Air Quality and ClimateCambridge IGCSE Chemistry: Revision notes
Section 1
What is the composition of clean, dry air?
Clean, dry air is a mixture of gases with a precise composition:
| Gas | Percentage |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Noble gases and carbon dioxide | ~1% |
This composition is relatively constant in the atmosphere. The main noble gases present are argon, neon, and helium. Carbon dioxide, despite being essential for photosynthesis and plant growth, makes up only about 0.04% of clean air. Understanding this baseline composition is crucial for identifying and quantifying air pollutants.
Examiners expect you to know the percentages of N₂ and O₂ precisely. If asked about air composition, always state 'approximately 78% nitrogen and 21% oxygen' to show understanding of the remaining 1%.
Section 2
Where do air pollutants come from?
Air pollutants are released into the atmosphere from various sources related to human activity and natural processes:
| Pollutant | Sources |
|---|---|
| Carbon dioxide (CO₂) | Complete combustion of all carbon-containing fuels (coal, oil, natural gas, petrol, wood) |
| Carbon monoxide (CO) | Incomplete combustion of carbon-containing fuels, mainly from car engines |
| Particulates | Incomplete combustion, industrial processes, dust, pollen |
| Methane (CH₄) | Decomposition of vegetation, animal digestion (especially in cattle), landfill sites |
| Oxides of nitrogen (NOₓ) | Car engines where high temperatures cause nitrogen and oxygen to react |
| Sulfur dioxide (SO₂) | Combustion of fossil fuels that contain sulfur compounds (coal and oil) |
Key distinction: Complete combustion of hydrocarbons produces CO₂ and H₂O only, whilst incomplete combustion produces CO and particulates because there is insufficient oxygen.
Students often confuse which combustion produces which pollutant. Remember: complete combustion produces CO₂ (not a local pollutant), whilst incomplete combustion produces CO (very toxic) and particulates.
Think of incomplete combustion like a candle with insufficient oxygen—it produces sooty, black smoke (particulates and CO). Complete combustion is a clean, bright flame (just CO₂ and H₂O).
Section 3
What are the adverse effects of air pollutants on human health and the environment?
Different air pollutants cause distinct environmental and health problems:
| Pollutant | Adverse Effects |
|---|---|
| Carbon dioxide (CO₂) | Increased global warming and climate change; traps thermal energy and prevents it escaping to space |
| Carbon monoxide (CO) | Toxic gas; binds to haemoglobin in blood, preventing oxygen transport; causes poisoning and death at high concentrations |
| Particulates | Respiratory problems including asthma and bronchitis; can carry toxic substances deep into lungs; increased risk of lung cancer |
| Methane (CH₄) | Increased global warming and climate change; greenhouse gas approximately 25 times more potent than CO₂ over 100 years |
| Oxides of nitrogen (NOₓ) | Form acid rain (causing damage to buildings, aquatic ecosystems, forests); contribute to photochemical smog; cause respiratory problems |
| Sulfur dioxide (SO₂) | Forms acid rain when dissolved in water; damages buildings, aquatic life, and vegetation |
Both CO₂ and methane are greenhouse gases because they absorb and re-emit thermal radiation, preventing heat loss from Earth's surface.
When answering questions about pollutant effects, link each pollutant to its specific consequence. Examiners award marks for precise connections—for example, CO binds to haemoglobin, not just 'it's toxic'.
If asked why CO is dangerous: CO competes with O₂ to bind to haemoglobin in red blood cells. Since CO binds more strongly than O₂, it reduces the oxygen-carrying capacity of blood, leading to oxygen starvation of tissues and organs.
Section 4
How do greenhouse gases cause global warming?
Greenhouse gases cause global warming through a specific mechanism involving thermal energy:
The greenhouse effect mechanism:
-
Absorption: Greenhouse gases (CO₂ and CH₄) in the atmosphere absorb thermal (infrared) radiation emitted from Earth's surface.
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Re-emission: These gases absorb the thermal energy and re-emit it in all directions, including back towards Earth's surface.
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Trapping: Without greenhouse gases, thermal energy would escape to space. However, greenhouse gases trap this energy in the lower atmosphere.
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Heat accumulation: More thermal energy is retained near Earth's surface, leading to increased average global temperature and climate change.
Why methane is particularly concerning: Although methane is produced in smaller quantities than CO₂, it is approximately 25 times more effective at trapping thermal energy over a 100-year period, making it a potent climate-forcing agent.
The carbon cycle connection: Increased atmospheric CO₂ from burning fossil fuels disrupts the natural carbon cycle, preventing the carbon from being removed from the atmosphere at the same rate it is added.
