Acidification & Eutrophication > Acidification

Acidification

Because sulphur emissions have started to fall you may think that the danger of acidification is over. However, acidification continues to get worse in many areas - albeit at a slower rate than before. The problem will only be solved when emissions of acidifying pollutants fall below a level that nature can tolerate and neutralize.

Effects

Acidification of water. Back in the fifties it was discovered that fish were disappearing from lakes and waterways in southern Scandinavia. Today, some 14,000 Swedish lakes are affected by acidification, with widespread damage to plant and animal life as a consequence. The damage is extensive in large parts of Scandinavia, but also occurs in parts of the United Kingdom and in the Alps.

Soil depletion. Sensitivity to acidification is greatest in areas where the minerals in the soil weather slowly. When the soil becomes acidified its essential nutrients are leached out, which reduces the fertility of the soil. The acidification process also releases metals that can harm the micro-organisms in the soil that are responsible for decomposition, as well as birds and mammals higher up the food chain, including man.

Plants and animals disappear. The sensitivity of individual species to air pollutants and acidification varies. The most sensitive groups include fish, lichens, mosses, certain fungi and small aquatic organisms.

Forest damage. From the yearly European survey it appears that every fourth tree examined can be classified as damaged (the loss of leaves or needles exceeding 25 per cent). This damage to the forest has many causes, but most researchers agree that acidification of the soil and high concentrations of ground-level ozone are important contributing factors.

Corrosion. Corrosion is greatly accelerated by sulphur dioxide and nitrogen oxides. These are converted into strong acids which attack new and old buildings, bridges, monuments, rock carvings, etc. Worst affected are objects and structures of easily weathered materials, such as limestone and sandstone.

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Causes

By far the most dominant cause of the acidification problem is the airborne deposition of sulphur, nitrogen oxides and ammonia. The harvesting of biomass is also of some importance.

Sulphur deposition, in the form of sulphuric acid, is generally the most important cause of acidification. Most sulphur comes from burning coal and oil, during which the sulphur in the fuel is converted into sulphur dioxide.

Anthropogenic (man-made) emissions of sulphur in Europe rose sharply from the end of the Second World War until the end of the 1970s. Between 1980 and 2003, emissions from land-based sources in Europe fell by almost 75 per cent - from 53 to 14 million tonnes of sulphur dioxide a year. At the same time, however, emissions from international shipping in European waters have nearly doubled, from 1.7 to 3 million tonnes a year.

Deposition of airborne nitrogen compounds - nitrogen oxides and ammonia - contribute to acidification and are also the dominant cause of eutrophication of many ecosystems on land and at sea.

Nitrogen oxides are always produced during combustion. Emissions from land-based sources in Europe have fallen from 23 million tonnes a year in 1980 to 15 million tonnes a year in 2000, a reduction of just over 30 per cent. About half the emissions in Europe come from the transport sector, and most the rest from combustion plants. In the case of nitrogen oxides a large part of the emission reduction from land-based sources has been offset by rising emissions at sea. As with sulphur dioxide, these have almost doubled since 1980.

The main source of ammonia emissions is agriculture. The amount of ammonia that evaporates depends primarily on how manure is handled during storage and spreading. According to statistics, European emissions have fallen by 25 per cent between 1990 and 2000, from 7.5 to 5.6 million tonnes per year.

Finally the harvesting of biomass, i.e. normal forestry operations, also leads to soil acidification.

Long-range transport

The figures presented here are based on EMEP data for 2003, and show the situation for two pollutants and two selected countries - Germany as an example of a net exporter of pollutants, i.e. one that exports more pollutants than is being imported, and Sweden as an example of a net-importer of pollutants.

In common to both these countries - and in fact to most European countries - is the fact that most of the depositions of sulphur and oxides of nitrogen emanate from outside their own territory.

Another similarity is that an increasing share of the depositions originates from international shipping.

