Atmospheric Pollutants

This lesson covers the main atmospheric pollutants, their sources, and their effects on the environment and human health, as required by AQA GCSE Chemistry specification (5.9.3). You need to understand which pollutants are produced by human activities (particularly burning fuels), how they form, and the problems they cause. This is a key topic that links chemistry to real-world environmental issues.

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What Are Atmospheric Pollutants?

Atmospheric pollutants are substances released into the air that have harmful effects on the environment and/or human health. Most atmospheric pollutants are produced by the combustion (burning) of fossil fuels — coal, oil, and natural gas — in power stations, vehicles, factories, and homes.

The main atmospheric pollutants you need to know are:

Pollutant	Chemical Formula	Main Source
Carbon dioxide	CO₂	Complete combustion of any carbon-containing fuel
Carbon monoxide	CO	Incomplete combustion of carbon-containing fuels
Sulfur dioxide	SO₂	Combustion of fuels containing sulfur impurities (especially coal)
Oxides of nitrogen (nitrogen oxides)	NOₓ (NO and NO₂)	High-temperature combustion in vehicle engines and power stations
Particulates (soot)	C (solid carbon)	Incomplete combustion of fuels, especially diesel engines
Water vapour	H₂O	Combustion of hydrogen-containing fuels (all hydrocarbon fuels)

Exam Tip: Water vapour is a product of combustion but is not usually considered a "pollutant" in the same way as the others. However, it is a greenhouse gas. Carbon dioxide is both a product of complete combustion AND a greenhouse gas. Make sure you can distinguish between pollutants that are toxic (CO, SO₂, NOₓ, particulates) and those that are primarily greenhouse gases (CO₂, H₂O).

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Complete vs Incomplete Combustion

The pollutants produced when a fuel burns depend on whether combustion is complete or incomplete.

Complete Combustion

Complete combustion occurs when there is a plentiful supply of oxygen. The products are carbon dioxide and water:

hydrocarbon + oxygen → carbon dioxide + water

For example, methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete Combustion

Incomplete combustion occurs when there is a limited supply of oxygen. Instead of all the carbon being converted to CO₂, some forms carbon monoxide (CO) and/or carbon particles (soot/particulates):

hydrocarbon + limited oxygen → carbon monoxide + water hydrocarbon + very limited oxygen → carbon (soot) + water

For example, methane with limited oxygen:

2CH₄ + 3O₂ → 2CO + 4H₂O

Or with very limited oxygen:

CH₄ + O₂ → C + 2H₂O

graph TD
    A["Combustion of<br/>Hydrocarbons"] --> B{"Oxygen<br/>supply?"}
    B -->|"Plentiful"| C["Complete Combustion"]
    B -->|"Limited"| D["Incomplete Combustion"]
    C --> E["CO₂ + H₂O"]
    D --> F["CO + H₂O"]
    D --> G["C (soot) + H₂O"]

    style C fill:#27ae60,color:#fff
    style D fill:#c0392b,color:#fff
    style E fill:#2ecc71,color:#fff
    style F fill:#e74c3c,color:#fff
    style G fill:#e74c3c,color:#fff

Exam Tip: You must be able to write balanced equations for both complete and incomplete combustion of simple hydrocarbons (methane, propane, butane, octane). Remember that incomplete combustion produces carbon monoxide and/or carbon (soot), not carbon dioxide.

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Effects of Each Pollutant

Carbon Dioxide (CO₂)

• Source: Complete combustion of any fuel containing carbon.
• Effect: A greenhouse gas that contributes to global warming and climate change.
• CO₂ is not toxic at normal atmospheric concentrations but its accumulation in the atmosphere is causing the enhanced greenhouse effect.

Carbon Monoxide (CO)

• Source: Incomplete combustion of carbon-containing fuels — particularly in car engines and faulty gas boilers.
• Effect: Carbon monoxide is a colourless, odourless, toxic gas. It binds to haemoglobin in red blood cells, preventing them from carrying oxygen. This can cause headaches, dizziness, unconsciousness, and death at high concentrations.
• CO is particularly dangerous because it cannot be detected by human senses — this is why carbon monoxide detectors are recommended in homes with gas appliances.

Sulfur Dioxide (SO₂)

• Source: Combustion of fuels containing sulfur impurities, especially coal and some types of oil. Sulfur is present as an impurity in many fossil fuels.
• Effect: SO₂ dissolves in water in the atmosphere to form sulfuric acid (H₂SO₄), which falls as acid rain. Acid rain damages buildings (especially limestone and marble), vegetation, and aquatic life by lowering the pH of lakes and rivers.
• SO₂ can also cause respiratory problems in humans, particularly for people with asthma.

Oxides of Nitrogen (NOₓ)

• Source: Formed when nitrogen and oxygen in the air react at the high temperatures inside vehicle engines and power station furnaces. (Note: the nitrogen comes from the air, not from the fuel.)
• Effect: Nitrogen oxides contribute to acid rain (forming nitric acid, HNO₃), photochemical smog, and respiratory problems. Nitrogen dioxide (NO₂) is a brown, irritating gas that can trigger asthma attacks and damage lung tissue.

Particulates (Soot)

• Source: Incomplete combustion, especially from diesel engines, coal fires, and industrial processes.
• Effect: Particulates are tiny solid particles suspended in the air. They cause:
  • Respiratory problems — particulates can penetrate deep into the lungs, causing or worsening conditions like asthma, bronchitis, and lung cancer.
  • Reduced visibility — particulates in the air scatter light, causing smog and haze.
  • Global dimming — particulates in the upper atmosphere reflect sunlight back into space, reducing the amount of light reaching the surface.
  • Surface coating — particulates settle on buildings and surfaces, causing discolouration and damage.

Exam Tip: You may be asked to explain why incomplete combustion is more dangerous than complete combustion. The answer is that incomplete combustion produces carbon monoxide (toxic) and particulates (cause respiratory problems), neither of which are produced by complete combustion.

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Summary Table of Pollutants and Their Effects