Acid Deposition : Acid Rain, Mist and Fog

Acid deposition is a general name for a number of phenomena, namely acid rain, acid fog and acid mist. This means it can imply both wet and dry (gaseous) precipitation. Acid deposition is a rather well known environmental problem, for example acid fog killed several thousand people in London in 1952.

Acid deposition is concerned with long-range rather than local effects. Pollutants are mixed in the atmosphere and therefore usually cannot be attributed to any local source. Pollutants are generally more dispersed and of lower concentrations than local ground level pollutants.

Acid deposition typically has a pH below 4, but this may be as low as 1.5 under seriously acidic conditions. It primarily consists of two types of compounds, namely sulphuric acid (H₂SO₄) and nitric acid (HNO₃).

Sulphuric acid is formed by conversion of sulphur dioxide emitted from power stations, melting processes, home fires, car exhausts and other sources. It contributes about 70% to the overall acidity of deposition.

Reaction mechanism: SO₃ + H₂O -> H₂SO₄

Nitric acid is formed from nitrogen oxide (NO_x) emissions from fossil fuel combustion. It contributes about 30% to the overall acidity of deposition.

Reaction mechanism: NO₂ + OH^- -> HNO₃

Acid rain has various environmental and health effects, for example:

- Chocking plant leave pores (forest loss)

- Corroding stone and brick walls of buildings and monuments

- Corroding paper and rubber objects

- Altering soil chemistry (soil acidification, loss of plant nutrients)

- Altering the chemical balance of lakes and streams

- Disrupting fish gill operation (fish deaths)

- Deteriorating human breathing disorder (asthma, bronchitis, lung oedema)

When people die of acid deposition it is usually caused by access mucous production in the bronchi, leading to chocking from a lack of oxygen, or a heart attack.

Acid deposition in various countries

Acid deposition is a transboundary environmental problem. This basically means that emissions in one country may affect forests and structures in a neighbouring country. Therefore, international agreements were made, such as the Sulphur emissions Reduction Protocol (1979) and the Convention on Long-Range Transboundary Air Pollution (1983).

Some examples of countries that experience(d) acid deposition, either from their own sources or from transboundary air pollution:

- Britain: smog episodes around London, particularly in 1952

- Germany: acid mists in central Germany and the Black Forest area, acid cold smog from Poland and former Czechoslovakia in 1985

- Greece: intense industrialization in the Athens area causes deterioration of ancient monuments such as the Parthenon by acid deposition

- Italy: damage to Venice structures from acid deposition

- Scandinavia: 15% of acid rain caused by Great-Britain

- Scotland: episodes of black acid snow in the Cairngorm mountains in 1984

- The Netherlands: corrosion of bells of the Utrecht Dom tower since 1951

- United States: acid rains disrupts forest ecosystems and pollutes surface waters, industrial fossil fuel combustion processes are adapted to prevent sulphur dioxide emissions.