Air Quality, Pollutants and Climate Change
Air quality for IGCSE Chemistry 0620: clean air composition, CO, CO2, CH4, SO2 and NOx sources and effects, the greenhouse effect and catalytic converters.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
This is the longest subtopic in Chemistry of the Environment and a regular home for the 6-mark extended response: name pollutants, give their sources, state their effects, then evaluate a strategy. The marking is table-shaped (each pollutant needs its source and its effect, correctly paired), and answers that scramble the pairs (“sulfur dioxide from car exhausts causes global warming”) collapse fast.
Clean air: the baseline numbers
Clean, dry air is approximately 78% nitrogen and 21% oxygen, with the remainder mostly argon plus about 0.04% carbon dioxide. The 78 and 21 are direct recall marks. Anything else in the air (CO, SO2, NOx, particulates, extra CO2 and methane) counts as a pollutant when it harms health or the environment.
The pollutant table
| Pollutant | Source | Effect |
|---|---|---|
| Carbon monoxide (CO) | Incomplete combustion of carbon-containing fuels | Toxic gas: binds to haemoglobin, preventing oxygen transport |
| Carbon dioxide (CO2) | Complete combustion of fossil fuels | Greenhouse gas: higher levels cause climate change |
| Methane (CH4) | Livestock (cattle) digestion; decomposition of vegetation/waste | Greenhouse gas |
| Sulfur dioxide (SO2) | Combustion of fossil fuels containing sulfur compounds | Acid rain |
| Oxides of nitrogen (NOx) | Car engines (N2 and O2 react at high temperature) | Acid rain; photochemical smog; respiratory problems |
Two pairings need care. Carbon monoxide comes from incomplete combustion (too little oxygen), and the word incomplete is part of the mark. Oxides of nitrogen do not come from the fuel: the high temperature inside an engine makes atmospheric nitrogen and oxygen react, which is exactly why unreactive nitrogen becomes a problem only there.
Acid rain gets its own consequence list: corrodes limestone buildings and metal structures, damages trees, acidifies lakes and harms aquatic life. Reducing it means using low-sulfur fuels or removing sulfur dioxide from power station flue gases with calcium oxide or calcium carbonate (flue gas desulfurisation: a basic oxide neutralising an acidic one).
The greenhouse effect, phrased the 0620 way
The mark scheme rewards a specific sequence, not a vibe about “trapping heat in the ozone layer” (ozone is a different issue entirely):
- The Earth’s surface radiates thermal energy (infrared).
- Greenhouse gases (carbon dioxide and methane) absorb this energy and re-emit it in all directions.
- Less thermal energy escapes to space, so the atmosphere warms.
- Rising greenhouse gas concentrations increase the effect, causing global warming and climate change.
Consequences worth citing: melting ice caps and rising sea levels, flooding, more extreme weather, and damage to habitats and crops.
Cleaning up: converters and strategies
Catalytic converters sit in vehicle exhausts. Over a hot platinum/rhodium catalyst, the toxic gases convert to less harmful ones:
2CO + 2NO → 2CO2 + N2
Carbon monoxide is oxidised to carbon dioxide; nitrogen monoxide is reduced to nitrogen, a tidy redox example, kin to those in chemical reactions. The evaluation point: converters remove CO and NOx but still emit CO2, so they do nothing for climate change.
Climate change strategies the syllabus expects: greater use of renewable energy (solar, wind, hydroelectric) in place of fossil fuels, improved fuel efficiency and electric vehicles, planting trees (photosynthesis removes CO2), reducing livestock farming and food waste (methane), and carbon capture. The fuel side of this story (why we burn fossil fuels at all) sits in organic chemistry’s fuels section.
Worked exam question
Car engines release carbon monoxide and oxides of nitrogen. (a) Explain how each pollutant forms. [2] (b) State one harmful effect of each. [2] (c) Write the equation for the reaction in a catalytic converter that removes both gases. [2]
Model answer: (a) Carbon monoxide forms by incomplete combustion of the fuel, where oxygen supply is limited (1); oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperature inside the engine (1). (b) CO is toxic: it binds to haemoglobin and prevents oxygen being carried (1); NOx causes acid rain / photochemical smog / breathing problems (1). (c) 2CO + 2NO → 2CO2 + N2: species correct (1), balanced (1).
