Acidification and transboundary air pollution: causes and impact

Acidification is caused by the contamination of the air with sulphur dioxide, ammonia and oxides of nitrogen. These substances can result in the acidification of soil and water and can affect plants and materials. Agriculture, traffic and industry are the main sources of acidifying substances.

What is acidification?

The acidification of soil or water is a result of the emission of gases by industry, agriculture, electricity plants and road traffic. The emissions consist of, among other things, sulphur dioxide (SO₂), oxides of nitrogen (NO_x), ammonia (NH₃) and volatile organic compounds (VOCs). These gases react with each other and are then converted into other substances. The resulting substances include nitric acid (HNO₃), sulphuric acid (H₂SO₄) and ammonia (NH₄⁺). They form aerosols which are known as "secondary particulate matter". Exposure to these substances can result in damage to ecosystems and materials, as well as damage to human health.

What is transboundary air pollution?

Air pollution is no respecter of national borders. The longer a substance is present in the atmosphere, the longer the distance it can travel. The distance that air pollution can spread depends on several factors. Important factors are the amounts emitted, chemical reactions, meteorological conditions and deposition rates. Approximately 45% and 30% of acid deposition and nitrogen deposition in the Netherlands come from other countries.

Sources of acidifying substances

The main sources of acidifying substances are agriculture, traffic and industry.

Agriculture accounts for more than 90% of ammonia emissions in the Netherlands. The main emission sources are animal accommodation, the application of manure and chemical fertilisers, pasturage and manure storage.
Traffic is the main source of oxides of nitrogen, accounting for over 65% of emissions in the Netherlands in 2002. Industry and the energy sector are other important sources.
Industry emits most sulphur dioxide (approximately 50%). Sulphur dioxide is mainly released during the combustion of coal and oil.
VOCs are mainly emitted by traffic and industry.

Impact of acidifying substances

The acidifying substances enter plants and trees through leaves and roots, making them more susceptible to disease. Acid deposition also affects rivers and lakes, and ultimately the animals that live in them or drink from them.
Acidification also affects groundwater. Two-thirds of Dutch drinking water comes from the soil, so this constitutes a threat to public health. Excessive nitrate concentrations in drinking water are particularly harmful for babies. Excessive concentrations of aluminium can cause Alzheimer's disease.

Acidification abatement policy

At the international level, 31 countries, including all the EU member states, have made agreements in the Gothenburg Protocol about emission ceilings for 2010.
The EU member states agreed on national emission ceilings in the NEC directive on 23 October 2001.

The national objectives in the fourth National Environmental Policy Plan are commitments that are stricter than the international agreements. The thinking behind this was to establish a safety margin for coping with adverse developments (VROM, 2001).

For a extensive overview of all acidification objectives, see: Acidification and transboundary air pollution: policy

What is potential acid?

The term "potential acid" is used to indicate the amount of acidification. Potential acid is defined as the maximum acidification that sulphur dioxide, nitrogen oxides and ammonia can cause in soil and water. Actual acidification in soil and water can be lower. This depends on a number of processes and on the uptake of the substances by plants.

The potential of a substance to cause acidification is expressed in acid equivalents per hectare (a.e./ha). An acid equivalent is the amount of acid (H⁺ in mol/ha) that can be formed in soil or water. Here: 1 mol sulphur dioxide forms 2 mol acid, 1 mol oxides of nitrogen forms 1 mol acid and 1 mol ammonia forms 1 mol acid.

There is sometimes confusion about the acidification effect of ammonia. In the atmosphere, ammonia neutralises acid. However, if ammonia (or ammonium) enters the soil, it can be converted into nitric acid, which causes acidification.

References

EU (2001). Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2001 in respect of national emission ceilings for certain air pollutants (NEC directive) (link to PDF file).
UNECE (1999). Protocol accompanying the 1979 Convention on long-range transboundary air pollution, Gothenburg, 30-11-99 (Trb. 2000, 66). Gothenburg Protocol Web Page.
VROM (2001). Fourth National Environmental Policy Plan. Working on sustainability: Where there's a will there's a world. Ministry of Housing, Spatial Planning and the Environment, The Hague.

Relevant sections and indicators in the Environmental Data Compendium

Relevant information outside of the Environmental Data Compendium

See the Convention on long-range transboundary air pollution and IIASA for information about international agreements for reducing transboundary air pollution.
See the Environmental Protection Agency for the situation in the United States.