Acid rain

The atmosphere is a thin film of gases which envelops our planet and makes it habitable. Without this layer, there would be no life on earth. We know that the health of plants and animals including humans depends upon an unpolluted atmosphere, yet we are putting all kinds of poisonous waste into it. Gases produced when fossil fuels are burnt cause acid rain which can damage forests, lakes, rivers, the land and the plants and animals living there. Developed countries which are dependent on fossil fuels to maintain their high standards of living, are the main culprits.

The meaning of acid rain

Acid rain is the popular term used to describe acid precipitation. As well as rain, it includes mist, snow and dry deposition. Dry deposition refers to pollutants which are deposited on the environment before they have a chance to be absorbed by the moisture in the atmosphere.

Measuring acidity

The pH scale is used as a measure of a substance's acidity or alkalinity. 7 is neutral, less than 7 is acidic and more than 7, alkaline.

How the rain becomes acid

Rainfall is naturally acid because it absorbs carbon dioxide in the atmosphere and becomes a weak carbonic acid with a pH between 5 and 6. The major causes of acid rain are the sulphur dioxide and nitrogen oxides produced when fossil fuels such as coal, oil and gas are burned. Sulphur dioxide and nitrogen oxides are released into the atmosphere where they can be absorbed by the moisture and become weak sulphuric and nitric acids, sometimes with a pH of around 3. Most natural gas contains little or no sulphur and causes less pollution.

The main sources of pollution

Coal fired power stations are the major producers of sulphur dioxide, although all processes that burn coal and oil contribute.

Vehicles, especially cars, are responsible for most of the nitrogen oxides in the atmosphere. Some come from the vehicle exhaust itself, but others form when the exhaust gases react with the air.

Exhaust gases also react with strong sunlight to produce poisonous ozone gas which damages plant growth and in some cases, human health.

Sulphur is one of the chemical elements that make up the Earth. It can come from natural sources including volcanic eruptions, sea spray and minute sea creatures called plankton. In the world as a whole as much as 50% of the sulphur dioxide in the air comes from natural sources of sulphur. In Europe, however, only 15% comes from natural sources.

Acid rain as an international problem

It was once thought that pollution caused only local problems but we know that air pollution can be dispersed by the wind causing environmental problems far from the source of the pollution. The building of tall chimneys which was supposed to prevent problems by dispersing pollutants over a wider area, has made the problem worse for distant areas lying downwind. Norway and Sweden, for example, receive far more pollutants than they produce, much of them from Britain.

Environmental damage caused by acid rain

The built environment: Acid rain corrodes metal and stone work making the maintenance of buildings more costly. The major threats are to old historic buildings.

Farming: As the land becomes more acid, it is less suitable for growing crops and yields can be reduced. The crops themselves can be damaged, for example, pollen on maize affected by acid rain does not germinate so well and cannot fertilise the female plant.

Trees and forests: Sulphur dioxide interferes with the process of photosynthesis. Coniferous trees seem to be most at risk from acid rain because they do not shed their needles at the end of each year. On a healthy conifer needles can be traced back for up to 7 years but trees affected by acid rain often have needles from only the last 2-3 years. If a tree loses over 65% of its needles, it will probably die.

Young trees in soils affected by acid rain often show abnormally rapid growth. This is because the nitrogen in the pollutants acts as a fertiliser. However, the root systems do not develop as well as in trees that have to collect their nutrients from a larger area and the trees are more easily blown over. Also, they are short of other vital nutrients and the wood can be very soft making the trees more prone to attacks from insects.

When the soil becomes acid, toxic minerals like aluminium and cadmium are washed out by water passing through the soil(leaching). These minerals are taken in by the trees causing their growth to suffer.

Damaged coniferous trees can be recognised because the extremities of the trees die, especially the crown which is most exposed. Needles drop and leave the tree looking very thin. Branches on some trees droop. In most cases acid rain does not kill the tree. It is one more pressure on the tree which becomes stressed and more likely to suffer damage from insects, fungi, frost, wind and drought. Although deciduous trees generally do not suffer as much, research is showing that their growth is also affected.

Water courses and lakes: Water courses and lakes are affected by acid rain which falls directly into them and from water which runs into them. The problem is worst in spring when snow melts. The pollution accumulated over the winter period is suddenly released as an 'acid surge' just at the time when many young fish and insects are most vulnerable. A healthy lake has a pH of about 6.5 and supports a rich variety of wildlife. They are all dependent upon one another in a complex food web. As a lake becomes more acidified, the fish population declines and the birds which feed on the fish, also decline. For a while there is usually an increase in the number of insects in the lake as they are not eaten by fish. However, as the acidity increases, the number of species found declines. The lake becomes 'dead' when the pH reaches about 4.5.

Other factors contributing to acidification of the environment

Farming and forestry can also increase acidification. When plants grow they take up nutrients from the soil and it becomes more acid. When they die and rot back into the soil the nutrients are replaced and the soil becomes less acid. In farming and forestry the plants are harvested, not left to rot back into the soil which gradually becomes more acid. The removal of a whole tree including the branches and roots can be equivalent to the accumulation of 60 years of acid rain. Only taking the trunks reduces this to the equivalent of 20 years of acid rain.

When areas are prepared for forestry they are often drained and deep ploughed. This allows more oxygen into the soil, the minerals are oxidised and the soil becomes more acid.

Some areas are more prone to damage from acid than others

Some environments are able to neutralise the effects of acid rain. This is referred to as the buffering capacity. Chalk and limestone areas are very alkaline and can neutralise acids very effectively. The underlying rocks over much of Scandinavia, Scotland and northern Canada are granite. They are naturally acid, and have a very low buffering capacity. It is in these areas that there is the worst damage from acid rain.

Finding solutions

The only satisfactory way to prevent damage to the environment from acid rain is to reduce the amount of pollution from burning fossil fuels. However, this will take a long time and immediate measures are needed to safeguard the environment.

Immediate action: Many lakes are limed. Crushed limestone is put into the lakes from boats or helicopters where the lakes are difficult to reach. This is very effective and life in the lakes recovers quite quickly although the poisonous minerals settle on the bottom where they can continue to damage the plants and animals that live there. Liming is an expensive process and needs to be repeated regularly, usually every two or five years.

Liming the land is more difficult because there is so much more of it. Also, when it is covered by forest, the soil itself is very inaccessible.

Long term actions