Acid Rain and Global Dimming

This lesson covers two important environmental consequences of atmospheric pollution: acid rain and global dimming, as required by AQA GCSE Chemistry specification (5.9.3). You need to understand how these phenomena are caused, what their effects are, and how they can be reduced. Both topics are frequently examined and require you to link chemical knowledge to real-world environmental issues.

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What Is Acid Rain?

Acid rain is rain (or any form of precipitation) that is more acidic than normal. Normal rain is slightly acidic (pH approximately 5.6) because carbon dioxide in the atmosphere dissolves in rainwater to form carbonic acid:

CO₂ + H₂O → H₂CO₃

Acid rain has a pH significantly lower than 5.6, typically between pH 3 and pH 5, due to the presence of stronger acids formed from atmospheric pollutants.

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Causes of Acid Rain

Acid rain is caused primarily by two pollutant gases dissolving in rainwater:

1. Sulfur Dioxide (SO₂)

Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned, especially coal. In the atmosphere, SO₂ reacts with water and oxygen to form sulfuric acid (H₂SO₄):

SO₂ + H₂O → H₂SO₃ (sulfurous acid)