Acidifying gases

Acid deposition of sulphur and nitrogen compounds mainly derives from emissions of SO₂, NO_X 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 NO_X 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:

where:

A is the acidification index in Mmole

m_i is the emission of pollutant i in tonnes

M_i is the mole weight [tonne/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 2009, the most important acidification factor in Denmark was ammonia nitrogen and the relative contributions for SO₂, NO_X and NH₃ were 6 %, 36 % and 58 %, respectively. However, with regard to long-range transport of air pollution, SO₂ and NO_X are still the most important pollutants.

Read more about the acidifying gasses:

• SO₂, Sulphur dioxide
• NO_X, Nitrogen oxides
• NH₃, Ammonia

Other air pollutants

Read more about other air pollutants:

• NMVOC, Non Methane Volatile Organic Compounds
• CO, Carbon monoxide
• PAH, Polycyclic Aromatic Hydrocarbons
• PM, Particulate Matter

Heavy metals

Total emissions for 1990 and 2009 are shown with the corresponding reduction since 1990 in the table below.

In general, the most important sources of heavy metal emissions are combustion of fossil fuels and waste, but tyre and brake wear in the transport sector is the dominating source fom Cu, Pb and Zn. Further, national navigation has a large contribution to the national emission of Ni. The heavy metal emissions have decreased substantially in recent years, except for Cu. The reductions from 1990 to 2009 span from 30 % to 92 % for Zn and Pb, respectively. The emission of Cu has increased 36 % in the same period. 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.

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.

• Pb
• Cd
• Hg