(These are Short Notes based on Oheads originally presented in a lecture for BIOL246 by Dr R T Leah - these provide some background for BIOL202)

The problem of 'Acid Rain'

The term "acid rain" was first used by a British chemist, Robert Angus Smith, about 120 years ago in a book about pollution from coal burning in British cities.

He suggested that pollutants in the air were dissolving in rainwater making it acidic, causing damage to buildings and historic monuments. However, it was not until the 1970s that long range air transport of acidity in Europe was discovered by Swedish scientists.

Following this, Scandinavian research showed that the main source of acidity was sulphur and that this can have widespread effects on the natural environment as well as buildings.

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What causes acid rain?

We now know that several pollutant gases are responsible.

These include:

• sulphur dioxide, mainly from power stations,

• oxides of nitrogen from motor vehicles and the industrial burning of fossil fuels (gas, oil and coal).

• Another form of nitrogen pollution, ammonia, is emitted especially in rural areas, e.g. from manure spread on farmland and from farm animals.

It is subsequent chemical changes of these pollutant gases that create the acidity as sulphuric and nitric acid in the atmosphere.

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The complexity of the Problem

It is not always easy to see the effects of acidification or to know if they have been caused by air pollution.

Many areas are naturally quite acid. Their acid soils contain few minerals or they are resistant to weathering.

The way land is managed can also make acidification problems worse, e.g. trees can increase the amount of acid deposited.

Ozone

Metal mobilisation - aluminium

Mercury

Determining trends in noisy data

Geographical Variation

Close to large air pollution sources, (eg metal smelters) more obvious soil acidification effects from acid deposition are found.

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Capacity

(to neutralise inputs)

ie amounts of

Na K

Ca Mg

Al

Intensity

(of inputs)

Sensitivity

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Critical Loads

To help quantify effects and relate them to the acid deposited, an "effects based" approach, known as critical loads, has been developed.

The critical load is a measure of sensitivity of the environment to pollutants.

It is defined as "the quantity of pollution that a part of the environment can tolerate without harmful effects occurring".

Deposition above that limit may damage plants and animals. Where acidity critical loads are large, more acid deposition can be tolerated, but areas with small acidity critical loads are very sensitive to acidification.

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Acidification of soils

Acidification of mineral soils can release forms of aluminium that may damage plants.

It also affects fine plant roots and fungi (mycorrhizae) that help plants obtain nutrients from the soil.

This may lead to decreased plant growth or changes in plant communities. Populations of other soil micro-organisms may also change, with a shift towards acid tolerant species.

As a result, a number of soil processes can slow down.

For example, the breakdown of plant litter becomes slower, leading to surface accumulation.

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Acidification of Freshwaters

Acidification of soils gradually leads to acidification of waters draining from them. Eventually the streams and lakes fed by water from these soils may themselves become acid.

Acidity and toxic aluminium can cause harm to aquatic animals. In acidified waters the diversity and size of invertebrate and fish populations decline. Acid tolerant species become dominant. In extreme cases lakes and rivers become fishless.

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Acidification of Lakes

The chemistry of lake water is largely controlled by the chemistry of the water draining from the surrounding soils and rocks

Lakes that are now acid were often unacidified before the mid-1800s, when emissions of oxides of sulphur and nitrogen were small.

We know this from looking at the remains of diatoms which are found in lake sediments.

Marked changes in the species occur. These can be linked to the start of the industrial revolution and the high pollutant emissions in this century. The presence of carbon particles in some lake sediments gives further evidence of the causes of change and helps identify the combustion processes (eg coal and oil burning) involved.

While many lakes have not yet become acidified, some are likely to be affected in the future as their ability to neutralise acidity becomes exhausted.

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What are the impacts in the UK?

Studies also showed widespread acid deposition to sensitive buildings, such as cathedrals made of limestone.

In the UK, as in many other European countries, there has been acidification of soils and surface waters.

