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examBoard: Cambridge
examType: IGCSE
lessonTitle: Acid Rain Formation and Causes
Acid rain is precipitation (rain, snow, or fog) that is unusually acidic due to atmospheric pollution. While normal rain is slightly acidic with a pH of about 5.6 (due to naturally occurring carbon dioxide forming carbonic acid), acid rain typically has a pH between 4.2 and 4.4, or even lower in severely affected areas.
Key Definitions:
Normal rain has a pH of about 5.6 because carbon dioxide in the air naturally combines with water to form a weak carbonic acid. Acid rain, however, can have a pH as low as 3 or 4, making it significantly more acidic and harmful to ecosystems and structures.
The pH scale is logarithmic, meaning each whole number decrease represents a tenfold increase in acidity. For example, rain with a pH of 4 is 10 times more acidic than rain with a pH of 5 and 100 times more acidic than rain with a pH of 6.
Acid rain forms when certain air pollutants react with water, oxygen and other chemicals in the atmosphere. The two primary pollutants responsible for acid rain are sulphur dioxide (SO2) and nitrogen oxides (NOx).
Sulphur dioxide undergoes a series of chemical reactions in the atmosphere to form sulphuric acid:
This sulphuric acid then falls as precipitation, contributing to acid rain.
Nitrogen oxides (mainly NO and NO2) also contribute to acid rain formation:
These acids lower the pH of precipitation, resulting in acid rain.
The pollutants that cause acid rain come from both natural and human sources, though human activities have significantly increased their concentration in the atmosphere.
Human activities account for over 70% of sulphur dioxide and 90% of nitrogen oxides in the atmosphere in industrialised regions. Natural sources dominate in remote areas, but their global impact is much smaller.
The burning of fossil fuels in power stations and industrial facilities is the largest contributor to acid rain formation. Here's how different industries contribute:
Coal-fired power stations are major sources of SO2 emissions. Coal naturally contains sulphur, which is released as sulphur dioxide when burned. A single large power station can emit tens of thousands of tonnes of SO2 annually. Natural gas power plants produce less SO2 but still emit significant NOx.
Vehicles are the primary source of nitrogen oxides. The high temperatures in car engines cause nitrogen and oxygen in the air to combine, forming NOx. Diesel engines typically produce more NOx than petrol engines. In urban areas, transport can contribute up to 60% of all NOx emissions.
Metal smelting operations, particularly for copper, zinc and lead, release significant amounts of sulphur dioxide. The ore processing involves heating metal sulphides, which releases sulphur dioxide as a by-product. Mining activities also expose sulphur-containing minerals to air and water, leading to acid formation even without combustion.
Sudbury was once known as one of the world's most severe examples of acid rain damage. Home to some of the largest nickel smelting operations in the world, by the 1960s, approximately 100 square kilometres around Sudbury had become a barren landscape with acidified lakes and damaged vegetation. The area was so desolate that NASA used it to train astronauts for moon missions due to its lunar-like appearance.
In the 1970s, a 381-metre "Superstack" chimney was built to disperse emissions higher in the atmosphere. While this reduced local pollution, it spread acid rain over a wider area. Since the 1980s, emission controls and regreening efforts have helped restore the ecosystem, with SO2 emissions reduced by over 90%. This case demonstrates both the severe impact of industrial emissions and the potential for recovery with proper environmental management.
Acid rain is not evenly distributed around the world. It tends to be concentrated in industrialised regions and can travel hundreds of kilometres from its source.
One of the most challenging aspects of acid rain is that it often affects areas far from where the pollutants were emitted. Tall smokestacks, originally built to reduce local pollution, disperse pollutants high into the atmosphere where they can be carried by prevailing winds across national boundaries.
For example, emissions from the UK and Germany have historically contributed to acid rain in Scandinavia, while US emissions affect Canada. This transboundary nature makes acid rain an international environmental issue requiring cooperation between countries.
The "Black Triangle" refers to a region where the borders of Germany, Poland and the Czech Republic meet. During the Cold War era (1950s-1980s), this area had some of the highest concentrations of coal-burning power plants and heavy industry in Europe. The region burned high-sulphur brown coal (lignite) with minimal emission controls.
By the 1980s, over 75% of the forests in parts of the region were damaged or dying. Acid rain pH values as low as 2.7 were recorded. Following the fall of communism in 1989, environmental regulations were strengthened and many polluting facilities were modernised or closed. Today, emissions have decreased by over 80% and the forests are slowly recovering, though the soil remains acidified in many areas.
Several factors influence how severe acid rain becomes in a particular region:
The conversion of primary pollutants (SO2 and NOx) to acids is accelerated by other atmospheric components:
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