Clean air - Danish efforts
Sulphur and acidification
Sulphur dioxide is together with soot the classical urban pollution, which causes respiratory trouble, soiling and material disintegration. Sulphur pollution arises mainly because organic materials, comprising both fossil fuels and bio fuels, contain sulphur. During combustion the sulphur is oxidised to sulphur dioxide (SO2). In the atmosphere it is further transformed to sulphate (SO4 -2) that is deposited either as salts or as sulphuric acid and is a significant reason why precipitation polluted with sulphur becomes acid.
"That the fish have died in thousands of lakes is something we have known for many years. But not until recently have we been able to establish that drinking water from springs and wells may, in consequence of acidification, contain sufficient amounts of toxic heavy metals to be a threat to health. That forest trees on acidified land may begin to show slower growth is so far only a suspicion - it will be at least another two decades before we know for certain" |
Until the early 1970's it was the general political assumption that sulphur pollution
was a local problem and could be solved by cleaner fuels in residential furnaces and high
stacks dispersing the pollution from power plants. It was, however, a short-sighted
solution. In June 1981 Statens Naturvådsverk (Swedish Environmental Protection Agency)
published a report where the problem was spelled out as shown in the box.
At that time 18,000 of Sweden's 85,000 large lakes were acidified and half of them to a degree that seriously harmed fish life. The Swedish area where the critical load for acidification is exceeded is now more than halved, from above 80% to below 40%.
Emissions of sulphur dioxide
Danish emissions of sulphur dioxide peaked around 1970 and have since been reduced to
around 1/10. The pattern is by and large the same as for Europe as a whole.
(Sources: EMEP, Risø, NERI)
Concentrations of sulphur dioxide
The reduction of emissions of sulphur has been a contributing factor for the heavy
decrease of concentrations in Danish cities. Around 1970 the yearly average in Copenhagen
was about 80 µg/m3. Today it is below 5 µg/m3. For comparison, the
EU limit value is 20 µg/m3.
(Source: NERI)
Danish emissions
In Denmark the sulphur content in oil has been regulated since 1972 and SO2 emissions from power plants and combined power-heating plants since 1984. The rules have been tightened several times, and taxes on sulphur were introduced in 1998. By use of cleaner fuels and desulphurisation of flue gas in power plants it has been possible to reduce Danish emissions of sulphur dioxide from nearly 600,000 tons per year to below 50,000 tons. Thus the target set for 2010 in the ECE and EU agreements (55,000 tons) is already achieved.
To this must, however, be added a significant, but not yet regulated contribution from shipping in the Danish waters. In 1990 to 2000 it was 133,000 tons per year and thus more than twice the contribution from landbased Danish sources. This contribution, however, is of minor importance for the air quality in urban areas, since ferries use cleaner fuel in harbours.
Sulphur dioxide in Danish cities
Efforts to reduce airborne sulphur pollution have contributed significantly to a reduction in local levels. It does not exclude, however, that under special meteorological conditions high peak values can appear. In the 19th century the average concentration of sulphur dioxide in the centre of Copenhagen must have been nearly 100 µg/m3 during the winter. When measurements proper were initiated in the beginning of the 1960's it was about 80 µg/m3. The impacts can still be seen in the form of deteriorated sandstone monuments and corroded statues. Today the levels of sulphur dioxide are less than 5 µg/m3 and thus significantly below both current and planned limit values.
A corresponding development has been seen in other cities in Denmark and abroad, caused by several conditions: less sulphur in fuel oil and coal, regulation of emissions from power plants, and increased use of natural gas that does not contain sulphur. A contributing factor is increasing use of district heating often based on combined power and heat production in large plants with high stacks and flue gas desulphurization.
Extensive damages to ecological systems focus attention on transboundary sulphur pollution. The phenomenon, however, is complicated, and with decreasing emissions of sulphur the largest acidifying contribution today comes from nitrogen. Also pollution with ozone can be important. The recent years' growing occurrence of the "red" Norway Spruce in Denmark may be related to climate change.