The atmosphere and pollution
Composition today
The current atmosphere is roughly 78 % nitrogen, 21 % oxygen, 0.9 % argon, 0.04 % carbon dioxide, plus traces of water vapour and other noble gases.
Evolution over 4.6 billion years
- Volcanic outgassing released mostly CO₂, water vapour and small amounts of nitrogen, methane and ammonia.
- Water vapour condensed as Earth cooled, forming the oceans. CO₂ dissolved into the oceans and was later locked into carbonate rocks (limestone) and fossil fuels.
- Photosynthesising bacteria and plants removed CO₂ and released O₂. Oxygen levels rose to today's 21 % over the last ~2 billion years.
Greenhouse effect
Visible light from the Sun passes through the atmosphere and warms Earth's surface. Earth re-emits that energy as infrared. Greenhouse gases (CO₂, CH₄, H₂O vapour) absorb the infrared and re-emit it in all directions, including back towards the surface. This keeps Earth ~33 °C warmer than it would otherwise be — but adding extra greenhouse gases by burning fossil fuels and rearing livestock enhances the effect, raising global temperatures.
Other air pollutants
| Pollutant | Source | Problem |
|---|---|---|
| CO | Incomplete combustion | Toxic — binds to haemoglobin, prevents O₂ transport |
| SO₂ | Burning sulfur-rich fuels | Acid rain (H₂SO₃ / H₂SO₄), respiratory irritant |
| NOₓ | High-temperature combustion (engines) | Acid rain, photochemical smog |
| Particulates | Diesel, wood smoke | Respiratory disease, global dimming |
CCEA tip
For "explain how a greenhouse gas warms the atmosphere", you must mention BOTH "absorbs the infrared (re-emitted from Earth)" AND "re-emits it in all directions / back to the surface". One half scores B1.
AI-generated · claude-opus-4-7 · v3-ccea-combined-science-leaves