As acid rain is called precipitation , whose pH is about 4.2-4.8, making it lower than the pH-value of 5.5 to 5.7 which in pure rain water through the natural carbon dioxide content of the atmosphere adjusts.
The main cause of acid rain is air pollution , particularly from acidic exhaust gases . Acid precipitation also occurs in the vicinity of active volcanoes . Acid rain damages nature and the environment and is a major cause of forest damage . The connection between environmental pollution and industrialization in Europe was observed in the Scandinavian countries, for example. Acid rain is also caused by transmission (distribution of pollutant emissions in the atmosphere by wind, etc.) to distant regions. This means that the source and recipient of acid rain are often different states. Another example is the transnational pollution in East Asia since the 1990s.
In addition to acid rain, damage to plants caused by fog (acid fog) must also be taken into account. Fog water is often significantly more acidic (has a lower pH value) than rain water, as fog absorbs pollutants from the air more efficiently than rain.
Air pollution from exhaust gases is primarily responsible for acid rain . The sulfur pollutants in the atmosphere come mainly from the combustion of sulfur-containing fossil fuels ( coal , petroleum and petroleum products) that are used as heating and fuel. They also come from volcanic exhalations . Most of the sulfur is present in the atmosphere as sulfur dioxide , the primary product of the oxidation of organic substances containing sulfur .
- (S) org + O 2 → SO 2
The sulfur dioxide concentrations in the atmosphere over Germany are currently below 25 micrograms per cubic meter. The atmosphere contains sulfur trioxide in lower concentrations than sulfur dioxide , which is formed by the oxidation of sulfur dioxide with oxygen (dioxygen, O 2 ) and is particularly contained in volcanic exhalations:
- 2 SO 2 + O 2 → 2 SO 3
- SO 2 + H 2 O → H 2 SO 3
- SO 3 + H 2 O → H 2 SO 4
Cloud water as well as precipitation contain these acids (in addition to carbon dioxide) in different concentrations and are therefore weaker or more acidic (pH values around 5.5 to 4.2).
Furthermore, during combustion - especially at high temperatures (see Zeldovich mechanism ) - nitrogen oxides (NO x ) are produced by converting the nitrogen contained in the fuel and in the air . Together with water and oxygen, these form both nitric acid (HNO 2 ) and nitric acid (HNO 3 ). There are different chemical reactions here:
- 2 NO 2 + H 2 O → HNO 2 + HNO 3
- N 2 O 4 + H 2 O → HNO 2 + HNO 3
About two thirds of the sulfur oxides and about one third of the nitrogen oxides are responsible for the acidification of the precipitation.
Acid rain can damage plants through the acidification of the soil and has been causally linked to new types of forest damage. The forests mainly affected are in regions with frequent and abundant rainfall, which also have relatively low annual average temperatures. In Germany, this applies particularly to forests in higher elevations in the low mountain ranges and the Alps.
The acidification of the soil disrupts the natural composition. There are toxic heavy metal ions and aluminum ions released that kill off the fine roots of the trees. This causes disturbances in the water and nutrient balance of the tree, and its resistance is greatly reduced. The affected trees are more susceptible to disease and natural stress. Under normal conditions, harmless effects such as ground frost or pest infestation can cause considerable damage to the weakened tree. First of all, the damage affects the leaf or needle crowns of the trees, the leaves or needles are shed. This leads to a crown defoliation. In addition, canopy drought can occur and the tree can eventually die. The acid rain also prevents young trees from growing.
As a result, acid rain is one of the causes of forest damage. However, the clinical pictures that have occurred are very different. In addition to healthy stocks in comparable locations, there are also heavily damaged stocks. Therefore, in addition to acid precipitation, other causes of tree damage are now suspected, such as global warming or the susceptibility of certain tree species to soil acidification. The causes could be proven to be a lack of minerals, damage to weakened trees by fungi, bacteria and forestry errors. Acid rain is one of several factors, the effects of which led to an intense public discussion, especially in the 1980s under the term forest dieback . With the forest condition report an attempt is made in Germany to present scientific material for this.
Waters are increasingly polluted by acid input. The acid input takes place less directly via the acid precipitates than more indirectly via the tributaries. Metal cations accumulate in the water as a result of the soil runoff and as a result of the acid input, e.g. B. Al 3+ , which act as cell toxins and can lead to species depletion.