Examiners expect you to explain the mechanism, not just state that CO₂ causes warming. Use the phrase 'absorbs and re-emits thermal energy' or 'prevents thermal energy escaping to space' to show understanding of the physics.
A greenhouse works by the same principle: glass lets light in but traps thermal energy inside. Similarly, greenhouse gases let light through but trap heat in Earth's lower atmosphere.
Section 5
What strategies reduce air pollution and climate change?
Multiple strategies can reduce environmental damage from air pollutants and greenhouse gas emissions:
Strategies to reduce climate change (CO₂ and methane):
- Planting trees: Photosynthesis removes CO₂ from the atmosphere; trees act as a carbon sink
- Reducing livestock farming: Decreases methane emissions from animal digestion
- Decreasing fossil fuel use: Reduces CO₂ emissions from combustion
- Increasing hydrogen and renewable energy: Provides clean energy alternatives (solar, wind, hydroelectric) that produce no greenhouse gases during use
Strategies to reduce acid rain (SO₂ and NOₓ):
| Strategy | How it works |
|---|---|
| Catalytic converters | Convert harmful gases (CO and NOₓ) into less harmful products; use platinum/palladium catalysts |
| Low-sulfur fuels | Reduce the sulfur content in fossil fuels before combustion, so less SO₂ is produced |
| Flue gas desulfurisation (FGD) | Remove SO₂ from exhaust gases using calcium oxide (CaO); SO₂ reacts with CaO to form calcium sulfite or sulfate |
Example catalytic converter reaction: 2CO + 2NO → 2CO₂ + N₂ (at high temperature with platinum catalyst)
This reaction shows how the toxic gases CO and NO are converted into non-toxic products: CO₂ and N₂.
When writing about catalytic converters, always mention the catalyst material (platinum/palladium), the reactants (CO and NOₓ), and the products (CO₂ and N₂). Showing the balanced equation is excellent exam technique.
In flue gas desulfurisation, SO₂ is removed by reacting it with calcium oxide: SO₂ + CaO → CaSO₃. This prevents SO₂ from being released into the atmosphere where it would form acid rain.
Section 6
What is photosynthesis and how does it relate to air quality?
Photosynthesis is the process by which plants remove carbon dioxide from the atmosphere, directly improving air quality and providing oxygen.
Word equation: Carbon dioxide + Water → Glucose + Oxygen (in the presence of chlorophyll, using energy from light)
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Key requirements for photosynthesis:
- Chlorophyll: The green pigment in chloroplasts that absorbs light energy
- Light energy: From the sun, providing the energy to drive the reaction
- Carbon dioxide: Taken from the atmosphere through stomata
- Water: Taken from the soil through roots
Role in air quality:
- Removes CO₂ from the atmosphere, reducing greenhouse gas concentration
- Produces oxygen, improving air composition
- Acts as a carbon sink, storing carbon in plant biomass
- Why planting trees is an effective climate change strategy
Note: The oxygen produced in photosynthesis comes from the splitting of water molecules (H₂O), not from CO₂. This was demonstrated through isotope labelling experiments.
Always write out both the word equation and the symbol equation for photosynthesis when asked. The symbol equation shows that 6 moles of CO₂ react with 6 moles of H₂O to produce 1 mole of glucose and 6 moles of O₂.
Students sometimes think the oxygen in O₂ comes from CO₂. In fact, it comes from water (H₂O). The carbon from CO₂ ends up in the glucose molecule.
Must Know
- Clean air composition: Approximately 78% nitrogen, 21% oxygen, 1% noble gases and carbon dioxide
- Sources of pollutants: CO₂ from complete combustion; CO and particulates from incomplete combustion; methane from decomposition and digestion; NOₓ from car engines; SO₂ from burning sulfur-containing fossil fuels
- Greenhouse effect mechanism: CO₂ and CH₄ absorb thermal (infrared) radiation and re-emit it, trapping heat near Earth's surface and preventing thermal energy escaping to space, causing global warming
- Health and environmental effects: CO is toxic (binds to haemoglobin); particulates cause respiratory disease and cancer; NOₓ and SO₂ form acid rain; all greenhouse gases increase climate change risk
- Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (removes CO₂, acts as a carbon sink, and produces oxygen)
- Reduction strategies: Trees and renewable energy reduce climate change; catalytic converters and low-sulfur fuels reduce acid rain; the catalytic converter reaction is 2CO + 2NO → 2CO₂ + N₂
That's the notes covered.
Carry on to the next subtopic.