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Topic: Climate in Cities. Basics.
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Areas in danger from acid rain

Acid rain occurs mainly in the highly industrialised regions of the world where emissions of sulphur dioxide and nitrogen oxides are large.  However, long range transport of pollutants by the wind means that acid rain can affect regions thousands of kilometers away from its original source.

Long-range transport

In the 18th Century acid rain fell mainly in towns and cities close to the source of the pollution.   Since the 1950's high chimneys have been used to disperse air pollution and these have greatly improved air quality in urban areas.   The improvement in local air quality has, however, come at a cost.  Air pollution is now blown far away from its source, high into the atmosphere and is transported by the wind to other countries and to remote regions where it falls as acid rain.  For example, in Sweden and Norway, around 90% of the acid deposition comes from other countries, primarily the United Kingdom, Germany and Poland and from emissions from ships.

1. The distribution of 600 largest emitters of SO2 in Europe. At the top of the list are two large coal-fired power stations in Bulgaria. Together they emit nearly 600,000 tons of sulphur dioxide a year - as much as the combined total from all the following countries: Austria, Belgium, Denmark, Finland, the Netherlands, Norway and Sweden.
Source: Barrett M., 2000, The worst and the best. Atmospheric Emissions from Large Point Sources in Europe, SENCO, UK, Swedish NGO Secretariat on Acid Rain.

Regions in danger

The areas most in danger from acid rain are those where acid rocks are found on the surface, where rainfall is high and where there are many sources of SO2 and NOx.  The increasing demand for electricity and the rise in the number of motor vehicles in recent decades has meant that emissions of acidifying pollutants have increased dramatically, particularly since the 1950's (read also about natural emissions of SOand NOx).


The heavily industralised regions in Northern and Central Europe are at most risk of damage from acid rain because the sources of pollution are close.  Rain in these areas is generally acidic (pH 4.1 to 5.1).  Comparison of Figures 1. and 2. shows that even though the Scandinavian Countries don't emit much SO2, they are also threatened by acid rain.

2. The risk of acid rain in Europe in 1993.
Source: UNEP GRID-Arendal, based on European Atlas of Environment and Health (

The acidity and pollutant content of precipitation in Scandinavia are greatest in the southern parts of Sweden.  Deposition of acidifying substances depends, however, not only on the acidity of the rainfall but also on the amount of rain.  As a result, the greatest acid deposition occurs in the rainy southwest part of Sweden.

3. Total deposition (i.e. wet and dry) of sulphur (left map) and nitrogen (right map) in Sweden, in 1996. Units: mg per m2 per year.
Source: Swedish Environmental Protection Agency

North America

The highly urbanised and industralisd regions of Eastern Canada and the Northeastern USA are at most risk of acid rain damage.  Sources of SO2 in the USA are concentrated in Ohio, Indiana and Illinois and the prevailing winds carry it to New England and Canada. Figures 4. and 5. show areas with the highest deposition of acidifying substances. The areas that combine acid rain and low soil neutralising capacity are found on high ground east of the Mississippi.

4. Deposition of sulphate (in kg ha-1) in the USA in 2002.
 Source: National Atmospheric Deposition Program (NRSP-3)/National Trends Network. (2004). NADP Program Office, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820.

5. Deposition of nitrate (in kg ha-1) in the USA, in 2002.
Source: National Atmospheric Deposition Program (NRSP-3)/National Trends Network. (2004). NADP Program Office, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820.

Acid rain causes major problems to aquatic life.  It has resulted in the loss of Atlantic salmon in Nova Scotia and Maine and the reduction in fish diversity in northwest Pennsylvania. 41% of lakes in the Adirondack region of New York are either chronically or episodically acidic. In Canada, acid rain has resulted in large losses of fish and aquatic communities in over 30,000 sensitive lakes in Ontario and Quebec.


Acid rain is now emerging as a major problem in the developing world, especially in parts of Asia and the Pacific region where energy use has surged and the use of sulphur-containing coal and oil is very high. Problems associated with acid rain started in China and India in the 1970's when industrial development was very intense.  An estimated 34 million metric tons of sulphur dioxide were emitted in Asia in 1990, over 40% more than in North America.  Acid deposition levels are particularly high in areas such as southeast China, northeast India, Thailand, and the Republic of Korea, all of which are near or downwind of major urban and industrial centers.

The effects are already being felt in the agricultural sector.  Researchers in India found that wheat growing near a power plant where sulphur dioxide deposition was almost five times greater than the critical load (see below)  suffered a 49% reduction in yield compared with wheat growing 22 kilometers away. In southwestern China, a study in Guizhou and Sichuan provinces revealed that acid rain fell on two thirds of the agricultural lands.  Here 16% of the crop area sustained some level of damage.  A study of pines and oaks in both rural and urban areas in the Republic of Korea showed significant decline in growth from 1970 as a result of acid rain.

Negative effects of acid rains have also been seen in Brazil and Venezuela.

Critical loads and target loads

6. Sequence of eight maps showing where the 5th percentile maximum critical load of sulphur (acidity critical load) has or will be exceeded between 1960 and 2010.  White areas indicate regions where the critical load is not exceeded or there is no data.  Red areas show where sulphur deposition is much greater than the amount allowed.  In these areas the acidifying effect of sulphur is the largest.  For detailed explanations look on the web site mentioned below. 
Source: Coordination Center for Effects (CCE) at RIVM   and click: "data" on the left bar, and then "critical loads databases and maps". For an explanation of the 5th percentile click on "Methods and models" on the left bar.

Critical loads are the maximum amount of pollutants that ecosystems can tolerate without being damaged.  They are the amount that will not cause chemical changes leading to long-term harmful effects in the most sensitive ecological systems. In order to obtain values for the critical loads, an ecosystem has to be chosen and then a suitable indicator species is selected to represent the ecosystem. A chemical limit is subsequently defined as the concentration at which the indicator species will die. In forests the indicators are trees, and in freshwaters they are fish.
In order for critical loads to be used, target loads need to be set for different areas in order to try and halt the acidification processes. Target loads have been defined as "the permitted pollutant load determined by political agreement". Therefore, target loads can be either higher or lower than the critical load values.

In Europe in 1990 around 93 million hectares of land were affected by levels of acid deposition that exceeded the critical load.  To achieve the desired limits and reduce the damage caused by acid rain, it is necessary to reduce acidifying pollution in parts of Europe by 80-90% compared with 1990 levels.

Related pages:

You will learn about acid rain origin in:
Climate in cities - More - Unit 3 - Origin
Read about natural emissions of SO2 and NOx in:
Climate in cities - Basics - Unit 3 - What is it?
Learn more about emissions of acidifying substances in various parts of the world in: 
Climate in cities - Basics - Unit 3 - What is it?
More information on wet and dry deposition in:
Climate in cities - More - Unit 3 - Origin

About this page:

author: Anita Bokwa - Jagiellonian University, Cracow, Poland supporter: Anna Gorol
educational reviewing: Michael Seesing - University of Duisburg, Duisburg, Germany
last update: 2004-08-05

Última modificación: miércoles, 27 de febrero de 2019, 14:37