Atmospheric Pollutants

What Is Released When Fuels Burn?

When fossil fuels such as petrol, diesel, coal and natural gas burn, they do more than produce energy - they release a range of gases and particles that affect human health and the environment.

Hydrocarbon fuels are made only of carbon (C) and hydrogen (H). When they react with oxygen during combustion, a variety of products can form depending on how much oxygen is available.

Products of Combustion

1. Complete Combustion (Enough Oxygen)

When plenty of oxygen is present, hydrocarbons burn “cleanly.”

Main products:

• Carbon dioxide (CO₂)
• Water vapour (H₂O)

These products form because all carbon and hydrogen atoms are fully oxidised.

2. Incomplete Combustion (Limited Oxygen)

If oxygen supply is restricted, combustion is incomplete and harmful substances are produced.

Possible products:

• Carbon monoxide (CO) – a poisonous gas
• Soot / carbon particles – tiny solid carbon pieces
• Unburned hydrocarbons – fuel molecules that didn’t react fully
• Carbon dioxide and water – still formed, but in lower amounts

Why does this happen?

Not all carbon gets fully oxidised, so some escapes as soot or becomes carbon monoxide rather than carbon dioxide.

3. Other Pollutants from Fossil Fuels

Sulfur dioxide (SO₂)
Produced if the fuel contains sulfur impurities (common in coal and some oils).

Nitrogen oxides (NOₓ)
Formed when nitrogen and oxygen in air react at very high temperatures, such as inside car engines or power-station furnaces.

NOₓ includes:

• NO (nitrogen monoxide)
• NO₂ (nitrogen dioxide)

Carbon Particulates (Soot) 

Effects on humans

• Can become lodged in the lungs
• Linked to breathing difficulties and long-term respiratory disease

Effects on the environment

• Reflect sunlight away from Earth
• Contribute to reduced global sunlight levels, a process often referred to as global dimming

Carbon Monoxide (CO)

Carbon monoxide is especially dangerous because:

• It attaches to haemoglobin more strongly than oxygen does
• This means the blood can’t carry enough oxygen around the body
• This may lead to fainting, organ damage, coma, or death
• It is colourless and odourless, so it is extremely hard to detect without specialised equipment

Acid Rain

Acid rain forms when:

• Sulfur dioxide (SO₂) reacts with water in clouds → sulfuric acid
• Nitrogen oxides (NOₓ) react with water in clouds → nitric acid

These acids fall to the ground as acid rain.

Environmental consequences

• Makes lakes and streams acidic → kills fish and aquatic organisms
• Damages trees by weakening leaf surfaces and removing nutrients from soil
• Erodes buildings made from limestone or marble
• Can degrade outdoor sculptures
  

Health Effects of SO₂ and NOₓ

Even aside from acid rain, breathing in sulfur dioxide or nitrogen oxides can:

• Trigger asthma attacks
• Irritate airways
• Cause long-term respiratory issues

These gases dissolve in the moisture lining the respiratory system, forming mild acids that irritate tissues.

Recall

Application

Challenge (HT Only)