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Air pollutants

Acidifying gasses

Acid deposition of sulphur and nitrogen compounds mainly derives from emissions of SO2, NOX and NH3. The effects of acidification show up in a number of ways, including defoliation and reduced vitality of trees, declining fish stocks in acid-sensitive lakes and rivers.
SO2 and NOX can be oxidised into sulphate (SO4--) and nitrate (NO3-) - either in the atmosphere or after deposition - resulting in the formation of two and one H+, respectively. NH3 may react with H+ to form ammonium (NH4+) and by nitrification in soil NH4+ is oxidised to NO3- and H+ are formed.
Weighting the individual substances according to their acidification effect total emissions in terms of acid equivalents can be calculated as:

where  mi is the emission of pollutant i in ton
          Mi is the mole weight [ton/Mmole] of pollutant i

The actual effect of the acidifying substances depends on a combination of two factors: The amount of acid deposition and the natural capacity of the terrestrial or aquatic ecosystem to counteract the acidification. In areas where the soil minerals easily weather or have a high chalk content, acid deposition will be relatively easily neutralised.
Figure 2.1 shows the emission of Danish acidifying gases in terms of acid equivalents. In 1990 the relative contribution in acid equivalents was almost equal for the three gases. In 2003 the most important acidification factor in Denmark was ammonia nitrogen and the relative contributions for SO2, NOX and NH3 were 7 %, 38 % and 55 % respectively. However, regarding long range transport of air pollution SO2 and NOX are still the most important pollutants.

acidifying gasses

Read more about the acidifying gasses:


Other air pollutants

Read about other air pollutants :

  • PM, Particulate Matter

Heavy metals

Total emissions for 1990 and 2004:

( Kg ) As Cd Cr Cu Hg Ni Pb Se Zn
1990 1504 1139 6326 10253 3339 25244 122074 4470


2004 657 577 1161 8447 1064 9546 5254 1837 23412
Reduction in % 56 49 82 18 68 62 96 59 34

In general the most important sources of heavy metal emissions are combustion of fossil fuels and waste. The heavy metal emissions have decreased substantially the last years. The reductions span from 18 % and 96 % for Cu and Pb, respectively. The reason for the reduced emissions is mainly the increased use of gas cleaning devices at power and district heating plants (including waste incineration plants). The large reduction in the Pb emission is due to a gradual shift towards unleaded gasoline being essential for catalyst cars.
According to the UNECE Heavy Metal Protocol the priority metals are Pb, Cd and Hg and the objective is to reduce the emissions of these heavy metals.


Henrik Bruun


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