Acid rain

When acid rain is called precipitation, the pH value is lower than the pH value in pure water by the natural carbon dioxide content of the atmosphere adjusts ( pH ≤ 5.5 ).

Main cause of acid rain is air pollution, especially acid-forming gases. Acid rain harms nature and the environment and is a major contributor to the so-called forest dieback.

In addition to the acid rain also the damage to plants by fog (acid mist) must be considered. Fog water is often significantly more acidic (has a lower pH ) than rain water as fog efficiently absorbs pollutants from the air as rain.

Acid rain is often caused by transmission (distribution of pollutant emissions in the atmosphere by wind, etc. ) in distant regions. This means that the polluter and the receiver of acid rain are often different States. An example of this is the transnational pollution in East Asia.

  • 3.1 combat the symptoms
  • 3.2 Tackling the causes


The main reason for acid rain, air pollution is caused by gases emitted. In particular, through the use of sulfur-containing fossil fuels such as coal and fuel oil caused sulfur oxides (SOx ) are the water and possibly oxygen sulfuric acid ( H2SO4). Primarily produced by the combustion of fuels containing sulfur initially sulfur dioxide, which eventually forms sulfuric acid with water and oxygen. Two reaction mechanisms are possible:

Continue to emerge at every lightning and at each combustion nitrogen oxides ( NOx) by conversion of the nitrogen contained in the fuel and in the air. This form water and oxygen nitric acid ( HNO3).

The sulfuric acid is responsible for approximately ⅔, the nitric acid to about ⅓ for the acidification of precipitation.

Global impact may also during the combustion of the above fossil fuels released carbon dioxide ( CO2). The natural CO2 content of the atmosphere is so low that it significantly increases the combustion of fossil fuels by about 0.03 %. This leads next to the greenhouse effect and to a further reduction of the above-mentioned natural pH of the rain, as this more carbonic acid can be formed.

Carbon dioxide ( CO2) dissolves in water to form CO2 (aq ) and responds to approximately 0.2 % with water to form carbonic acid ( H2CO3 ), which is present in turn (partially) in dissociated form:

In tropical areas, organic acids, such as formic acid ( HCOOH ), have a major role in the lowering of the pH of precipitation.


On plants

Acid rain can damage plants by soil acidification and was believed to be responsible with new types of forest damage in connection. The affected mainly forests are located in regions with frequent and abundant precipitation, which also have relatively low annual average temperatures. This is especially true on forests in higher altitudes of the mountains and the Alps in Germany.

By acidification of the soil 's natural composition is disturbed. Are released toxic heavy metal ions and aluminum ions, which kill off the fine roots of trees. This allows faults occur in the water and nutrient balance of the tree, and its resistance decreases greatly. The affected trees are more susceptible to diseases and natural stresses. Under normal conditions, harmless effects such as ground frost or pest infestation can cause considerable damage to the weakened tree. Due to the acid rain not only the young trees are prevented from growing, but also severely damaged the adult trees. First, the injury affects the leaf or needle crowns of the trees, the leaves or needles are discarded. This leads to defoliation. Moreover tops drought can occur and eventually die of the tree.

This acid rain is one of the causes of novel forest decline. However, the illness which arose images are very different. It is found next to healthy stocks in comparable positions severely damaged. Therefore, also other causes of tree damage in addition to the acid rain by now suspects such as climate change or natural variability in susceptibility to soil acidification. Could be detected as well as other causes mineral deficiency, damage to trees weakened by fungi, bacteria and silvicultural error. Acid rain is one of several factors led their effects especially in the 1980s, the term deforestation to an intense public debate.

On waters

Waters are increasingly impacted by acid deposition. Here, the acidity is less done directly on the acid deposition as more indirect tributaries. Through the floor drainage and as a result of acid deposition accumulate in the water - metal cations, eg Al3 , in which act as cellular poisons and lead to species impoverishment can.

The geology of the catchment areas of rivers and lakes continues to have a major impact on acidification. Standing in the catchment area mainly rocks, which hardly neutralizing the acid rain effect ( eg granite, gneiss, sandstone), these waters are particularly affected by acidification. Conversely, waters that have large limestone deposits in the catchment area, hardly 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 dissolve, for example from a certain pH, the carbonate shells of mussels and snails, which according to the following equations per kg of carbon dioxide 2.27 kg of calcium carbonate are dissolved:

In building

Acid rain attacks especially sandstone and limestone, but also concrete structures. Thus, the weathering of buildings is progressing much faster, and numerous buildings and cultural monuments are so badly damaged or destroyed.

Marble dissolves in acids, since it is composed of calcium carbonate. When acid rain hits marble, created a variety of damage. These include roughened surfaces, removal of material, and loss of fine carved structures. The destruction can affect the entire surface or selectively occur at reactive sites. The calcium carbonate reacts with the oxonium ions in acid rain. In this reaction it decomposes to calcium ion, carbon dioxide and water:

Then the sulfate ions of the sulfuric acid react with the calcium ions and coat the marble or limestone with a white layer of gypsum:

The rain carries with time from a portion of the gypsum crust. This results in small cracks and increasing erosion.

The restoration of damaged cultural property and buildings is very expensive. Alone for the Westminster Abbey in London up to 10 million pounds were designed to end in 1990 to eliminate the acid rain and fog damage ( " smog " ) caused.

In Money does not outweigh the damage to the cultural treasures of the earth. Taj Mahal in India and the Acropolis in Athens had also suffered from the action of acid, such as the Canadian Parliament building, the Capitol in Washington or the Cologne Cathedral.


Combat the symptoms

As a countermeasure, trying in many parts of Europe ( banned in Switzerland ) to neutralize the acidity with lime. In many places this large amounts of lime are scattered by helicopter.

In the vicinity of lime or cement plants with poor dust, the unintentional emission of lime dust in extreme cases, even lead to the reversal of the phenomenon and there is basic rain. Such an effect may also occur when large amounts of ash or other alkaline dust get into the air, such as a result of large-scale forest fires or volcanic eruption.

Address the root causes

It has passed since the 80's to be desulfurized flue gases. Here, the SO2 is removed from the exhaust gas and mostly converted to CaSO4 (gypsum), which can be used as a deposit or FGD gypsum. This is by means of transportation such as automobiles, airplanes, etc. However, not possible. Therefore, it is removed from fuels such as gasoline, diesel, kerosene and natural gas of sulfur by means of special procedure whereby the entry of SO2 was significantly reduced in the atmosphere, at least in the industrialized countries. Studies by researchers at the Institute for World Forestry in Hamburg have to calculate the putative acidification based on dynamic models suggest that the limits of the international cooperation program for detecting and monitoring the effects of air pollutants (ICP Forests) in 2020 with a few local exceptions, no longer be exceeded.

The removal of nitrogen oxides turns out, however, considerably more difficult. These arise from a certain temperature at all combustion processes, where no work is done in nitrogen-free atmosphere. This is hardly possible or only with great effort, as our atmosphere to 78.09 % composed of nitrogen. Therefore, the nitrogen oxide must be removed from the exhaust gas, which occurs in automotive catalytic converters, but not 100% (see the lambda window). Nitrogen oxides, together with the UV radiation from the sun are the main cause for the formation of ground-level ozone. So you would either need other catalysts, or you would have the internal combustion engine, for example, replaced by an electric motor.