Atmospheric Pollutants

This lesson covers atmospheric pollutants produced by burning fossil fuels as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the sources and effects of particulates, carbon monoxide, sulfur dioxide and nitrogen oxides, as well as how catalytic converters help reduce pollution.

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Where Do Atmospheric Pollutants Come From?

Most atmospheric pollutants are produced by burning fossil fuels, particularly in:

• Vehicle engines (cars, lorries, buses, aircraft)
• Power stations (burning coal, oil or gas)
• Industrial processes (factories, refineries)
• Domestic heating (gas boilers, open fires)

When fuels burn, they can release a range of harmful substances depending on the conditions of combustion and the impurities in the fuel.

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The Main Pollutants

1. Particulates (Soot / Carbon Particles)

Particulates are tiny solid particles released during incomplete combustion of fossil fuels, especially diesel.

Aspect	Detail
Source	Incomplete combustion of hydrocarbons (especially in diesel engines)
Composition	Mainly unburned carbon (soot), plus other microscopic particles
Health effects	Cause and worsen respiratory diseases (asthma, bronchitis, lung cancer); particles penetrate deep into the lungs
Environmental effects	Global dimming — particulates in the atmosphere reflect sunlight back into space, reducing the amount reaching Earth's surface; they also make buildings and surfaces dirty

Exam Tip: Particulates cause two main problems: (1) respiratory/health issues and (2) global dimming. Make sure you mention both if asked for effects.

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2. Carbon Monoxide (CO)

Carbon monoxide is a toxic gas produced by incomplete combustion of hydrocarbons.

Aspect	Detail
Source	Incomplete combustion — particularly in vehicle engines and faulty gas appliances
Properties	Colourless, odourless — cannot be detected without a CO alarm
How it harms	Binds to haemoglobin in red blood cells more strongly than oxygen; reduces the blood's oxygen-carrying capacity
Effects	Headaches, dizziness, nausea; in severe cases death from oxygen starvation
Prevention	Regular servicing of gas appliances; adequate ventilation; CO alarms

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3. Sulfur Dioxide (SO₂)

Sulfur dioxide is produced when sulfur impurities in fossil fuels burn.

Aspect	Detail
Source	Burning fossil fuels (especially coal and heavy fuel oil) that contain sulfur impurities: S + O₂ → SO₂
Health effects	Irritates the respiratory system; causes breathing difficulties; worsens asthma
Environmental effects	Causes acid rain — SO₂ dissolves in rainwater to form sulfurous acid (H₂SO₃) and can be further oxidised to sulfuric acid (H₂SO₄)

Acid Rain

Acid rain has serious environmental consequences:

Effect of Acid Rain	Detail
Damage to trees and plants	Acid leaches nutrients from soil; damages leaves
Acidification of lakes and rivers	Kills aquatic organisms (fish, invertebrates)
Damage to buildings and statues	Acid reacts with limestone and marble (CaCO₃), causing erosion
Corrosion of metals	Bridges, railings and other metal structures corrode faster

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4. Nitrogen Oxides (NOₓ)

Nitrogen oxides (mainly NO and NO₂, collectively written as NOₓ) are formed when the nitrogen and oxygen in the air react at the very high temperatures inside vehicle engines.

Aspect	Detail
Source	High temperatures in car engines cause N₂ + O₂ → 2NO; NO is then oxidised to NO₂
Health effects	Irritate the respiratory system; trigger asthma attacks; NO₂ is a toxic brown gas
Environmental effects	Contribute to acid rain (forms nitric acid, HNO₃, in rainwater); contribute to photochemical smog at ground level

graph TD
    FF["Burning Fossil Fuels"] --> P["Particulates (C)<br/>Incomplete combustion"]
    FF --> CO["Carbon monoxide (CO)<br/>Incomplete combustion"]
    FF --> SO2["Sulfur dioxide (SO₂)<br/>Sulfur impurities burn"]
    FF --> NOx["Nitrogen oxides (NOₓ)<br/>High temperature in engines"]

