Short note: Acid rain causes and impacts

Acid rain is rain, snow, fog or dust that is more acidic than normal, with pH below about 5.6. (Normal rain is slightly acidic, pH ≈ 5.6, because of dissolved CO₂.) It forms when sulphur dioxide and nitrogen oxides in the air react with water vapour to form sulphuric and nitric acids.

Causes

• Sulphur dioxide (SO₂) – from burning coal and oil in thermal power plants, industries, brick kilns, smelting of ores.
• Nitrogen oxides (NOₓ) – from vehicle exhaust, power plants and high-temperature combustion.
• Natural sources – volcanoes, lightning, decaying vegetation (smaller share).

Formation (simplified):

• SO₂ + H₂O + O₂ → H₂SO₄ (sulphuric acid)
• NOₓ + H₂O + O₂ → HNO₃ (nitric acid)

These acids fall as wet deposition (rain, snow) or dry deposition (acidic gases and particles), sometimes hundreds of kilometres away from the source.

Impacts (consequences)

1. Aquatic life – lakes and rivers become acidic; fish eggs and insects die; aluminium released from soil is toxic to fish.
2. Forests and crops – leaves damaged, nutrients (Ca, Mg) washed out of soil, trees weakened (e.g. forest dieback in Europe).
3. Soil – loss of fertility and of useful microbes.
4. Buildings and monuments – marble and limestone corrode (e.g. Taj Mahal yellowing; risk to stone temples); metals rust faster.
5. Human health – the SO₂ and NOₓ gases that cause acid rain (and the sulphate and nitrate particles they form) also cause asthma and bronchitis; acidic water can leach lead and copper from pipes.

Control: low-sulphur fuels, flue gas desulphurisation (scrubbers), catalytic converters in vehicles, more renewable energy, and liming of acidified lakes.