Mark-by-mark: (a) “from the exhaust” is a location, not a formation mechanism. Incomplete combustion and high-temperature N2/O2 reaction are the scoring phrases. (b) “bad for you” fails; the haemoglobin detail or a named environmental effect scores. (c) unbalanced or with NO2 in place of NO, the equation loses at least one mark.
The mistakes that cost marks
- Scrambled source-effect pairs. SO2 → acid rain; CO2/CH4 → climate change; CO → toxic. The 6-marker is lost fastest by cross-wiring these.
- “Carbon monoxide is a greenhouse gas.” Its credited harm is toxicity via haemoglobin. The greenhouse gases on the syllabus are carbon dioxide and methane.
- NOx “comes from the petrol.” It forms from atmospheric nitrogen and oxygen at engine temperatures. Sulfur dioxide is the one that comes from impurities in the fuel.
- Greenhouse effect mixed up with the ozone layer. Ozone depletion is a separate phenomenon. The 0620 greenhouse answer is absorb and re-emit thermal energy, less escapes to space.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”Cars make poisonous smoke" | "Incomplete combustion produces carbon monoxide, which is toxic" |
| "CO2 traps heat in the ozone" | "Carbon dioxide absorbs and re-emits thermal energy radiated from the Earth, reducing energy lost to space" |
| "Acid rain is caused by pollution" | "Sulfur dioxide from burning sulfur-containing fuels dissolves in rainwater, forming acid rain" |
| "Catalytic converters clean the exhaust" | "The catalyst converts carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen: 2CO + 2NO → 2CO2 + N2” |
Build the pollutant table from memory weekly until the pairs are automatic. It is the highest-leverage half-page in the topic. For a structured plan covering the whole syllabus at this level of precision, start with a free trial lesson and we will map your gaps in the first hour.
Test yourself
Do these three from memory. The answers are hidden until you click.
Q1 (2 marks). State the approximate percentages of the two main gases in clean, dry air.
Show answer
• 78% nitrogen [1] • 21% oxygen [1]
Q2 (3 marks). Sulfur dioxide is an air pollutant. State its source, its harmful effect, and one method of reducing the amount released by power stations.
Show answer
• source: combustion of fossil fuels containing sulfur compounds [1] • effect: acid rain [1] • reduction: flue gas desulfurisation, where sulfur dioxide reacts with calcium oxide or calcium carbonate (or: use low-sulfur fuels) [1]
Q3 (3 marks). Describe how carbon dioxide and methane cause the Earth’s atmosphere to warm.
Show answer
• the Earth’s surface radiates thermal energy (infrared) [1] • greenhouse gases absorb this energy and re-emit it (in all directions) [1] • less thermal energy escapes to space, so the temperature of the atmosphere rises [1]
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Frequently asked questions
What is the composition of clean, dry air?
Approximately 78% nitrogen and 21% oxygen. The remaining 1% is mostly argon with about 0.04% carbon dioxide. The 78/21 figures are a recurring one-mark recall question.
Which pollutants do I need, with sources and effects?
Carbon monoxide (incomplete combustion, toxic), carbon dioxide (combustion, greenhouse gas), methane (livestock and decay, greenhouse gas), sulfur dioxide (burning sulfur-containing fuels, acid rain), and oxides of nitrogen (car engines, acid rain and photochemical smog).
How does a catalytic converter work?
A hot platinum/rhodium catalyst converts carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen: 2CO + 2NO → 2CO2 + N2. It removes the toxic gases but still releases CO2.
How should I describe the greenhouse effect in 0620 language?
Greenhouse gases such as CO2 and methane absorb, then re-emit, thermal (infrared) energy radiated from the Earth's surface, reducing the energy escaping to space. Increased concentrations trap more energy, raising global temperatures.