C9 - Chemistry of the Atmosphere

• For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today:

  • about four-fifths (approximately 80%) nitrogen

  • about one-fifth (approximately 20%) oxygen

  • small proportions of various other gases, including carbon dioxide, water vapour and noble gases.
• Theories about what was in the Earth’s early atmosphere and how the atmosphere was formed have changed and developed over time. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.
• One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans
• At the start of this volcanic period the Earth’s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.
• Volcanoes also produced nitrogen which gradually built up in the atmosphere and there may have been small proportions of methane and ammonia.
• When the oceans formed carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere.
• Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis.
• Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
• Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis.
• Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon.
• As the oceans formed, carbon dioxide dissolved to form soluble carbonate compounds so its amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, e.g. limestone.
• 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) (equation for photosynthesis)
• Precipitating is A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction.
• Coal is a fossil fuel which was formed from trees which were in dense forests in low-lying wetland areas.
• Crude oil and natural gas were formed from simple plants and tiny animals which were living in oceans and lakes. These small organisms died and their remains sank to the bottom where they were buried under sediments. 
• Over millions of years, heat and pressure turned the remains of the organisms into crude oil and natural gas. Natural gas contains the smallest molecules and is often found on top of crude oil, trapped under sedimentary rock.
• Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases.
• Greenhouse Effect:

  1. electromagnetic radiation at most wavelengths passes through the Earth's atmosphere
  2. the Earth absorbs most of the radiation and warms up
  3. the Earth radiates energy as infrared radiation
  4. some of the infrared radiation goes into space
  5. some of the infrared radiation is absorbed by greenhouse gases in the atmosphere
  6. the lower atmosphere warms up
• Greenhouse Gases Include:

  • Carbon Dioxide
  • Methane
  • Water Vapour
• Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth’s atmosphere to increase at the surface and that this will result in global climate change.
• it is difficult to model such complex systems as global climate change. This leads to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence and which may be biased.
• An increase in average global temperature is a major cause of climate change