Main phenomena

Acidification
Acid pollution (or acid rain) is caused by acid pollutants (SO₂, NOx, NH₃, HCl, HF) emitted by human activities. Some of these pollutants are deposited close to the emission source, but others are carried hundreds or even thousands of kilometres away. These pollutants are depsoited in wet or dry form. While being carried through the air, these pollutants are transformed. SO₂ and NOx change into sulphates (SO₄^2-) and nitrates (NO₃^2-) if the atmosphere is dry, or into sulphuric acid (H₂SO₄) and nitric acid (HNO₃) where the atmosphere is humid.

Large-scale phenomena of acid pollution can be seen in the acidification of water in Scandinavian and Canadian lakes. The pH of the water has turned acid, bringing massive changes to the fish populations. Some rain has a pH of between 3 and 4, whereas pure water has a pH of 5.6.

Acid rainfall has damaging effects on materials, forest ecosystems and fresh water ecosystems.

Eutrophication
Eutrophication is a disruption of the biological balance of the soils and water, due to an excess of nitrogen especially of atmospheric origin (NOx and NH₃), greater than the ecosystems are able to absorb.

Photochemical pollution
Photochemical pollution (or photo-oxidising pollution) is a series of complex phenomena leading to the formation of ozone and other oxidising compounds (hydrogen peroxide, aldehydes, peroxy acetyl nitrate or PAN) from primary pollutants (known as precursors - oxides of nitrogen and volatile organic compounds or VOCs) and energy provided by ultra-violet (UV) radiation from the sun. These phenomena occur in the layers of air close to the ground, and in the free troposphere. The ozone formed at this level is called ground-level or "bad ozone" because of its damaging effect on human health and vegetation. Conversely, the ozone in the stratosphere (at an altitude of 19-30 km), is called "good ozone" because it protects us from solar UV radiation.

Surprisingly, the concentrations of ozone measured far from sources of precursors (built-up areas, for example) are higher than those measured near the sources. This is because, in a city for example, the emissions of NO (mainly from traffic) are high. Ozone is destroyed by NO. The NO acts as a sink of ozone because it uses it up. If the cloud of pollutants formed in the city moves to the countryside, or if emissions of NO are reduced, the concentration of ozone increases because the ozone is no longer consumed.
Photochemical pollution is a characteristic phenomenon of summer anticyclonic situations.

Ozone has damaging effects on human health, forest and agricultural ecosystems. In addition, this phenomenon of photo-oxidising pollution is closely linked to acid rain.

The greenhouse effect
The greenhouse effect is a natural phenomenon involving the absorption of very long wave infra-red (IR) radiation, reflected from the surface of the earth, by compounds present in the atmosphere: CO₂, CH₄, H₂O, O₃, N₂O, CFCs. Part of the IR radiation is not reflected back into space. The result is energy absorption. This energy is transformed into heat. Most of these compounds are present in natural form, which has enabled the development and survival of life on Earth. The average temperature on earth is 15°C; if the natural greenhouse effect did not exist, the average temperature would be -18°C.

Since the industrial revolution, there has been an increase in the concentration of greenhouse gases :

• CO₂ mainly from industrial, domestic and transport combustion.
• CH₄ mainly from farming practices: e.g. rice growing , livestock.
• N₂O mainly from farming practices.
• CFCs (now banned), HFCs, PFCs, SF₆.

Depletion of the ozone layer
Ozone is the main component in the upper atmosphere at an altitude of 25 km. The ozone layer is said to be "good ozone" because it absorbs UV radiation from the sun and so protects us from the risks of skin cancer and other genetic mutations. It also protects the photosynthesising activity of plants.

An abnormal lowering in concentrations of ozone at the South pole at the end of the polar winter, when the sun returns, was discovered in 1980. At the end of the southern winter, when the sun returns, the ozone content diminishes by 40 to 60%. The maximum deficiency is about 20 - 25 km above ground level.

Many compounds can destroy ozone (OH, H, NO, Cl, Br, HO₂). A high correlation was found between ozone deficiency and concentrations of ClO. The presence of the radicals Cl and ClO in the stratosphere is due to the natural emission of methylene chloride by oceans and to chlorofluorocarbons (CFCs) emitted by human activities. CFCs are very stable molecules. They are transported into the stratosphere where they release chlorine, thus disturbing the natural balance which governs the presence of ozone at this altitude.

The annual phenomenon of depleted concentration of ozone is more marked at the South pole than at the North pole because of the different conditions prevailing there. At the South pole, a vortex appears during the winter. The temperatures are around -80° to -100°C. Clouds then contain fine crystals of ice which fix the chlorine in the form of HCl and NO₂ClO. As soon as the sun returns, its UV radiation releases the radicals Cl and ClO which quickly react with the ozone. At the North pole no such vortex is formed. Instead, there are a multitude of small holes.

Depleted concentrations of ozone in the stratosphere could have effects on the climate and biological systems.