10.3 Air quality and climate
- Syllabus
- 0620–2026–2027
- Topic
- 10.3
- Level
- —
Learning objectives
10.3.1Composition of clean, dry air as• State the composition of clean, dry air as approximately 78% nitrogen, N2, 21% oxygen, O2 and the remainder as a mixture of noble gases and carbon dioxide, CO210.3.2Air pollutant sources: CO2 from• State air pollutant sources: CO2 from complete combustion; CO/particulates from incomplete combustion; methane from decomposing vegetation and animal digestion; nitrogen oxides from car engines; sulfur dioxide from sulfur-containing fossil fuels10.3.3Pollutant effects: CO2 and methane• State pollutant effects: CO2 and methane increase global warming/climate change; CO is toxic; particulates increase respiratory/cancer risk; nitrogen oxides cause acid rain, photochemical smog and respiratory problems; sulfur dioxide causes acid rain10.3.7Greenhouse gases carbon dioxide and• Describe how the greenhouse gases carbon dioxide and methane cause global warming, (a) the absorption, reflection and emission of thermal energy (b) reducing thermal energy loss to space10.3.4Reduction strategies: for climate• Explain reduction strategies: for climate change, plant trees, reduce livestock, use less fossil fuel, increase hydrogen/renewables; for acid rain, use catalytic converters, low-sulfur fuels and flue gas desulfurisation with calcium oxide10.3.5Photosynthesis as the reaction between• Describe photosynthesis as the reaction between carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and using energy from light10.3.6Word equation for photosynthesis• State the word equation for photosynthesis, carbon dioxide + water → glucose + oxygen10.3.8How oxides of nitrogen form in car• Explain how oxides of nitrogen form in car engines and describe their removal by catalytic converters, e.g. 2CO + 2NO → 2CO2 + N210.3.9Symbol equation for photosynthesis• State the symbol equation for photosynthesis, 6CO2 + 6H2O → C6H12O6 + 6O2
State the composition of clean, dry air
| Gas | Formula | Approximate percentage by volume |
|---|---|---|
| nitrogen | N₂ | 78% |
| oxygen | O₂ | 21% |
| noble gases and carbon dioxide | noble gases; CO₂ | the remaining about 1% |
Clean, dry air is a mixture, not a compound. 'Dry' means water vapour has been excluded, and 'clean' means pollutant amounts are not being included in this standard composition.
The two main percentages total about 99%, so noble gases and carbon dioxide together make up the small remainder.
Do not assign the whole remaining 1% to carbon dioxide: most of that remainder is noble gases. The figures are approximate, not exact integers for every sample.
Match each air pollutant to its source
| Pollutant | Required source |
|---|---|
| carbon dioxide, CO₂ | complete combustion of carbon-containing fuels |
| carbon monoxide, CO | incomplete combustion of carbon-containing fuels |
| particulates | incomplete combustion of carbon-containing fuels |
| methane, CH₄ | decomposition of vegetation and waste gases from digestion in animals |
| oxides of nitrogen, NOₓ | car engines |
| sulfur dioxide, SO₂ | combustion of fossil fuels containing sulfur compounds |
Complete combustion has enough oxygen and forms carbon dioxide. Incomplete combustion has insufficient oxygen and can form carbon monoxide and solid particulates.
For a source question, name both the pollutant and the precise process or material that produces it—for example, sulfur dioxide from burning a sulfur-containing fossil fuel.
Nitrogen oxides form from nitrogen and oxygen in the hot engine; they are not described as coming from nitrogen compounds in petrol.
Match air pollutants to their adverse effects
| Pollutant | Required adverse effect |
|---|---|
| carbon dioxide | increased global warming, leading to climate change |
| methane | increased global warming, leading to climate change |
| carbon monoxide | toxic gas |
| particulates | increased risk of respiratory problems and cancer |
| oxides of nitrogen | acid rain, photochemical smog and respiratory problems |
| sulfur dioxide | acid rain |
Keep each cause-and-effect pair exact. If a question asks for one effect, state a listed effect for that named pollutant rather than giving a general statement such as 'damages the environment'.
Carbon monoxide harms people because it is toxic. Carbon dioxide and methane are linked here to global warming and climate change, not acute poisoning.
