Chemistry of the Atmosphere
Earth's atmosphere, greenhouse gases, climate change, and pollution.
In this topic
Earth's Atmosphere
The atmosphere today consists of approximately: - 78% nitrogen (N₂) - 21% oxygen (O₂) - ~1% argon (Ar) - 0.04% carbon dioxide (CO₂) - Variable amounts of water vapour
How the atmosphere evolved: Phase 1 (first billion years): Volcanic activity released CO₂, water vapour, and nitrogen. Very little oxygen. Phase 2: Water vapour condensed to form oceans. CO₂ dissolved in oceans and was locked up in sedimentary rocks and fossil fuels. Phase 3: Photosynthesising organisms (first algae, then plants) produced oxygen. O₂ levels gradually increased, CO₂ levels decreased.
Key Points
- The early atmosphere was mostly CO₂ (like Mars and Venus today)
- Photosynthesis was key to producing the oxygen we have today
- Carbon is locked in fossil fuels, limestone, and ocean sediments
- Greenhouse gases: CO₂, methane, water vapour
Example Questions
3Explain how the level of oxygen in the atmosphere increased over billions of years.
Early photosynthesising organisms (algae and later plants) used carbon dioxide and released oxygen as a by-product. Over billions of years, the oxygen level gradually built up. Some oxygen was also used up by organisms in respiration and in oxidation reactions, but photosynthesis produced more than was consumed.
[3 marks]
3Describe three human activities that increase the concentration of greenhouse gases in the atmosphere.
1) Burning fossil fuels (for transport, energy, industry) releases CO₂. 2) Deforestation reduces photosynthesis, so less CO₂ is removed. 3) Agriculture (cattle farming and rice paddies) produces methane.
[3 marks]