Pollutants That Affect Air Quality (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Combustion of fossil fuels

• The combustion of fossil fuels is a major source of atmospheric pollution

• Fossil fuels include coal, oil, natural gas, oil shales, and tar sands

• These fuels are mainly hydrocarbons, containing only carbon and hydrogen

  • Fuels may also contain small amounts of sulfur as impurities

• When burned, the fuels release energy but also a variety of pollutants that can harm the environment and human health

Combustion products

• Burning fossil fuels releases several gases into the atmosphere:

  • Carbon dioxide (CO₂) – produced during complete combustion

  • Water vapour (H₂O) – also from complete combustion

  • Carbon monoxide (CO) – produced during incomplete combustion when oxygen is limited

  • Oxides of nitrogen (NO and NO₂) – formed from nitrogen and oxygen in the air at high temperatures in engines

  • Sulfur dioxide (SO₂) – produced from sulfur impurities in the fuel

  • Solid particles and unburned hydrocarbons can also be released, forming soot (carbon particulates)

Complete vs incomplete combustion

• Complete combustion happens when there is plenty of oxygen:

hydrocarbon + oxygen → carbon dioxide + water

example: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

• Incomplete combustion happens when there is not enough oxygen

hydrocarbon + oxygen → carbon monoxide + water 

example: 2CH₄ + 3O₂ → 2CO + 4H₂O

• Incomplete combustion is common in engines and poorly ventilated stoves

Formation of key pollutants

Carbon monoxide (CO)

• Carbon monoxide is produced when fuels burn with limited oxygen 

• The carbon in the fuel is only partially oxidised

  • For example:

2CH₄ + 3O₂ → 2CO + 4H₂O

Soot (carbon particles)

• Soot forms when there is very little oxygen, causing hydrocarbons to break down without fully combusting

  • For example: 

C₈H₁₈ + 8½O₂ → 8C (soot) + 9H₂O

Sulfur dioxide (SO₂)

• Some fossil fuels contain sulfur as an impurity

• During combustion, this sulfur reacts with oxygen to form sulfur dioxide:

S + O₂ → SO₂

Oxides of nitrogen (NO and NO₂)

• Oxides of nitrogen form when nitrogen and oxygen from the air react at the extreme temperatures achieved in car engines and power stations. 

  • For example:

N₂ + O₂ → 2NO

2NO + O₂ → 2NO₂

Carbon monoxide and health

• Carbon monoxide is an extremely poisonous gas as it combines with haemoglobin in blood and prevents it from carrying oxygen

• It is particularly dangerous as it is colourless and odourless

  • This makes it difficult to detect carbon monoxide

• At low concentrations, symptoms include:

  • Faster heart rate (to compensate for reduced oxygen delivery to cells)

  • Dizziness

  • Tiredness

• At high concentrations, cells receive insufficient oxygen for respiration and release less energy, causing loss of consciousness, coma or death

Acid rain

• Acid rain is commonly caused by:

  • Sulfur dioxide, SO₂

  • Oxides of nitrogen, NO_x

Sulfur dioxide

• Sulfur dioxide is a colourless, pungent-smelling gas produced when fuels containing sulfur impurities are burned

• The main sources are coal burned in power stations and diesel fuels used in ships and heavy vehicles

• Sulfur dioxide dissolves in water in clouds to form sulfuric acid, which falls as acid rain

Oxides of nitrogen

• Oxides of nitrogen form when nitrogen and oxygen react at the high temperatures reached in vehicle engines and power stations

• They contribute to acid rain and photochemical smog, and cause respiratory problems, particularly for people with asthma

Effects of acid rain

• Acid rain corrodes metal structures and buildings and statues made of carbonate rocks

• It damages aquatic organisms in rivers, ponds and lakes

• It pollutes crops and water supplies, and irritates the lungs, throat and eyes

Particulates

• Particulates are tiny solid particles released by human activity

• Sources include soot from diesel engines and dust from roads and industry

• Soot forms during incomplete combustion when very little oxygen is available

Health and environmental effects

• Larger particulates clump together to form soot

  • The soot then falls back to the ground and blackens buildings and statues, accelerating corrosion

• Smaller particulates are inhaled and penetrate deep into the lungs

  • These smaller particulates cause damage that can lead to heart disease and lung cancer