Chemistry of the Atmosphere
The chemistry of the atmosphere covers how the Earth's atmosphere has changed since the planet formed about 4.6 billion years ago to reach today's composition of roughly 80% nitrogen and 20% oxygen, with small proportions of carbon dioxide, water vapour and other gases. It also covers the greenhouse effect, the evidence linking human activity to recent climate change, the possible consequences of that change, and the pollutant gases and particulates released when fuels are burned.
Method
- Learn today's approximate atmospheric composition: about 80% nitrogen, about 20% oxygen, with small, roughly fixed amounts of other gases including argon and carbon dioxide (about 0.04%), plus a variable amount of water vapour.
- Describe the early atmosphere: intense volcanic activity released mostly carbon dioxide, together with water vapour, nitrogen and small amounts of methane and ammonia, and there was little or no oxygen.
- Explain how carbon dioxide levels fell over billions of years: as the Earth cooled, water vapour condensed to form the oceans, into which carbon dioxide dissolved; carbon dioxide was also locked away in sedimentary carbonate rocks and, later, in fossil fuels, and was taken up by early photosynthesising algae and plants.
- Explain how oxygen levels rose: photosynthesising algae and plants removed carbon dioxide from the atmosphere and released oxygen as a by-product, gradually building up the oxygen level over billions of years until it reached a level that could support animal life.
- Describe the greenhouse effect: certain gases in the atmosphere, called greenhouse gases (including carbon dioxide, methane and water vapour), absorb infrared radiation that the Earth's surface re-radiates, and then re-emit that energy in all directions, including back towards the surface, which keeps the Earth significantly warmer than it would otherwise be.
- Link human activity to an enhanced greenhouse effect: burning fossil fuels for energy and transport releases extra carbon dioxide, while activities such as livestock farming, rice cultivation and landfill sites release extra methane, and most of the scientific community agrees this has driven recent increases in global temperature.
Worked example
Scientists believe the early Earth's atmosphere contained almost no oxygen, but today's atmosphere is about 20% oxygen. Explain how this change is thought to have happened.
- State the starting point: the early atmosphere, formed largely from volcanic gases, was mostly carbon dioxide with little or no oxygen present.
- Identify the process responsible for producing oxygen: photosynthesis, first carried out by early algae and, later, by plants.
- Explain what photosynthesis does to the atmosphere: it removes carbon dioxide from the atmosphere and releases oxygen gas as a product.
- State the timescale and outcome: this process continued for roughly two billion years, so oxygen built up gradually rather than suddenly, eventually reaching a concentration high enough to allow animal life to evolve.
- Final answer: oxygen accumulated in the atmosphere because photosynthesising organisms steadily converted atmospheric carbon dioxide into oxygen over an extremely long period of geological time.
Practice questions
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Q1State the approximate percentage of nitrogen in today's atmosphere.Show answer
Answer: About 80%.
Q2Name the two gases released in large amounts by volcanic activity that formed most of the Earth's early atmosphere.Show answer
Answer: Carbon dioxide and water vapour.
Q3Give two processes that removed carbon dioxide from the early atmosphere.Show answer
Answer: Carbon dioxide dissolving into the newly formed oceans, and carbon dioxide being taken up by photosynthesising algae and plants (it was also locked into carbonate rocks and, later, fossil fuels).
Q4Name three greenhouse gases.Show answer
Answer: Carbon dioxide, methane and water vapour.
Q5Explain, in terms of infrared radiation, how greenhouse gases keep the Earth warm.Show answer
Answer: The Earth's surface re-radiates infrared radiation; greenhouse gases in the atmosphere absorb this radiation and then re-emit it in all directions, including back down towards the surface, trapping additional energy that would otherwise escape into space.
Q6Give two human activities that increase the amount of greenhouse gases in the atmosphere.Show answer
Answer: Burning fossil fuels for energy and transport (increases carbon dioxide), and livestock farming, rice growing or landfill sites (increase methane).
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe the theory of how the Earth's atmosphere has changed since it formed about 4.6 billion years ago to reach today's composition of roughly 80% nitrogen and 20% oxygen.
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Carbon dioxide and methane are both greenhouse gases, but methane traps more energy per molecule than carbon dioxide. Explain why increasing atmospheric methane from farming is still a smaller overall contributor to the greenhouse effect than increasing atmospheric carbon dioxide from burning fossil fuels.
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