How is acid rain composed

acid rain

As acid rain is the term used for precipitation with a pH value lower than the pH value that occurs in pure water due to the natural carbon dioxide content of the atmosphere (pH value ≤ 5.5).

The main cause of acid rain is air pollution, especially from acidic exhaust gases. Acid rain damages nature and the environment and is a major cause of so-called forest dieback.

Next to the acid rain must also prevent damage to plants from fog(acid fog) be considered. Fog water is often significantly more acidic (has a lower pH value) than rainwater, since fog absorbs pollutants from the air more efficiently than rain.

acid rain is caused by transmission (distribution of pollutant emissions in the atmosphere by wind etc.) often in distant regions. This means that the polluter and the recipient of acid rain are often different states. The transnational environmental pollution in East Asia is an example of this.


Main article: Air pollution

Air pollution from exhaust gases is primarily responsible for acid rain. Sulfur oxides (SOx) the sulfuric acid (H2SO4) form. When fuels containing sulfur are burned, sulfur dioxide is primarily produced, which ultimately forms sulfuric acid with water and oxygen. Two reaction mechanisms are possible:

This reaction mechanism only needs the water to form sulfuric acid from the sulfur trioxide:
2 SUN2 + O2 → 2 SO3
SO3 + H2O → H2SO4
This reaction mechanism takes place completely in aqueous solution, e.g. B. Water droplets in the fog:
SO2 + H2O → H2SO3
2 H2SO3 + O2 → 2 H2SO4

Furthermore, nitrogen oxides (NOx) by converting the nitrogen contained in the fuel and in the air. Together with water and oxygen, these form nitric acid (ENT3).

2 NO2 + H2O → ENT2 + ENT3
N2O4 + H2O → ENT2 + ENT3

About 2/3 of the sulfuric acid and about 1/3 of the nitric acid are responsible for the acidification of the precipitation.

The carbon dioxide (CO2) out. The natural CO2-The content of the atmosphere is so low at around 0.03% that it increases significantly due to the burning of fossil fuels. In addition to the greenhouse effect, this also leads to a further lowering of the natural pH value of the rain mentioned above, as more carbonic acid can be formed as a result.

Carbon dioxide (CO2) dissolves in water to form CO2(aq) and reacts to about 0.2% with water to form carbonic acid (H.2CO3), which in turn is (partially) in dissociated form:

CO2 + H2O → H2CO3

In tropical areas, organic acids, e.g. B. formic acid (HCOOH), have a significant role in lowering the pH value of precipitation.


On plants

Dead spruce trees in the Ore Mountains (1998)

Acid rain can damage plants through acidification of the soil and has been causally linked to tree damage (forest dieback). The forests mainly affected are in regions with frequent and abundant rainfall, which also have relatively low annual average temperatures. In Germany, this applies in particular to forests at higher altitudes in the low mountain ranges and the Alps. Since the clinical pictures that have occurred are very different (in addition to healthy stands there are also severely damaged stands in a comparable location), other causes of the tree damage are now suspected in addition to acidic precipitation (natural fluctuations in susceptibility to soil acidification, changes in the climate) or proven (Deficiency of minerals, damage to weakened trees by fungi, bacteria, silvicultural errors).

The acidification of the soil disrupts the natural composition of the soil. Poisonous heavy metal ions are released, which cause the fine roots of the trees to die off. This causes disturbances in the water and nutrient balance of the tree, and its resistance is greatly reduced. These trees are now more susceptible to disease and natural stress. Even ground frost or a pest infestation can now cause considerable damage. The acid rain not only hampers the growth of the young trees, but also the mature trees are severely damaged. First of all, the infestation affects the leaf or needle crowns of the trees, the leaves or needles are thrown off. This leads to crown defoliation. In addition, treetop drought can occur and the tree can eventually die. Acid rain is one of the causes of forest dieback.

On waters

Waters are increasingly polluted by acid input. The acid input takes place less directly via the acidic precipitation than more indirectly via the tributaries. Metal cations, e.g. Al, accumulate in the water due to the runoff and as a result of the acid input3+, which act as cell toxins and can lead to the impoverishment of species.

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 CO2-Concentration in the atmosphere can also lead to acidification of the oceans and could thus pose a threat to the continued existence of the oceanic biosphere, as, for example, from a certain pH value the calcareous shells of mussels and snails dissolve, according to the following equations per kg Carbon dioxide 2.27 kg calcium carbonate are dissolved:

CO2 + H2O → H2CO3 and
CaCO3 + H2CO3 → 2 HCO3 + Approx2+

On buildings

The acid rain attacks sandstone and limestone in particular, 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.

Marble dissolves in acids because it consists of calcium carbonate. When acid rain hits marble, a variety of damage occurs. This includes roughened surfaces, the removal of material and the loss of chiseled fine structures. The destruction can affect the entire surface or occur selectively at reactive points. The calcium carbonate reacts with the dissolved hydrogen ions in acid rain. In this reaction it breaks down into calcium ions, carbon dioxide and water:

CaCO3 + 2H+ → CO2 + H2O + approx2+

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:

Approx2+ + SO42− + 2H2O → CaSO4 + 2H2O

Over time, the rain wears away part of the gypsum 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 pounds were spent on Westminster Abbey in London alone to remove damage caused by acid rain or fog ("smog").

The damage to the earth's cultural treasures cannot be outweighed in money. The Taj Mahal in India and the Acropolis in Athens suffered from the effects of acid, as did the Canadian parliament building, the Capitol in Washington or the Cologne Cathedral.


Combating Symptoms

Forest liming by helicopter

As a countermeasure, attempts are being made in many areas of Europe (prohibited 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 unintentional emission of lime dust can, in extreme cases, even reverse the phenomenon and arise 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 1980s there has been a move towards desulphurising flue gases. The SO2 removed from the exhaust gas and mostly to CaSO4 (Plaster) implemented, which can be used or deposited. However, this is not possible with means of transport such as automobiles, airplanes, etc. For this reason, the sulfur is removed from fuels such as gasoline, diesel, kerosene and natural gas using special processes, which at least in the industrialized countries leads to the entry of SO2 into the atmosphere could be reduced significantly.

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 nitrogen oxides would have to be removed from the exhaust gas, which happens in car catalytic converters, but not 100% (see lambda window). Nitrogen oxides, along with UV radiation from the sun, are also the main cause of the formation of ground-level ozone. So you would either need other catalytic converters, or you would have to replace the combustion engine with an electric motor, for example.


  • Gerd Spelsberg: Smoke plague: 100 years of acid rain. Alano Verlag, Aachen 1984, ISBN 3-924007-05-5
  • Walter Jansen, Anke Block, Jürgen Knaack: Acid rain: causes, analytics, assessment. Metzler Verlag, Stuttgart 1987, ISBN 3-476-30291-1
  • Gregory S. Wetstone, Armin Rosencranz: World threat of acid rain: Defense attempts in Europe and North America; a study by the Environmental Law Institute for the German Marshall Fund of the United States. Dreisam publishing house, Freiburg i. Br. 1985, ISBN 3-921472-87-3
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