Clean air - Danish efforts
From emissions to impacts
Air pollution may create a number of environmental problems: poor living conditions for animals and plants, health problems with premature deaths among humans, material disintegration and climate changes.
Many compounds have the same impacts, and many sources emit the same compounds. Any reduction of pollution emissions will thus have a series of advantages. A reduction of emissions of sulphur dioxide will thus limit the ecological damages, the degradation of materials and the pressure on human health. It is more complicated for nitrogen oxide, which takes part in a series of chemical processes in the atmosphere.
A further complication is that different areas are not equally sensitive. In this connection the concept critical load is defined. Pollution levels below the critical load are expected not to cause significant impacts on nature and the environment.
The individual compounds come from many sources and have many different impacts that overlap and interact. Compounds with a lifetime in the atmosphere of a few days can be dispersed more than 1000 km. A reduction of the emissions can limit different impacts, but detailed mapping is necessary, when an effective abatement strategy is planned.
The critical load is not the same for different areas, but depends on vegetation, soil, climate etc. A calcareous soil in Denmark can, thus, be more robust than sensitive natural areas in e.g. Norway and Sweden.
Pollution sources
Air pollution is mainly caused by combustion, evaporation or biological processes.
Pollution from combustion
Any combustion forms new chemical compounds. The main constituent of the fuel, carbon, reacts with atmospheric oxygen and forms carbon dioxide (CO2). Some of the atmospheric nitrogen (N2) "burns" and forms nitrogen oxide (NOx). Combustion can further be incomplete, leading to formation of carbon monoxide (CO), soot/particles, polyaromatic hydrocarbons (PAH) and different volatile organic compounds (VOC). Finally most fuels contain impurities. Most important is the content of sulphur in coal and oil that burns to sulphur dioxide (SO2). But also incombustible constituents that may e.g. contain heavy metals are important.
A complex phenomenon
Air pollution starts with an activity that leads to emissions of chemical
compounds. They are dispersed and possibly transformed in the atmosphere, before they are
finally deposited and cause harmful impacts. The relations between the individual steps
are often complicated. That must be taken into account in the planning of technically and
economically efficient abatement measures.
The significance of long-range transport
Only a minor part of the sulphur emitted will be deposited in the same country. The
table shows how much sulphur is emitted in different countries, further the amount that is
deposited in the country itself, and finally the total amount that is deposited in the
country.
|
Emissions |
Deposited nationally |
Total deposition in country |
Denmark |
54 |
7 |
35 |
Sweden |
34 |
12 |
123 |
Norway |
15 |
5 |
76 |
United Kingdom |
813 |
263 |
333 |
Germany |
705 |
35 |
389 |
Belgium |
117 |
17 |
44 |
| (Rounded values for 1997 in 1000 t.) | (Data from EMEP) |
Pollution by evaporation
Volatile fuels (e.g. petrol) can be spilled or evaporate during transport, filling and running. Many processes in industry and crafts (e.g. surface treatment with paint) evaporate organic compounds.
Pollution from biological processes
Biologically related air pollution often arises from agriculture, where storing and use of manure and commercial fertilisers cause formation and emissions of ammonia (NH3), methane (CH4) and nitrous oxide (N2O). Methane is further formed by anoxic fermentation in the digestive system of domestic animals, especially ruminants, e.g. cows.
Methane and nitrous oxide are also formed by biological processes in wetlands. Human influence in the form of changes of land use can therefore change emissions. The same applies to evaporation of organic compounds from vegetation - especially coniferous. The border between natural and anthropogenic emissions of pollution is, thus, not clearcut.
Long-range transport of air pollution
The amount of air pollution that is emitted in a given country depends upon the size of the country, its degree of industrialisation, its energy sources and many other parameters. But it is absolutely not the same amount that is deposited. The possibility that contaminants are carried by the wind means that meteorological conditions are decisive. As appears from the table, a predominantly westerly wind means that countries like Norway and Sweden receive much more air pollution than they emit. The opposite applies for UK, Belgium and Germany. In all cases only a minor amount of the emitted pollution is deposited in the country itself.
In this game Denmark is for various reasons fortunate. The country's distribution on minor islands means that pollution sources are never located as closely as in foreign megacities or industrial areas. And Denmark has practically no strongly polluting industry. A flat landscape and much wind allow effective dispersion, and because of our predominantly westerly wind in combination with the larger cities being situated on east coasts, large parts of the pollution are blown out of the country. That, however, does not exclude that pollution can arise during special meteorological conditions. Also local urban areas can be strongly polluted - especially in case of heavy traffic.
In a street canyon pollution from cars is only dispersed with great difficulty, and high levels can build up. At an open motorway the same emission of contaminants is rapidly dispersed and thus causes much lower local concentrations. But all emissions pollute at longer distances.
A complex phenomenon
All these conditions demonstrate that a simple percentage reduction of national emissions is neither fair nor economically effective.
Although air pollution always starts with emissions of substances, normally much happens before the unwanted effects appear.
During dispersion the compounds can be transformed before they are deposited on soil and plants or inhaled by humans. Sometimes it is only after the compound has passed a food chain that the harmful effects appear.
Further a series of compounds are not only dispersed with the air, but also through other paths. Eutrophication, with resulting oxygen deficiency, is thus caused both by washing out of nutrients from agriculture and by deposition of nitrogen compounds from the air. For the inner waters the contaminants are also carried in the water.