Full chapter
Maintaining Air Quality
Connect atmospheric substances to their sources, effects and possible solutions.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
What is in air, and where do pollutants come from?
Separate the normal mixture from substances causing harm.
Dry air contains about 78% nitrogen and 21% oxygen by volume. Most of the remaining 1% is noble gases, mainly argon, with carbon dioxide and other trace gases. Dry air excludes water vapour; the water-vapour content of actual air varies.
| Substance | Source or formation | Main concern |
|---|---|---|
| Carbon monoxide, CO | Incomplete combustion of carbon-containing fuels when oxygen supply is insufficient | Toxic; reduces oxygen transport in blood |
| Nitrogen oxides, NO and NO2 | High-temperature reactions involving air during lightning and in internal combustion engines | Respiratory irritation and acid rain |
| Sulfur dioxide, SO2 | Volcanoes and burning fossil fuels containing sulfur compounds | Respiratory irritation and acid rain |
| Methane, CH4 | Livestock, decomposition without oxygen, and natural-gas leakage | Greenhouse gas |
| Ozone, O3 | Can form near ground level through sunlight-driven reactions involving other pollutants | Harmful to respiration at ground level |
| Unburned hydrocarbons | Fuel that escapes combustion, including engine exhaust and evaporation | Contribute to photochemical air pollution |
Distinguish CO from CO2. Both contain carbon and oxygen, but CO is a toxic incomplete-combustion product. Complete combustion produces CO2, which is important in the carbon cycle and greenhouse effect. A fuel containing no sulfur cannot directly supply sulfur dioxide simply by burning; identify which elements the source provides.
Check your understanding
Air is often quoted as 78% nitrogen and 21% oxygen. Do these figures apply directly to humid air?
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Not quite. They describe dry air; water vapour lowers the other percentages.
The figures are for dry air. In humid air, water vapour takes up part of the sample, so nitrogen and oxygen make up slightly smaller shares.
Explain harm through a chemical cause
Link the pollutant to the process and then the effect.
Make a guess. You are not marked.
Rain falls through air polluted with sulfur dioxide from burning fuels. How does its pH compare with rain in clean air?
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The pH is lower, as acids form in the rain
Sulfur dioxide dissolves in the rain and reacts to form acids, so the rain has more hydrogen ions and a lower pH.
Carbon monoxide binds strongly to haemoglobin and reduces the blood's ability to carry oxygen. It is not the same hazard as carbon dioxide's role in climate change. Because incomplete combustion can produce CO, a fuel burning does not guarantee that its exhaust contains only carbon dioxide and water.
Nitrogen dioxide and sulfur dioxide react in the atmosphere with water and oxygen to produce acidic substances that enter rain. Acid rain can damage plants and aquatic ecosystems and attack buildings containing carbonate stone. These gases also irritate the respiratory system; health effects are not confined to rainwater.
For a limestone surface, carbonate reacts with acid: CaCO3(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) + CO2(g). Solid material is gradually lost. This connects the visible damage to an acid-carbonate reaction rather than merely saying that pollution is bad.
Worked exampleUse source evidence
Two fuels release the same energy. One contains much more sulfur. What additional air-quality concern does it create?
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- Combustion can convert its sulfur compounds into sulfur dioxide.
- Sulfur dioxide contributes to acid rain and respiratory irritation.
- Equal useful energy does not imply equal pollutant emissions.
The higher-sulfur fuel can create greater SO2 pollution; compare sulfur content and any treatment of the exhaust.
Check your understanding
A student says carbon monoxide causes acid rain. Which correction is right?
Show the answer
Acid rain comes from sulfur dioxide and nitrogen oxides; CO is harmful as a toxic gas.
SO2 and NO2 react with water and oxygen to form acids. Carbon monoxide is dangerous because it binds to haemoglobin and reduces oxygen transport.
Exam-style question
Sulfur dioxide is released when fuels containing sulfur are burned. Explain how this can damage buildings. [2]
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Carbon cycling and greenhouse gases
Track the flows before judging whether they balance.
Photosynthesis takes carbon dioxide from air and incorporates its carbon into plant material. Respiration returns carbon dioxide as organisms release energy from organic compounds. Combustion of biomass and fossil fuels also returns carbon dioxide. These flows help regulate atmospheric carbon dioxide, but they do not guarantee a constant concentration if inputs exceed removals.
The central carbon-cycle flows
Photosynthesis transfers atmospheric carbon into living material; respiration and combustion return carbon dioxide to the air.
Carbon dioxide and methane are greenhouse gases. They absorb outgoing infrared radiation and contribute to warming of the atmosphere and surface. More greenhouse gas strengthens this effect. Fossil-fuel combustion and deforestation add CO2 or reduce its removal. Methane sources include livestock, landfill decomposition, wetlands and fossil-fuel leakage.
Potential consequences of increased greenhouse-gas levels include more extreme weather events and melting polar ice. Melting land ice contributes to rising sea level; floating sea ice and land ice should not be treated as identical sea-level mechanisms. Warming also changes rainfall patterns and stresses habitats.
For bioethanol, plant growth can take up carbon dioxide later released by burning the fuel. The comparison with fossil fuels depends on how the crop is grown, processed and transported and whether land-use changes release additional carbon. A carbon-flow explanation is stronger than the claim that renewable fuels produce no carbon dioxide.
Check your understanding
Why can atmospheric carbon dioxide rise even though plants photosynthesise?
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Burning fuels adds CO2 faster than plants and oceans remove it.
The natural cycle does not automatically cancel an extra human input. Burning fossil fuels adds carbon quickly, and clearing forests reduces how much is taken up.
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Issue | Key chemical connection |
|---|---|
| CO poisoning | Incomplete combustion; reduced oxygen transport |
| Acid rain | NO2/SO2 form acids; carbonate stone reacts |
| Greenhouse warming | CO2/CH4 affect outgoing infrared radiation |
Track carbon flows in amount as well as direction: atmospheric CO2 is steady only when additions and removals balance. Biofuel combustion emits CO2 even when plant growth offsets part of the release.
Scope
Learning outcomes
12. Maintaining Air Quality (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
12(a) Describe dry-air composition
- 78% nitrogen
- 21% oxygen
- Remainder noble gases, mainly argon, and CO2
12(b) Name atmospheric pollutants
- CO, CH4, NO, NO2, O3, SO2, unburned hydrocarbons
12(c) Identify pollutant sources
- CO from incomplete carbon-fuel combustion
- NOx from lightning and engines
- SO2 from volcanoes and fossil fuels
12(d) Explain pollutant effects
- CO toxicity
- NO2/SO2 acid rain
- Respiration and buildings
12(e) Explain the carbon cycle
- Photosynthesis
- Respiration
- Combustion
- Regulation of atmospheric CO2
12(f) Explain greenhouse-gas effects
- CO2 and CH4 sources
- Global warming
- Extreme weather and melting polar ice