The origin of the deposition of sulphur (left) and oxidized nitrogen (right) over Germany (above) and Sweden (below) in 2003.

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Affected areas

Oxides of sulphur and nitrogen can be carried long distances - thousands of kilometres - by the air. This means that the problem of acidification recognizes no borders. In Sweden and Norway around 90 per cent of the acid deposition comes from other countries, primarily the United Kingdom, Germany and Poland, and from international shipping.

There are two main factors that determine which areas are affected by acidification:

1. The amount of acid deposition.
2. The resistance of the soil.

When soil has a high content of easily weathered minerals it can absorb a relatively large amount of acid deposition without becoming acidic. But if the minerals in the soil do not weather easily, as is the case in large parts of the Scandinavian peninsula for example, there is little natural resistance. If the resistance of the soil is low then lakes are also sensitive to acid deposition.

The limits to what "nature can tolerate" are called critical loads. The critical load has been defined as "an exposure below which significant harmful effects on sensitive elements of the environment do not occur according to current knowledge".

Critical loads for acidity in Europe. The map shows the deposition of hydrogen ions that sensitive ecosystems (e.g. forest soils and surface waters) can tolerate without being acidified. At each load level 95 per cent of the ecosystems in the relevant square are protected.

In 1990 around 34 per cent of natural environment in Europe were affected by acid deposition that exceeded the critical load.

Emissions have however fallen since 1990. New calculations indicate that the exceeded area for acidification had shrunk to about 11 per cent by 2000.

For the year 2000, more than 20 per cent of the forest area in the EU25, or approximately one quarter of a million square kilometres, received acid deposition above the critical loads. By 2020 this is calculated to come down to about 12 per cent in the CLE scenario, and 3 per cent in the MTFR scenario (see maps below).

In order to achieve the political goal of non-exceedance of the critical loads it will be necessary to reduce emissions of sulphur dioxide and nitrogen compounds in parts of Europe by 80-90 per cent compared with 1990 levels.

It is also important to note that damage from acidification in an area can last for a long time, even when the critical limit is no longer exceeded.

Percentage of forest area receiving acid deposition above the critical loads for acidification. Black colour = 100% exceedance, blue = less than 5%. For the emission levels in the year 2000 (left), and for two projected EU emission levels for 2020: Current legislation (CLE; centre) and Maximum Technically Feasible Reduction (MTFR; right). For details and country-by-country figures, see factsheet.

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Liming as a countermeasure

If finely ground limestone (CaCO3) is added to water it raises the pH and increases resistance to acidification. The liming of lakes and waterways is carried out on a large scale in Sweden and Norway. In Sweden around 7500 lakes and 11,000 kilometres of waterways are now limed each year.

The goal of liming is partly to restore acidified lakes and waterways, and partly to increase the resistance of the lakes and waterways that are at risk but not yet affected.

Because the water in a lake is constantly being replaced, liming must be repeated every few years. In running water, lime dosing equipment is used to continuously add lime to the water.

In order to raise the pH in small waterways, and to increase the duration of the effect when lakes are limed, part of the lime is often spread on wetlands in the catchment area. This causes damage to plants, including killing off bog moss, but the area is limited and the benefits are generally considered to outweigh the harm.

Liming of soils
The acidification process in soils can be countered by liming. This raises the pH level and tops up reserves of exchangeable cations (increases the base saturation), while also reducing the concentration of free aluminium ions. Lime acts like a filter in the upper layer of the forest soil, where it can capture and neutralize future acid deposition before it has time to leach out base cations and/or dissolve toxic aluminium.

The effect of the added lime penetrates slowly into the soil, at roughly one centimetre per year, but on the other hand persists for a long time in the future. The liming of soil can therefore help counter the acidification of surface water in the long term. A dosage of 3-5 tonnes of lime per hectare is estimated to protect soil from acidification for 20-30 years with current levels of acid deposition in southern Sweden.

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Last modified 21 December 2005.

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