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How does acid rain form?

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Quick answer

Acid rain forms when sulfur dioxide and nitrogen oxides, mainly from burning fossil fuels, react with water vapour and oxygen in the atmosphere to create sulfuric and nitric acids. These acids mix with cloud droplets and fall as rain, snow, or dry particles, acidifying lakes, soils, and buildings far from the pollution source.

Acid rain forms when sulfur dioxide and nitrogen oxides from burning fossil fuels react in the atmosphere to produce sulfuric and nitric acids, which then fall to the ground in rain, snow, or dust. These pollutants can travel hundreds of kilometres, so the pollution that causes acid rain often originates far from where it lands.

Step by step

  1. Fossil fuels such as coal and oil are burned in power plants, factories, and vehicles, releasing sulfur dioxide and nitrogen oxides.
  2. These gases rise into the atmosphere and are carried by winds, sometimes across national borders.
  3. In the air, they react with water vapour, oxygen, and sunlight to form sulfuric acid and nitric acid.
  4. The acids dissolve in cloud droplets or stick to dust particles, making the cloud water acidic.
  5. When precipitation falls, it carries the acids to the ground as rain, snow, sleet, or fog.
  6. The acids can also settle as dry particles and later be washed into soils and water bodies.

Key things to understand

  1. Burning coal, oil, and gasoline releases sulfur dioxide and nitrogen oxides into the air.
  2. In the atmosphere, these gases react with water vapour, oxygen, and sunlight to form sulfuric and nitric acids.
  3. The acids dissolve in cloud droplets or attach to dust particles, making the precipitation acidic.
  4. When rain or snow falls, it carries these acids to the ground, where they can acidify lakes, soils, and streams.
  5. Acid rain can also corrode buildings and statues, and harm forests by leaching nutrients from the soil.

Labelled diagram

  1. Emission

    Burning coal and oil releases sulfur dioxide and nitrogen oxides into the air.

  2. Transport

    Winds carry these gases hundreds of kilometres, often across borders.

  3. Chemical reaction

    Gases react with water vapour, oxygen, and sunlight to form sulfuric and nitric acids.

  4. Cloud formation

    Acids dissolve in cloud droplets, making the water acidic.

  5. Deposition

    Acidic rain, snow, or dust falls to the ground, affecting soils and water.

How acid rain forms and falls

Real-world examples

  • In the 1980s, acid rain severely damaged lakes in Scandinavia, killing fish populations, even though much of the pollution came from factories in the UK and Germany.
  • The Adirondack Mountains in New York suffered acid rain that acidified hundreds of lakes, harming trout and other aquatic life.
  • In India, the Taj Mahal has been discoloured by acid rain and air pollution, which react with the marble surface.
  • Coal-burning power plants in China have contributed to acid rain in neighbouring countries like Japan and South Korea.

Common mistake

✗ Myth: Acid rain is just natural rain that is slightly acidic.

✓ Fact: All rain is slightly acidic due to carbon dioxide, but acid rain is far more acidic because of human-made pollutants. The term specifically refers to precipitation with a pH below about 5.0, caused mainly by sulfur dioxide and nitrogen oxides from burning fossil fuels.

People also ask

What causes acid rain?
Acid rain is caused by sulfur dioxide and nitrogen oxides released when fossil fuels are burned. These gases react in the atmosphere with water, oxygen, and other chemicals to form sulfuric and nitric acids. The acids then mix with rain, snow, or dust and fall to the ground, acidifying lakes, soils, and buildings.
How does acid rain affect plants and animals?
Acid rain harms plants by leaching essential nutrients like calcium and magnesium from the soil, and by releasing toxic metals such as aluminium that damage roots. In lakes, acidic water can kill fish eggs and disrupt the food chain, sometimes wiping out entire fish populations. Forests in affected areas may grow more slowly or die back.
What can be done to reduce acid rain?
Reducing acid rain requires cutting emissions of sulfur dioxide and nitrogen oxides. This can be done by burning less fossil fuel, using low-sulfur coal, installing scrubbers on smokestacks, and switching to renewable energy sources. International agreements like the 1979 Convention on Long-Range Transboundary Air Pollution have helped lower emissions in Europe and North America.
What is the pH of acid rain?
Normal rain is slightly acidic, with a pH around 5.6, because carbon dioxide in the air forms weak carbonic acid. Acid rain typically has a pH below 5.0, and in severe cases it can be as low as 4.0 or even lower, which is ten to a hundred times more acidic than normal rain.
Does acid rain harm humans directly?
Acid rain does not burn human skin on contact, but the pollutants that cause it, such as sulfur dioxide and nitrogen oxides, can irritate the lungs and worsen respiratory conditions like asthma. The acids can also release toxic metals from soil into drinking water, which poses a longer-term health risk.
Which countries are most affected by acid rain?
Acid rain has been a serious problem in parts of Europe, North America, and Asia, especially in regions with many coal-burning power plants and factories. The pollutants can cross national borders, so countries downwind often suffer even if they produce little pollution themselves.
Can acid rain be reversed?
Yes, reducing emissions of sulfur dioxide and nitrogen oxides allows ecosystems to recover over time. Lakes and soils can be treated with lime to neutralise acidity, but the most effective solution is cutting the pollution at its source, such as by using cleaner fuels and installing scrubbers on smokestacks.

Part of the guide: Water & Weather Phenomena →