Clean air is almost all nitrogen and oxygen. Burning fuels, running engines and farming add carbon monoxide, particulates, oxides of nitrogen and sulfur dioxide, each with its own source, its own harm and its own fix.
Clean, dry air is about 78% nitrogen, 21% oxygen and the remaining 1% is a mixture of noble gases (mainly argon) and carbon dioxide. Pollutants each have a source and an effect: carbon monoxide (incomplete combustion) is toxic; particulates (incomplete combustion) cause respiratory problems and cancer; oxides of nitrogen (car engines) cause acid rain, photochemical smog and respiratory problems; sulfur dioxide (burning fuels containing sulfur) causes acid rain; carbon dioxide and methane cause global warming. Acid rain is reduced by catalytic converters in vehicles, low-sulfur fuels and flue gas desulfurisation with calcium oxide. Extended: oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperature inside an engine, and a catalytic converter removes them: 2NO + 2CO → N2 + 2CO2.
Learn each pollutant as a pair: where it comes from, and what harm it does.
| Pollutant | Source | Adverse effect |
|---|---|---|
| carbon dioxide, CO2 | complete combustion of carbon-containing fuels | higher levels increase global warming, leading to climate change |
| carbon monoxide, CO | incomplete combustion of carbon-containing fuels | toxic gas |
| particulates (soot, C) | incomplete combustion of carbon-containing fuels | increased risk of respiratory problems and cancer |
| methane, CH4 | decomposing vegetation and waste gases from digestion in animals | higher levels increase global warming, leading to climate change |
| oxides of nitrogen, NO and NO2 | car engines | acid rain, photochemical smog, respiratory problems |
| sulfur dioxide, SO2 | combustion of fossil fuels that contain sulfur compounds | acid rain |
Carbon monoxide is toxic because it combines with haemoglobin in the blood, which then cannot carry oxygen. It has no colour or smell, so it is hard to detect without an alarm.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the approximate percentage of nitrogen in clean, dry air?
Which pollutant is toxic and is formed by incomplete combustion of fuels?
Which gas is the main cause of acid rain from burning fossil fuels that contain sulfur?
What is the main source of oxides of nitrogen in the air of a city?
Run the engine in the simulator and raise the temperature. Watch nitrogen and oxygen from the air combine, then follow the oxides through the converter.
Dissolve sulfur dioxide and nitrogen dioxide in rain water in the simulator and watch the pH fall. Then add limestone to the lake and to the chimney.
| Strategy | How it works | Pollutant reduced |
|---|---|---|
| catalytic converters in vehicles | convert oxides of nitrogen to nitrogen | NO, NO2 (and CO) |
| low-sulfur fuels | sulfur compounds are removed from the fuel before it is burned | SO2 |
| flue gas desulfurisation | calcium oxide in power station chimneys reacts with sulfur dioxide: CaO + SO2 → CaSO3 | SO2 |
Acid rain damages buildings and statues made of limestone, corrodes metals, makes lakes too acidic for fish and damages trees. Sulfur dioxide is an acidic oxide, so a basic oxide such as calcium oxide neutralises it.
A catalytic converter does not remove carbon dioxide: it makes more of it, from carbon monoxide. Carbon dioxide causes global warming, not acid rain; acid rain comes from sulfur dioxide and oxides of nitrogen. And the nitrogen in NOx comes from the air, not from the fuel.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) Which reaction takes place in a catalytic converter?
Which pollutant increases the risk of respiratory problems and cancer?
Which is the best method of reducing sulfur dioxide emissions from a coal-fired power station?
(Extended) Why are oxides of nitrogen formed in a car engine, even though petrol contains no nitrogen?
This skill is now recorded on your profile. Keep the chain going.