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Acid Rain

The acid rain menace is still with us, wider spread than ever and taking forms more difficult to control

Saurer Regen/Acid rain
The natural conditions for a plant are disrupted by acid rain

In the 1980s, alarm bells were ringing in northern Europe. Clouds of acid were travelling far and wide, damaging infrastructure, human health and ecosystems wherever they dropped their load. There was a widespread fear of a catastrophic 'forest death' in regions like Northern Europe.

Thanks to improvements in industrial and heating emissions, the forest death did not wreck havoc as feared across Western Europe. But does this mean the danger of acid rain has become history?

Far from it, says Andrew Bone. It has merely changed form and spread all around the globe.

Themain types of acid rain are sulphuric acid, nitric acid, and hydrochloric acid, which can also form ground-level ozone. Each have their own characteristics, sources, and counter-measures.

Since the beginning of the Industrial Revolution, it has been known that the burning of coal, which has large quantities of impurities such as sulphur, leads to deposition of sulphuric acid over a wide area and far from the source. This is because the sulphur dioxide as a gas only enters reactions to form sulphuric acid when in solution. The molecules become seeds of water droplets and form clouds, which will travel hundreds or thousands of kilometres before precipitating the acid in concentrated form.

Nitrogen oxides (NOx), on the other hand, enter reactions in the gaseous state. These reactions are complex, in part reversible, and very sensitive to prevailing conditions, such as the intensity of light and temperature. NOx emissions from industry and vehicles can lead to the formation of nitric acid and ozone, both of which are sources of acid rain. Both of these pollutants are increasing in most countries, due to growth in traffic and industrial development.

Whereas Western Europe has seen a decrease in sulphuric acid, and an increase in nitric acid and ozone, China is still gasping from the consequences of its rapid industrialisation. Its energy for industry and heating comes from the combustion of fossil fuels, mainly brown coal, a form of coal with a high percentage of impurities. It also has a burgeoning traffic problem, the source of NOx and ozone.

Other factors affect how much and where acid rain arises. An important moderating factor is the availability of neutralising calcium-rich dust in the atmosphere. When more roads were unpaved and more ash was released from home fires, more acid could be neutralised. With these sources of alkali greatly reduced in western Europe and America, there is less cleansing of acid before precipitation, increasing the acidic fallout during rainfall.

Ammonia is a pollutant which, although alkaline, undoes whatever good it might do by reacting with SO2 to produce ammonium sulphate, which forms nitric acid in soils. Another source of acid in the atmosphere is burning forests. South-east Asia's brown cloud is as bad as coal fires for acidification of rainfall.

However, there are improvements in some key areas, especially in Europe. Freshwater ecosystems far from acid sources are returning to normal pH ranges. The decrease in sulphur emissions has actually led to sulphur deficiency in some agricultural areas.

The German Bundesministerium für Ernährung und Landwirtschaft releases an annual report on the condition of Germany's forest (Deutschland Waldzustandsbericht). Overall, a general improving trend is observed. The percentage of trees classed as undamaged in 2013 was 39%, an increase of 2% over 2011. However, the trees classed as slightly damaged rose by 1% over the same period, to 36%. The total of badly damaged trees fell from 28% to 25%. Nothing to pop champagne about, but a positive trend nevertheless.

There is a considerable difference between recovery rates for different tree species in the report:

TreeSevere (%) 2003/ 2012trendSlight (%) 2003/ 2012trend

Bundesministerium für Ernährung und Landwirtschaft, Waldzustandsbericht 2013

Article by Andrew Bone, Wednesday, 14th September 2016