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Carbon dioxide, carbon monoxide and soot are not the only atmospheric pollutants produced by the combustion of hydrocarbon fuels. Sulfur dioxide and oxides of nitrogen may be produced too.

Sulfur dioxide

Many hydrocarbon fuels naturally contain sulfur impurities. When the fuels are burned, the sulfur oxidises to form sulfur dioxide gas:

sulfur + oxygen → sulfur dioxide

S(s) + O₂(g) → SO₂(g)

Sulfur dioxide dissolves in water in the clouds and reacts with oxygen in the air to form sulfuric acid. Overall:

H₂O(l) + SO₂(g) + ½O₂(g) → H₂SO₄(aq)

This falls from the clouds in acid rain.

Effects of acid rain

Acid rain damages the natural and built environment. For example, it:

• reacts with metals and rocks such as limestone, weakening and damaging buildings and statues
• damages the waxy layer on the leaves of trees, making it more difficult for trees to absorb the minerals they need for healthy growth
• makes rivers and lakes too acidic for some aquatic life to survive

Oxides of nitrogen

High temperatures are reached when fuels are burned in engines. At these high temperatures, nitrogen and oxygen from the air can react together to produce oxides of nitrogen. For example:

nitrogen + oxygen → nitrogen monoxide

N₂(g) + O₂(g) → 2NO(g)

Nitrogen oxide gas can be oxidised further in air to produce nitrogen dioxide gas, NO₂.

These two oxides of nitrogen are together called NOx. They are atmospheric pollutants. They can react in sunlight with other substances to produce a hazy, harmful smog.

Nitrogen dioxide is toxic. It can cause bronchitis and other lung diseases. It also dissolves in water in the clouds, forming an acidic solution that contributes to acid rain.