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Air Pollutants: Sources, Effects and Controls

Link prescribed air pollutants to specific sources, harmful effects and targeted emission controls.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • name some common atmospheric pollutants, e.g. carbon monoxide; methane; nitrogen oxides (NO and NO2); ozone; sulfur dioxide; unburned hydrocarbons
  • state the sources of these pollutants as — carbon monoxide from incomplete combustion of carbon-containing substances
  • state the sources of these pollutants as — nitrogen oxides from lightning activity and internal combustion engines
  • state the sources of these pollutants as — sulfur dioxide from volcanoes and combustion of fossil fuels
  • describe the reactions used in possible solutions to the problems arising from some of the pollutants named in (b) — the redox reactions in catalytic converters to remove combustion pollutants (see also 10(d))
  • discuss some of the effects of these pollutants on health and on the environment — the toxic nature of carbon monoxide

Outcome: build a source → pollutant → effect chain and choose a control that acts on that pollutant. Prerequisite: combustion and redox ideas.

1. Definition

An air pollutant is a substance present in air in an amount that harms health or the environment.

2. Key Ideas

The required pollutants are carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide and unburned hydrocarbons.

These pollutants do not all cause acid rain. Sulfur dioxide and nitrogen oxides are acid-rain precursors; the others have different primary effects in this course.

Retrieval check
What can form when a hydrocarbon burns with insufficient oxygen? Carbon monoxide and/or soot, with unburned hydrocarbons possible.

3. Detailed Explanations

PollutantSpecific sourceDecisive effect
COincomplete combustionreduces oxygen transport in blood
CH₄cattle, landfill decay, gas leakagegreenhouse gas
nitrogen oxideshot engines; lightningrespiratory irritation; acid-rain precursor
ground-level O₃reactions between pollutants in sunlightrespiratory irritation; plant damage
SO₂sulfur-containing fuels; volcanoesrespiratory irritation; acid-rain precursor
unburned hydrocarbonsincomplete combustion in enginesharmful; can help form ground-level ozone

A catalytic converter promotes redox conversions such as

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

Flue gas desulfurisation reacts acidic sulfur dioxide with calcium carbonate. The Acid Rain lesson develops that chemistry.

Two required methods for controlling air pollutionVehicle exhaust containing carbon monoxide, nitrogen oxides and unburned hydrocarbons passes through a catalytic converter to form carbon dioxide, nitrogen and water. Power-station flue gas containing sulfur dioxide passes over calcium carbonate to form calcium sulfite and carbon dioxide.Match each pollutant source to the correct controlVehicle exhaustCO + NOₓ + unburned hydrocarbonscatalytic converterCO₂ + N₂ + H₂OPower-station flue gasSO₂ in waste gascalcium carbonateCaCO₃CaSO₃ + CO₂
Match the emission source to its control: catalytic converters treat vehicle exhaust, while calcium carbonate removes sulfur dioxide from flue gas.

4. Common Mistakes

  • Carbon monoxide is toxic; carbon dioxide has a different greenhouse effect.
  • A catalyst is not a storage filter and is not used up.
  • Ground-level ozone is harmful; stratospheric ozone is protective.
  • Not every air pollutant causes acid rain: identify sulfur dioxide or nitrogen oxides before claiming that effect.
  • Catalytic converters treat vehicle exhaust; calcium carbonate treatment targets sulfur dioxide in flue gas.

5. Exam Tips

Write each causal link explicitly. “Cars cause pollution” is weaker than “incomplete combustion → carbon monoxide → reduced oxygen transport”.

6. Worked Examples

Modelled example 1

Model: faulty heater evidence

Core

Problem

A heater burns fuel with too little oxygen. Explain one health risk.
Study the worked solution

  1. Build the chain

    Method

    Identify incomplete combustion, carbon monoxide and reduced oxygen transport.

    Reason

    CO binds strongly to haemoglobin.

    Working

    insufficient O₂ → incomplete combustion → CO → lower oxygen transport.

Guided practice 2

Guided: changed emission stream

About 5 min

Problem

A bus exhaust contains carbon monoxide and nitrogen oxides. Select the control.

Try this before viewing the solution

Control

Hints

Hint 1: match the source
Identify whether the polluted stream is vehicle exhaust or power-station flue gas.
View solution step by step

  1. Match source and reactions

    Method

    Choose a catalytic converter.

    Reason

    It promotes the required vehicle-exhaust redox reactions.

    Working

    CO → CO₂; nitrogen oxides → N₂.

Challenge 3

Transfer: infer a source cautiously

Minimal support

Problem

Sulfur dioxide is detected. Does this alone prove fossil fuel was burned?

Try this before viewing the solution

Conclusion

Hints

Hint 1: consider another source
Recall one natural source of sulfur dioxide.
View complete solution

  1. Compare sources

    Method

    No; volcanoes also release sulfur dioxide.

    Reason

    A pollutant identity need not identify one unique source.

    Working

    Detection supports SO₂ presence, not a unique origin.

7. Mind Stretchers

Independent: Evaluate: “A catalytic converter cannot improve air quality because it can produce carbon dioxide.”

Show answer
It can form carbon dioxide while converting immediately harmful carbon monoxide, nitrogen oxides and unburned hydrocarbons into less harmful products. Carbon dioxide has a separate climate effect, so the statement ignores the pollutants actually controlled.

8. Quiz

Practise this lesson
Start focused practice, then continue to Acid Rain.