Chemistry of the Atmosphere

• Proportions of different gases in the atmosphere

  • ~80% nitrogen
  • ~20% oxygen
  • Small proportions of various other gases, i.e. CO2, H2O(g) and noble gases
• For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today
• The Earth's early atmosphere

  • Evidence is limited because of the time scale of 4.6 billion years and theories have changed/developed over time
  • 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
  • At the start of this period, the atmosphere may have been like the atmospheres of Mars and Venus today: mainly CO2 with little or no O2(g)
  • Volcanoes also produced nitrogen (N2) which gradually built up in the atmosphere & there may have been small proportions of methane (CH4) and ammonia (NH3)
  • Water vapour condensed to form the oceans, and CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere
• This theory is the only theory of the early atmosphere you need to know
• How oxygen increased

  1. Algae & plants produced the O2 that is now in the atmosphere by photosynthesis

  2. Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere

  3. Over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolve
• How carbon dioxide decreased

  1. Algae and plants decreased the % CO2 in the atmosphere by photosynthesis

  2. CO2 was also decreased by the formation of sedimentary rocks that contain carbon (e.g. limestone and coal) and by the production of fossil fuels from the remains of dead plants and animals when they decayed
• Greenhouse gases


  Maintain temperatures on earth high enough to support life
• Greenhouse gases

  • Water vapour
  • CO2
  • CH4
• Greenhouse gas effect

  1. Electromagnetic radiation at most wavelengths from the sun passes through the Earth's atmosphere

  2. The Earth absorbs some radiation and warms up

  3. Some heat is radiated from the Earth as infrared radiation

  4. Some of this IR radiation is absorbed by greenhouse gases in the atmosphere

  5. Atmosphere warms up leading to the greenhouse effect and global warming
• Human activities increase levels of CO2 & CH4
• Human activities that increase greenhouse gases


  • Driving (CO2)
  • Consuming electricity (CO2)
  • Raising livestock (cows - CH4)
  • Decay of organic waste in landfill sites (CH4)
• 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, leading 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
• Global climate change

  • An increase in average global temperature is a major cause
• Potential effects of global climate change

  • Extinction of species
  • Raising sea levels due to the melting of polar ice caps
  • Migration - people will move from areas suffering drought/flooding
  • Decrease in crop yield for all major world crops
• Carbon footprint


  The total amount of CO2 and other greenhouse gases emitted over the full life cycle of a product, service or event
• Reducing carbon footprint

  Reducing emissions of carbon dioxide and methane e.g. by using less electricity (so using less fossil fuels or using an alternative source of electricity), by reducing the amount of travel or transportation (of either goods or people) and by planting more trees
• Action to reduce carbon footprint may be limited because it's more difficult, can be more expensive and planting trees takes away land that could be used to grow crops
• Molecules collide with one another and with other particles (e.g., walls)
• Gas molecules have kinetic energy due to their random motion
• The atmosphere is made up of gases, which are mixtures of different elements.
• Combustion of fuels

  Major source of atmospheric pollutants
• Composition of most fuels

  • Carbon
  • Hydrogen
  • Sulfur
• Combustion of fuels

  1. Releases carbon dioxide

  2. Releases water (vapour)

  3. Releases carbon monoxide

  4. Releases oxides of nitrogen
• Incomplete/partial combustion

  • Releases solid particles of soot (carbon)
  • Releases carbon monoxide
  • Releases unburnt fuel
• Combustion of hydrocarbons

  • Releases energy
  • Oxidises carbon and hydrogen to form carbon dioxide and water
• Carbon monoxide


  • Toxic gas
  • Colourless
  • Odourless
• Sulfur dioxide and oxides of nitrogen


  • Cause acid rain
  • Cause respiratory problems in humans
• Particulates


  • Cause global dimming
  • Cause human health problems
• When molecules collide they transfer energy between them
• Kinetic theory states that gas molecules are constantly moving randomly at different speeds
• Air contains nitrogen gas (N2), oxygen gas (O2), argon gas (Ar), and small amounts of other gases such as carbon dioxide (CO2).
• Kinetic theory states that gas particles are constantly moving randomly at different speeds
• Temperature is a measure of how much kinetic energy there is within a substance