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Air Quality, Carbon Cycle and Climate

Learn air composition, pollutant source–effect chains, the carbon cycle, greenhouse gases and global warming.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

Show all 10 objectives
  • describe the volume composition of gases present in dry air as being approximately 78% nitrogen, 21% oxygen and the remainder being noble gases (with argon as the main constituent) and carbon dioxide
  • 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
  • discuss some of the effects of these pollutants on health and on the environment — the toxic nature of carbon monoxide
  • discuss some of the effects of these pollutants on health and on the environment — the role of nitrogen dioxide and sulfur dioxide in the formation of ‘acid rain’ and its effects on respiration and buildings
  • describe the carbon cycle in simple terms, to include — the processes of combustion, respiration and photosynthesis
  • describe the carbon cycle in simple terms, to include — how the carbon cycle regulates the amount of carbon dioxide in the atmosphere (see also 8.1(d))
  • state that carbon dioxide and methane are greenhouse gases and may contribute to global warming; give the sources of these gases and describe the potential effects of increased levels of these greenhouse gases, including more extreme weather events and melting of polar ice.

This lesson links pollutant source–effect chains, carbon flows and climate evidence without mixing up local air pollution and global warming.

1. Definition

An air pollutant is a substance present in the air at a concentration that causes harm.

  • Carbon cycle: movement of carbon between the atmosphere, living things, fuels and other stores.
  • Greenhouse gas: a gas in the atmosphere that can contribute to warming. Carbon dioxide and methane are the two named in this course.
  • Global warming: a long-term rise in average global temperature.
Course boundary

Know the named greenhouse gases, their sources and potential effects. The short infrared model below is included to build understanding, not as an extra explanation to memorise. Catalytic converters, flue-gas desulfurisation and ozone-layer depletion are G3 Chemistry extensions.

2. Key Ideas

Pollutant or processSourceRequired consequence
carbon monoxideincomplete combustionlowers the blood’s ability to carry oxygen
sulfur dioxidesulfur-containing fuels; volcanoesacid rain; respiratory irritation
nitrogen oxideshigh-temperature engines; lightningacid rain
methanedecay, cattle, landfill and gas leaksgreenhouse warming
photosynthesisgreen plantsremoves carbon dioxide from air
combustion and respirationfuels and organismsadd carbon dioxide to air

Clean, dry air is approximately 78% nitrogen and 21% oxygen. Almost all of the remainder is noble gases, mainly argon, with a small amount of carbon dioxide. Common atmospheric pollutants also include ozone and unburned hydrocarbons; do not invent a source if the question supplies none.

3. Detailed Explanations

Pollutant source–effect chains

Limited oxygen can cause incomplete combustion:

2C + O₂ → 2CO

Carbon monoxide is toxic because it reduces oxygen transport in blood. Sulfur dioxide and nitrogen dioxide can form acids in rainwater. Acid rain can irritate the respiratory system and react with carbonate stone in buildings, wearing it away.

Carbon cycle

Combustion and respiration transfer carbon to atmospheric carbon dioxide. Burning fossil fuels is an important human source. Photosynthesis transfers carbon dioxide into plant material:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Deforestation can increase atmospheric carbon dioxide even without immediate burning because fewer plants remove carbon dioxide by photosynthesis.

Required processes in the carbon cycleAn atmospheric carbon dioxide box is connected to plants, living organisms and fuels. Photosynthesis moves carbon dioxide from the atmosphere into plants. Respiration and combustion return carbon dioxide to the atmosphere.Carbon moves between the atmosphere and carbon storesAtmospheric carbon dioxideCO₂Plantscarbon stored in biomassLiving organismsrespiration releases CO₂Fuelscombustion releases CO₂photosynthesisremoves CO₂respirationadds CO₂combustionadds CO₂
Combustion and respiration add carbon dioxide to the atmosphere; photosynthesis removes it and stores carbon in plants.

Greenhouse gases and warming

Earth’s surface emits infrared radiation. Carbon dioxide and methane absorb and re-emit some of this infrared radiation. This simple model helps explain why increasing their concentration can reduce heat loss to space and raise average global temperature.

Greenhouse gases reduce infrared heat loss from EarthSunlight passes through the atmosphere and warms Earth. Earth emits infrared radiation upward. Greenhouse gases absorb some infrared radiation and re-emit it in different directions; some escapes to space and some returns toward the surface.Enhanced greenhouse effect: more IR is retainedSunAtmosphere: CO₂ and CH₄ absorb infraredGreenhouse gases re-emit IR in different directions.Earth's surfacesunlightIR from Earthsome IR escapessome IR returns
Follow the energy rather than memorising 'heat is trapped': short-wave radiation reaches Earth, the warmed surface emits infrared radiation, and greenhouse gases absorb and re-emit some of that outgoing infrared.

Potential effects include more extreme weather events and melting polar ice, which can contribute to sea-level rise and habitat disruption. Avoid claiming that every individual weather event is caused by global warming.

4. Common Mistakes

  • Including water vapour when the question specifies dry air.
  • Saying carbon dioxide is poisonous in the same way as carbon monoxide.
  • Naming carbon dioxide as the cause of acid rain.
  • Reversing photosynthesis and respiration in the carbon cycle.
  • Saying greenhouse gases simply “trap sunlight”. If you choose to explain the mechanism, refer to infrared radiation emitted by the warmed Earth.
  • Treating one weather event as proof of a long-term climate trend.
  • Forgetting argon when describing the remainder of dry air, or leaving ozone and unburned hydrocarbons out of a list of common pollutants.

5. Exam Tips

Use a causal chain

Write source or process → named gas → potential effect. For data questions, quote or compare the relevant values before explaining.