Soils most at risk of acid effects are those derived from the old, hard rocks of the north and west of Britain and some younger sandy deposits in the south and east.

The acid peats of the uplands are also vulnerable because their water chemistry is strongly influenced by the chemistry of incoming rainfall.

The amount of deposited acidity varies with rainfall and the concentration of pollutant gases and particles in the atmosphere. The largest deposition may be in areas remote from sources of the pollutants. In the UK, high deposition areas include Cumbria, Snowdonia and Galloway. Here the high rainfall washes more of the pollutants out of the air. Since these areas also contain acid sensitive soils, acidification of soils and freshwaters is more likely.

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International Problems

Similar changes to those seen in the UK, have taken place in soils and freshwaters in many areas of the northern hemisphere.

Major problems in Scandinavia (Southern Norway in particular) mainly from emissions from industrialised Europe.

In lakes in Norway and Sweden great changes to diatoms and fish populations have been observed.

Major problems in North East USA due to magnitude of outputs from major centres of population and industry

Major problems in Eastern Canada due to trans-boundary pollution from the USA.

Major problems in the 1960s and 70s in central USA and Canada from metal smelting at Sudbury

Inputs to sensitive areas in the UK are as large as those reported for Scandinavia.

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Mitigation

Cause - reduce acid emissions (eg by flue-gas desulphurisation)

Symptoms - neutralise emissions in situ in ecosystems (eg by liming)

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Recovery

• Simple calculations fail to take account of the slow recovery processes in soil and freshwater systems.

• Often where simple models predict recovery, more complex "dynamic" models show that recovery will be much slower. Further cuts in acidifying pollutants will be needed if the acidification process is to be reversed more rapidly.

• Even so, there is evidence that recovery in some freshwaters in Norway and elsewhere in Europe is already occurring.

• Improvements in water quality and changes in diatom and freshwater animal populations have been observed. It is believed that decreases in the sulphur emissions in recent years may already be having an effect, although improvements to most UK freshwaters are less certain.

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What is being done to solve the problem by legislation?

long-range transport of sulphur and nitrogen pollutants, international agreement was sought to control emissions.

Countries in the United Nations Economic Commission for Europe (UNECE) region drew up a Convention as the basis for pollution abatement and scientific collaboration.

1979 Convention on

Long Range Transboundary Air Pollution (LRTAP)

43 signatories

right around the Northern Hemisphere

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Legislation for Reductions in Acid Emissions (I)

Early Protocols for sulphur dioxide and oxides of nitrogen set simple targets for all countries:

• a 30% reduction of sulphur emissions by 1993

• decreasing emission levels of oxides of nitrogen to those of 1987 (by 1994)

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Legislation for Reductions in Acid Emissions (II)

In 1994, the new 'effects based' Protocol for sulphur, based on critical loads, was agreed in Oslo.

This defined a target for each European country based on its pollutant emissions, the costs of abatement and the contribution those emissions made to acid deposition on sensitive ecosystems across Europe.

The Multi-pollutant, Multi-effect Protocol

was signed in Gothenburg in 1999.

It takes into account effects of acidity, excess nutrient nitrogen and also photochemical oxidants (low level ozone).

Acidification can result from sulphur, oxides of nitrogen and ammonia, so all three of these atmospheric pollutants are included in the Protocol.

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What more can be done?

• Calculations show that to achieve recovery at the most sensitive sites, emissions of sulphur, oxides of nitrogen and ammonia must be decreased further still.
• The costs of this will be high
• For some sensitive areas, however, critical loads may be unattainable with current emission control technology, so recovery may not be possible through emission decreases alone in the foreseeable future.
• Other methods may be used to aid their recovery. For example, in many lakes in Scandinavia, the addition of lime has been used as an interim measure for neutralising acidification.

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Emissions Trading

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Conclusions

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Original material is Copyright University of Liverpool, 2003