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Acid rain is a pollution problem caused by the release of acidic gases into the atmosphere. It contributes to pollution in a variety of ways including:

• damage to plants and the wildlife
• erosion of limestone buildings/structures
• corrosion (rusting) of iron bridges/structures

The three main acidic gases responsible for lowering the pH of rainwater are non-metal oxides produced by the burning of fossil fuels like coal, oil and natural gas.

Sulfur dioxide (SO₂)

Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned.

When the gas sulfur dioxide dissolves in water an acidic solution is formed. Clouds are made from water so this gas rises and dissolves in the clouds to form an acid which falls as rain.

Nitrogen dioxide (NO₂)

Nitrogen dioxide is produced in cars with petrol engines. The spark plugs used to ignite the fuel provide enough energy to break the strong bonds between the nitrogen atoms allowing them to combine with oxygen. This also happens naturally during lightning storms.

Carbon dioxide (CO₂)

Carbon dioxide is produced from burning fossil fuels and another large contribution is made by cement manufacturing for use in new buildings. Carbon dioxide dissolves in oceans to cause ocean acidification which is harmful to shellfish and coral. The release of carbon dioxide is also linked to global warming.