Air pollutants

Acidifying gases

Acid deposition of sulphur and nitrogen compounds mainly derives from emissions of SO2, NOX and NH3. The effects of acidification may appear in a number of ways, including defoliation and reduced vitality of trees, and 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+ ions are formed.

Weighting the individual substances according to their acidification effect, total emissions in terms of acid equivalents can be calculated as:



A is the acidification index in Mmole

mi is the emission of pollutant i in Mg

Mi is the mole weight [Mg/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 lime content, acid deposition will be neutralised relatively easy.    

The figure 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 2016, the most important acidification factor in Denmark is ammonia nitrogen and the relative contributions for SO2, NOX and NH3 were 4 %, 35 % and 61 %, respectively. However, with regard to long-range transport of air pollution, SO2 and NOX are still the most important pollutants. 

Emissions of acidifying gases


Read more about the acidifying gasses:


Other air pollutants

Read more about other air pollutants:


Heavy metals

Total emissions for 1990 and 2016 are shown with the corresponding reduction since 1990 in the table below.

Emissions of heavy metals

In general, the most important sources of heavy metal emissions are combustion of fuels and waste. The heavy metal emissions have decreased substantially in recent years, except for Cu. The reductions span from 12 % to 91 % for Zn and Pb, respectively. The reason for the reduced emissions is mainly 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, the latter being essential for catalyst cars. The major source of Cu is automobile tyre and break wear (93 % in 2016) and the 31 % increase from 1990 to 2016 owe to increasing mileage.    

According to the UNECE Heavy Metal Protocol, the priority metals are Pb, Cd and Hg and the objective is to reduce emissions of these heavy metals.