Atmospheric pollutants

Carbon 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 to form sulfurous acid:

SO2(g) + H2O(l) → H2SO3(aq)

Oxygen in the air reacts with sulfurous acid to form sulfuric acid:

H2SO3(aq) + ½O2(g) → H2SO4(aq)

The mixture of the two acids falls from the clouds as acid rain.


Explain why sulfurous acid is oxidised when it forms sulfuric acid.

Sulfurous acid reacts with oxygen and gains oxygen in the reaction.

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 plants and trees, making them lose their leaves and die
  • makes rivers and lakes too acidic for some aquatic life to survive
Stone carvings damaged by acid rain
These stone carvings have been damaged by acid rain

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

Nitrogen dioxide

These two oxides of nitrogen are together represented by the formula 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.


Write a balanced equation for the reaction between nitrogen monoxide and oxygen, forming nitrogen dioxide. Include state symbols in your answer.

2NO(g) + O2(g) → 2NO2(g)

Sulfur dioxide is produced because of impurities in the fuel, but oxides of nitrogen are produced because of the high temperatures in engines.