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 relatively easily neutralised.

The figure below 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 2013, the most important acidification factor in Denmark is ammonia and the relative contributions for SO2, NOX and NH3 were 6 %, 36 % and 58 %, 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 2013 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 fossil fuels and waste. The heavy metal emissions have decreased substantially in recent years, except for Cu. The reductions span from 19 % 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 (94 % in 2013) and the 30 % increase from 1990 to 2013 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.

Henvendelse om denne sides indhold: 
Revideret 09.09.2015

Her finder du Institut for Miljøvidenskab

Vis detaljeret kort


Institut for Miljøvidenskab
Aarhus Universitet
Frederiksborgvej 399
Postboks 358
4000 Roskilde


Tlf: 8715 0000

EAN-nr: 5798000867000

Find vej til instituttet