas that is i.a. formed in bacterial decomposition of organic material. Earlier ammonia was only discussed in connection with odour problems, although in fact it is not the ammonia, but a series of degradation products that cause the greatest nuisances. In Denmark emissions of ammonia are almost exclusively due to agriculture - in particular evaporation in connection with application of nitrogen fertilisers (including manure). Only 2% are due to traffic.
In agriculture nitrogen in different forms of fertilisers and atmospheric deposition is added to the soil. In return, nitrogen is removed with crops and animal products. The nitrogen surplus constitutes the environmental problem proper. This includes the loss of nitrogen in the form of emissions of ammonia to the atmosphere, now in the order of 100,000 tons per year. It has been reduced by about 30% since 1985 i.a. by application of other foodstuffs, by sealing manure containers and by using dragging tubes in the distribution of the manure.
Further, in 2000-2001 a new ammonia action plan to limit the evaporation was adopted. It is expected that Denmark will be able to reach the EU and ECE target for emissions of ammonia in 2010 (39,000 tons) as a result of i.a. the Action Plan for the Aquatic Environment II.
Exceedance of critical loads
In Denmark the total area where the critical load is exceeded is only very modest, but it includes agricultural land, cities etc. Of the natural area proper, which constitutes 15- 20%, still about half is loaded above the critical limit of eutrophication load. In recent years a minor reduction seems to have occurred, but more exact inventories have not been drawn up.
Emissions of nitrogen oxides and ammonia
Danish emissions of nitrogen oxide peaked in the late 1980's, and have now fallen by
about 30%. According to recent inventories emissions of ammonia have fallen by about 30%
since 1985.
(Sources: Risø and NERI)
NO2 in urban and rural air
The concentration of NO2 in Danish air has also been reduced, but not as
fast as emissions.
(Source: NERI)
Total acidifying emissions
Sulphur dioxide, nitrogen oxide and ammonia all have an acidifying effect. The
emissions can be expressed with a common unit: acid equivalents. The sum has fallen in
Denmark by about 40% in the recent decade, and ammonia is now dominating. This, however,
does not give a direct impression of the actual load, which is strongly influenced by
transboundary pollution with a significant contribution from sulphur.
(Source: NERI)