Exam question 1: Explain why incomplete combustion is harmfulCore

A fuel burns with insufficient oxygen. Name a pollutant formed and explain one health effect.

Show Answer

Carbon monoxide forms. It reduces the blood’s ability to carry oxygen, so less oxygen reaches body tissues.

6. Worked Examples

Modelled example 1

Trace a carbon-cycle change

Core

Problem

A forested area is cleared but its trees are not burned. Explain why atmospheric carbon dioxide may still rise.
Study the worked solution

  1. Identify the lost process

    Method

    State that fewer living plants remain to photosynthesise.

    Reason

    Photosynthesis removes carbon dioxide from the atmosphere and stores its carbon in plant material.

    Working

    Forest clearance → lower total photosynthesis.

  2. Infer the atmospheric change

    Method

    State that less carbon dioxide is removed, so its concentration may rise.

    Reason

    The carbon-cycle balance changes even without immediate carbon release by burning.

    Working

    Reduced CO₂ uptake → possible atmospheric CO₂ increase.

Guided practice 2

Build a complete pollutant chain

About 5 min

Problem

A power station burns a sulfur-containing fuel. Identify the pollutant and explain one environmental effect.

Match source, gas and effect

Pollutant
Main required effect

Hints

Hint 1: source clue
The fuel is described as sulfur-containing.
Hint 2: complete consequence
Name a prescribed effect on respiration or buildings.
View solution step by step

  1. Name the pollutant

    Method

    Identify sulfur dioxide.

    Reason

    Combustion oxidises sulfur present in the fuel.

    Working

    sulfur-containing fuel → SO₂

  2. Complete the effect chain

    Method

    Link sulfur dioxide to acid rain and one consequence.

    Reason

    Acid rain reacts with carbonate stone and wears away buildings; sulfur dioxide and acid aerosols can also irritate the respiratory system.

    Working

    SO₂ → acid rain → carbonate stone in buildings wears away.

Common misconception 3

Separate acid rain from warming

Find and correct the mistake

Learner claim

A learner says sulfur dioxide is a greenhouse gas that causes warming, while methane causes acid rain. Correct the gas–effect pairings and give one consequence of each.

Match each gas to its effect

Sulfur dioxide
Methane

View solution step by step

  1. Correct sulfur dioxide

    Method

    Link sulfur dioxide to acid rain.

    Reason

    It contributes to acid formation in rainwater.

    Working

    SO₂ → acid rain → damage to carbonate stone in buildings.

  2. Correct methane

    Method

    Link methane to greenhouse warming.

    Reason

    Methane is a greenhouse gas and may contribute to global warming.

    Working

    CH₄ → enhanced greenhouse effect → global warming.

Examiner practice 4

Connect greenhouse gases to sources and effects

4 marks

Examination question

State one source of carbon dioxide and one source of methane. Then describe two potential effects of increased levels of greenhouse gases. [4 marks]

Write four distinct marking points

View solution step by step

  1. Give a carbon dioxide source

    1 mark

    Method

    Name combustion of a carbon-containing fuel.

    Reason

    Combustion transfers carbon from fuel into atmospheric carbon dioxide.

    Working

    Carbon dioxide source: combustion of fossil fuels.

  2. Give a methane source

    1 mark

    Method

    Name cattle, decay, landfill or natural-gas leaks.

    Reason

    Each is a recognised source of atmospheric methane.

    Working

    Methane source: decay of waste in landfill.

  3. State two potential effects

    2 marks

    Method

    Name two separate consequences rather than repeating “global warming”.

    Reason

    The syllabus expects possible outcomes of increased greenhouse-gas levels.

    Working

    More extreme weather events and melting polar ice.

Challenge 5

Interpret an increase

Minimal support

Single-variable transfer

Atmospheric methane concentration rises while carbon dioxide concentration is unchanged. Explain why greenhouse warming may still increase.

Reason about methane independently of carbon dioxide

Methane is
Higher methane may cause

Hints

Hint 1: independent contribution
Carbon dioxide is not the only greenhouse gas named in this course.
Hint 2: consequence
Use methane concentration → greenhouse contribution → possible warming.
View solution step by step

  1. Treat methane as a greenhouse gas

    Method

    Identify methane as a greenhouse gas.

    Reason

    Its possible contribution to warming does not require carbon dioxide concentration to change.

    Working

    More CH₄ → a greater greenhouse-gas contribution.

  2. Infer the warming effect

    Method

    State that greenhouse warming may increase.

    Reason

    The total greenhouse effect depends on contributions from multiple gases.

    Working

    Higher methane can strengthen warming even with unchanged CO₂.

7. Mind Stretchers

Mind stretcher 1: Correlation is not mechanismExtension

Two graphs rise over the same period. Why is that not enough by itself to prove one variable caused the other?

Show Answer

A correlation shows association, not a causal mechanism. Other variables may affect both trends; additional evidence and a scientifically plausible mechanism are needed.

Mind stretcher 2: Compare two interventionsExtension

Why can reducing methane emissions affect warming even when carbon dioxide emissions do not change?

Show Answer

Both gases can contribute to global warming. Lower methane concentration reduces one greenhouse-gas contribution, although carbon dioxide still contributes.

Mind stretcher 3: Keep the answer in scopeExtension

A question asks how sulfur dioxide harms the environment. What is the complete course-level answer?

Show Answer

Name its source, then state that it contributes to acid rain, which can irritate the respiratory system and react with carbonate stone in buildings. Emission-control chemistry is not required in this course.

8. Quiz

Check 1: Check your understandingCore

Check that you can write complete source → pollutant → effect chains, explain the carbon cycle, and link carbon dioxide and methane to their sources and potential warming effects without using an ozone-depletion explanation.

Start a quick check

Try this next: cover the Key Ideas table and rebuild every source → pollutant → effect chain from memory.