Sulfur dioxide causes acid rain; oxides of nitrogen have the wider listed set of acid rain, photochemical smog and respiratory effects.
Explain how greenhouse gases cause global warming
Carbon dioxide and methane are greenhouse gases because they reduce the loss of thermal energy from Earth to space.
| Stage | Thermal-energy change |
|---|---|
| 1 | Energy reaches Earth; the surface absorbs some and reflects some. |
| 2 | The warmed surface emits thermal energy towards the atmosphere and space. |
| 3 | Carbon dioxide and methane absorb some of this emitted thermal energy. |
| 4 | The gases emit thermal energy in different directions, including back towards Earth. |
| 5 | Less thermal energy escapes to space, so the average global temperature rises. |
A complete explanation links absorption and emission by greenhouse gases to reduced thermal-energy loss, then links that reduced loss to global warming.
Do not say greenhouse gases create energy or simply make more sunlight enter. Their required role is to absorb and re-emit thermal energy, reducing its loss to space.
Explain strategies that reduce climate change and acid rain
| Environmental issue | Strategy | Why it helps |
|---|---|---|
| climate change | plant trees | photosynthesis removes carbon dioxide from air |
| climate change | reduce livestock farming | lowers methane emissions from animal digestion |
| climate change | decrease fossil-fuel use | lowers carbon dioxide emissions from combustion |
| climate change | increase hydrogen and renewable energy such as wind and solar | replaces some fossil-fuel combustion |
| acid rain | use catalytic converters in vehicles | lowers emissions of oxides of nitrogen |
| acid rain | use low-sulfur fuels | lowers sulfur dioxide formation |
| acid rain | flue-gas desulfurisation using calcium oxide | neutralises/removes acidic sulfur dioxide |
Explain a strategy by naming the pollutant it reduces and the source or removal process it changes.
The two issues need different pollutant links: carbon dioxide and methane for climate change; oxides of nitrogen and sulfur dioxide for acid rain.
Calcium oxide is used in flue-gas desulfurisation to remove sulfur dioxide; it is not added to the fuel to prevent combustion.
Describe the reaction of photosynthesis
Photosynthesis is the reaction in which carbon dioxide and water produce glucose and oxygen, in the presence of chlorophyll and using energy from light.
| Role | Required item |
|---|---|
| reactants | carbon dioxide and water |
| products | glucose and oxygen |
| condition | chlorophyll present |
| energy source | light |
A complete description includes both reactants, both products, chlorophyll and light energy. It also explains why photosynthesis removes carbon dioxide from the atmosphere.
Chlorophyll and light are required for the reaction but are not written as reactants or products. Respiration runs in the opposite overall direction and must not be substituted.
Write the word equation for photosynthesis
carbon dioxide + water → glucose + oxygen
Write the reactants on the left and the products on the right. The arrow means 'produces'; it does not mean the reaction is being written backwards as respiration.
Chlorophyll and light energy may be written above or near the arrow as conditions, but they are not joined to either side with a plus sign.
A list such as 'carbon dioxide, water, glucose, oxygen' is not a word equation: the plus signs and correctly directed arrow are essential.
Explain nitrogen oxides in car engines and catalytic converters
At the high temperatures in a car engine, nitrogen and oxygen from the air gain enough energy to react and form oxides of nitrogen such as nitrogen monoxide, NO.
| Substance entering converter | Change | Less harmful product |
|---|---|---|
| nitrogen monoxide, NO | reduced / loses oxygen | nitrogen, N₂ |
| carbon monoxide, CO | oxidised / gains oxygen | carbon dioxide, CO₂ |
A representative catalytic-converter equation is: 2CO + 2NO → 2CO₂ + N₂.
The two pollutants react together on the catalyst surface: carbon monoxide removes oxygen from nitrogen monoxide, so both are converted into less harmful products.
Nitrogen oxides form in the hot engine, not in the catalytic converter. The converter removes them after combustion; it does not stop nitrogen and oxygen entering the engine.
Write the symbol equation for photosynthesis
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The coefficients balance 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side.
Read the equation as six carbon dioxide molecules reacting with six water molecules to form one glucose molecule and six oxygen molecules.
Do not change the subscripts inside CO₂, H₂O, C₆H₁₂O₆ or O₂ when balancing. Change only the coefficients placed in front of whole formulas.