8.12 - Sulfur dioxide and acid rain

From gas to acidic rain

Sulfur dioxide, SO₂, can dissolve in water droplets and make them acidic. Reactions involving water and oxygen in the atmosphere can also form acids from SO₂. Rain carrying this additional acidity has a lower pH than normal rain: acid rain.

Normal rain is already slightly acidic, partly because carbon dioxide dissolves in it. Acid rain means extra acidity, not simply any rain with pH below 7. Here we focus on sulfur dioxide as the pollutant source.

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Water, soil and living things

When acidic rain or runoff enters a lake, it can lower the water's pH. Fish, eggs and other organisms tolerate only a limited pH range. Conditions outside that range can reduce survival and disrupt food webs.