Chemistry of the atmosphere — section overview
Section C9 covers the history of Earth's atmosphere, current composition, air pollution and the causes and effects of climate change.
The early atmosphere
The early Earth (~4 billion years ago) had an atmosphere dominated by volcanic gases:
- CO₂ (major component)
- H₂O vapour (condensed to form oceans)
- NH₃ and CH₄ (smaller amounts)
- Little or no oxygen
Changes over time:
- Oceans formed as water vapour condensed → CO₂ dissolved in oceans → CO₂ decreased
- Photosynthesis by algae → O₂ increased → CO₂ decreased further
- Organisms evolved to use O₂ in aerobic respiration
Current atmospheric composition
| Gas | Approximate % |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Argon (Ar) | ~0.9% |
| Carbon dioxide (CO₂) | ~0.04% |
| Other | traces |
Greenhouse gases and climate change
Greenhouse gases: CO₂, CH₄ (methane), H₂O vapour, N₂O
Greenhouse effect: greenhouse gases absorb infrared (IR) radiation emitted by Earth's surface, re-radiating some back → warming the Earth.
Enhanced greenhouse effect: human activities (burning fossil fuels, deforestation, farming) increase greenhouse gas concentrations → additional warming → global climate change.
Effects of climate change:
- Rising sea levels (ice melting, thermal expansion of oceans)
- More extreme weather events
- Species extinctions and habitat loss
- Changes in agriculture and crop yields
Air pollution
Carbon monoxide (CO): toxic; produced by incomplete combustion; binds to haemoglobin.
Particulates (soot): cause respiratory problems; absorb sunlight → local cooling.
Sulfur dioxide (SO₂): from burning fossil fuels containing sulfur → dissolves in rainwater → acid rain → damages ecosystems and buildings.
Nitrogen oxides (NOₓ): from high-temperature combustion (car engines); react with O₂ and H₂O → nitric acid → acid rain.
Common exam mistakes in C9
- Greenhouse effect is essential for life — without it Earth would be ~−18°C; the enhanced greenhouse effect from human activities is the problem
- CO₂ directly causes the greenhouse effect — it absorbs and re-emits IR radiation (not UV)
- Ozone depletion ≠ climate change — ozone depletion (caused by CFCs) and climate change are different problems
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