    P --> HD["Health: respiratory disease"]
    P --> GD["Environment: global dimming"]
    CO --> HB["Binds to haemoglobin<br/>→ oxygen starvation"]
    SO2 --> AR1["Acid rain"]
    SO2 --> RD["Respiratory irritation"]
    NOx --> AR2["Acid rain"]
    NOx --> SM["Photochemical smog"]

    style FF fill:#d32f2f,color:#fff
    style P fill:#795548,color:#fff
    style CO fill:#e65100,color:#fff
    style SO2 fill:#f9a825,color:#000
    style NOx fill:#4a148c,color:#fff
    style HD fill:#c62828,color:#fff
    style GD fill:#37474f,color:#fff
    style HB fill:#bf360c,color:#fff
    style AR1 fill:#1b5e20,color:#fff
    style RD fill:#880e4f,color:#fff
    style AR2 fill:#1b5e20,color:#fff
    style SM fill:#4e342e,color:#fff

Exam Tip: NOₓ are NOT produced from impurities in the fuel. They form because nitrogen and oxygen from the air react at the very high temperatures inside engines. This is different from SO₂, which comes from sulfur impurities in the fuel.

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Reducing Atmospheric Pollution

Catalytic Converters

Modern cars are fitted with catalytic converters in the exhaust system. These contain a catalyst (usually platinum and rhodium on a honeycomb structure) that converts harmful gases into less harmful ones:

Harmful Gas In	Converted To	Equation
Carbon monoxide (CO)	Carbon dioxide (CO₂)	2CO + 2NO → 2CO₂ + N₂
Nitrogen monoxide (NO)	Nitrogen (N₂)	2CO + 2NO → 2CO₂ + N₂
Unburned hydrocarbons	Carbon dioxide + water	Oxidised to CO₂ and H₂O

The catalytic converter effectively:

• Reduces CO to CO₂ (less toxic).
• Reduces NO to N₂ (harmless — already 78% of the atmosphere).

Other Methods

Method	Pollutant Reduced	How It Works
Desulfurisation	SO₂	Flue gas desulfurisation in power stations — SO₂ is removed by reacting with calcium oxide or calcium carbonate
Cleaner fuels	SO₂	Using low-sulfur fuels or removing sulfur before burning
Particulate filters	Particulates	Diesel particulate filters (DPFs) trap soot particles in the exhaust
Electric vehicles	All exhaust pollutants	No combustion → no direct exhaust emissions
Public transport / cycling	All pollutants	Fewer vehicles on the road

Exam Tip: Catalytic converters convert CO → CO₂ and NO → N₂. They do NOT remove CO₂ or SO₂. For SO₂ reduction, mention desulfurisation at power stations or using low-sulfur fuels.

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Summary Table of Pollutants

Pollutant	Formula	Source	Health Effects	Environmental Effects
Particulates	C (soot)	Incomplete combustion	Respiratory disease, lung cancer	Global dimming
Carbon monoxide	CO	Incomplete combustion	Binds to haemoglobin → oxygen starvation, death	—
Sulfur dioxide	SO₂	Sulfur impurities in fuel	Respiratory irritation, asthma	Acid rain
Nitrogen oxides	NOₓ	N₂ + O₂ at high temperatures	Respiratory irritation, asthma	Acid rain, smog

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Summary

• Burning fossil fuels releases particulates, CO, SO₂ and NOₓ.
• Particulates cause respiratory disease and global dimming.
• CO is toxic — it binds to haemoglobin and prevents oxygen transport.
• SO₂ (from sulfur impurities) and NOₓ (from N₂ + O₂ at high temperature) cause acid rain.
• Catalytic converters reduce CO and NO by converting them to CO₂ and N₂.
• Other solutions: desulfurisation, particulate filters, low-sulfur fuels, electric vehicles and reducing car use.

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Worked Examples

Worked Example 1 — Forming sulfur dioxide and acid rain

Question: Coal contains small amounts of sulfur. Write equations for (a) the combustion of sulfur to form SO₂ and (b) SO₂ reacting with water to form sulfurous acid.