The geology of the catchment areas of rivers and lakes continues to have a major influence on acidification. If there are mainly rocks in the catchment area that hardly have a neutralizing effect on acid rain (e.g. granite , gneiss , sandstone ), these waters are particularly affected by acidification. Conversely, bodies of water that have large limestone deposits in the catchment area hardly have any problems with acidification.
The increase in the CO 2 concentration in the atmosphere can also lead to an acidification of the oceans and could thus pose a threat to the continued existence of the oceanic biosphere , as, for example, above a certain pH value, the calcareous shells of mussels and snails dissolve, whereby 2.27 kg of calcium carbonate can be dissolved per kg of carbon dioxide according to the following equations:
- CO 2 + H 2 O → H 2 CO 3 and
- CaCO 3 + H 2 CO 3 → 2 HCO 3 - + Ca 2+
The sulfuric acid rain particularly attacks sand-lime brick and limestone , but also concrete structures . As a result, the weathering of buildings progresses much faster, and numerous buildings and cultural monuments are severely damaged or destroyed. If fluorine gets into the air - e.g. B from exhaust gases from the chemical industry - the acidified rain can also burn window glass. At the Cologne Cathedral , this phenomenon could be observed in a medieval church window.
Marble is also attacked because it consists of calcium carbonate. When acid rain hits marble, a variety of damage occurs. This includes roughened surfaces, removal of material and loss of chiseled fine structures. The destruction can affect the entire surface or occur selectively at reactive points. The calcium carbonate reacts with the oxonium ions in acid rain. In this reaction, it breaks down into calcium ions, carbon dioxide and water:
- CaCO 3 + 2 H 3 O + → CO 2 + 3 H 2 O + Ca 2+
Then the sulphate ions of the sulfuric acid react with the calcium ions and coat the marble or limestone with a white layer of plaster of paris:
- Ca 2 + + SO 4 2− → CaSO 4
Over time, the rain wears away part of the plaster crust. This leads to small cracks and increasing erosion.
The restoration of damaged cultural assets and buildings is very costly. Up to 1990, up to £ 10 million was spent on Westminster Abbey in London alone to clean up damage caused by acid rain or fog (“ smog ”).
Many of the original sculptures have now been moved indoors and replaced outdoors by copies or casts.
As a countermeasure, attempts are being made in many areas of Europe (not regulated by law in Switzerland) to neutralize acidification with lime . In many places, large amounts of lime are scattered by helicopter for this purpose.
In the vicinity of lime or cement works with poor dedusting , the unintended emission of lime dust can, in extreme cases, even reverse the phenomenon and result in alkaline rain . Such an effect can also occur when large amounts of ash or other basic dusts get into the air, for example as a result of large-scale forest fires or a volcanic eruption .
Combating the causes
Since the 1970s, efforts have been made internationally for coordinated measures. These efforts eventually led to the Convention on Long-Range Transboundary Air Pollution . As a result, since the 1980s there has been a move towards desulphurising the flue gases in the large fossil fuel power plants . The SO 2 is removed from the exhaust gas and mostly converted to CaSO 4 ( gypsum ), which can be used as FGD gypsum or disposed of. However, this is not possible with internal combustion engines in cars, airplanes, etc. For this reason, the sulfur is removed from fuels such as gasoline, diesel, kerosene and natural gas by means of special processes before use. As a result, the input of SO 2 into the atmosphere in the western industrialized countries could be reduced considerably. Studies by researchers at the Institute for World Forests in Hamburg on the calculation of acidification based on dynamic models indicate that the limit values of the international cooperation program for the recording and monitoring of the effects of air pollution (ICP Forests) will probably no longer be in 2020 with a few local exceptions be crossed, be exceeded, be passed.
The removal of nitrogen oxides , on the other hand, is much more difficult. These arise from a certain temperature in all combustion processes, provided that work is not carried out in a nitrogen-free atmosphere. This is hardly possible or only possible with great effort, since our atmosphere consists of 78.09% nitrogen . Therefore, the formation of nitrogen oxides in the exhaust gas would have to be reduced, which happens in vehicle catalytic converters , but is not 100% possible (see lambda window ). Nitrogen oxides, together with the sun's UV radiation, are also the main cause of the formation of ground-level ozone . So you either need different catalysts than the previous ones, or you have to use the internal combustion engines z. B. replace it with electric motors .
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