Air quality and climate
Composition of clean air, sources and effects of pollutants, catalytic converters and climate change.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 10.3.1State the composition of clean dry air and identify common atmospheric pollutants and their sources.
- 10.3.2Describe the adverse effects of pollutants and strategies for reducing them.
6 minute read
Air quality and climate
Clean, dry air is roughly 78% nitrogen, 21% oxygen, with the remaining ~1% mostly argon and a small, currently-rising amount of carbon dioxide. Everything in this topic is really a question about what happens when combustion adds extra substances to that mixture.
Where the main pollutants come from
- Carbon monoxide (CO) forms from the incomplete combustion of fuels, when there isn't enough oxygen to fully oxidise carbon to CO₂. It is toxic because it binds to haemoglobin in red blood cells more strongly than oxygen does, reducing the blood's oxygen-carrying capacity.
- Sulfur dioxide (SO₂) forms when fuels containing sulfur impurities (especially coal and some petrols) burn. It dissolves in atmospheric water to form acid rain, which damages plants, corrodes buildings, and acidifies lakes.
- Oxides of nitrogen (NOₓ) form inside car engines, where the high temperature of combustion is enough to make unreactive atmospheric nitrogen react directly with oxygen — this is a thermal reaction, not from impurities in the fuel. NOₓ also contributes to acid rain and to photochemical smog.
- Particulates (soot/carbon) form from incomplete combustion and contribute to respiratory problems and global dimming.
- Carbon dioxide (CO₂), from complete combustion of any fossil fuel, is not toxic but is the main greenhouse gas driving climate change.
Reducing the impact
Catalytic converters in car exhausts use a platinum/rhodium catalyst to convert CO and NOₓ into less harmful gases: 2CO + 2NO → 2CO₂ + N₂. This is a genuine chemical solution — it converts pollutants into other products rather than just filtering them out.
Think of it like this
Think of combustion pollutants as the "unwanted side dishes" of getting energy from a fuel: complete, oxygen-rich combustion serves up mostly CO₂ and water, while starved-of-oxygen or too-hot combustion serves up CO, soot and NOₓ as well.
Worked examples
Method, step by step
Explain, using a word or symbol equation, how a catalytic converter reduces air pollution from a car exhaust.
- 1Exhaust gases contain the pollutants carbon monoxide and nitrogen monoxide.
- 2On the catalyst surface, these react with each other: 2CO + 2NO → 2CO₂ + N₂.
- 3Both products are far less harmful than the pollutants they replace.
The catalytic converter reacts carbon monoxide with nitrogen monoxide (2CO + 2NO → 2CO₂ + N₂), converting two toxic pollutants into carbon dioxide and harmless nitrogen gas.
Common misconceptions
- Thinking oxides of nitrogen come from impurities in the fuel like sulfur dioxide does — they actually form from the air's own nitrogen and oxygen reacting due to the heat of the engine, regardless of fuel purity.
- Confusing carbon monoxide and carbon dioxide — CO is toxic and comes from incomplete combustion, CO₂ is non-toxic (to breathe, in normal concentrations) but is the main greenhouse gas.
- Thinking a catalytic converter filters pollutants out — it chemically converts them into less harmful gases via a reaction on the catalyst surface.
In the exam
- When explaining CO formation, always say "incomplete combustion due to insufficient oxygen" — a vague "burning fuel" answer will not get the mark.
- For NOₓ formation, the trigger word examiners look for is "high temperature" inside the engine, not fuel composition.