Lecture 5

• Current value of stratospheric ozone concentration - dissapearing over Antarctica in southern hemisphere
• Atmosphere: the thin layer of gases that surrounds Earth. Composed of several sub-layers that differ in density, gas composition and temperature
• Nitrogen:

  Added to atmosphere by:

  • Decay and burning of organic matter
  • Volcanic eruptions
  • Chemical breakdown of rocks
  • Fossil fuel burning

  Removed from atmosphere by:

  • Biological processes (nitrogen fixation)
  • Being washed away (rain or snow)
• Oxygen:

  • produced by vegetation through photosynthesis
  • removed by organic and inorganic processes e.g. breathing
• Water Vapour:

  • amount present determines humidity
  • source of all clouds, precipitation
• Carbon Dioxide:

  • absorbs and re-emits IR radiation (outgoing/longwave)
  • "Greenhouse Effect": keeping the lower atmosphere and Earth's surface warm
• Ozone:

  • excellent absorber of UV (incoming)
  • Naturally present in our atmosphere
• Atmosphere Structure:

  • we live in the Troposphere - starts at the surface of the Earth and can extend up to 20km
  • next layer is the Stratosphere - between 20km and 50km above the Earth's surface
• Temperature: heated by huge amounts of Sun's incoming high energy solar radiation by atoms of oxygen and nitrogen. Extremely low density of molecules
• The stratosphere is compromised almost entirely of nitrogen and oxygen. It contains relatively high concentrations of ozone - unlike the troposphere
• How do you scientists measure ozone: measured in Dobson units (DU)

  • determined by measuring the concentration of ozone molecules in a column of air that extends from the Earth's surface to the top of the atmosphere
  • areas with values less than 220 Dobson Units - experienced severe ozone destruction
• How is Ozone created:

  • formed when high energy ultraviolet radiation from the sun breaks apart molecular oxygen
  • an oxygen atom combines with an oxygen molecule forming O3 - Ozone.
• Role of Ozone in the Atmosphere: When the ozone layer is intact it absorbs...

  • 50% of UV-A radiation
  • 90% of UV-B radiation
  • All of UV-C radiation
• Why is the ozone disappearing? - CFCs

  • occurs when the normally stable CFCs hit the lower temperatures and higher amounts of UV radiation in the stratosphere, could break them down
  • once broken down, they could attack and destroy ozone molecules
• 3 conditions needed for CFCs to efficiently destroy ozone molecules:

  1. UV radiation
  2. Extremely low temperatures
  3. A surface on which the ozone-destruction process can occur
• Montreal Protocol on Substances that Deplete the Ozone Layer (1987):

  • an action plan developed to phase out CFCs - administered by the U.N.
  • by 2009, every country had signed the agreement
  • individual governments developed their own policies for reduction of CFCs
• Canada and CFCs:

  • Canada banned CFCs in most aerosol products in the late 1970s
  • 1980 - Canada passed its first regulation on CFCs
  • 2001 - accelerated phase-out plan was implemented
• The Kigali Amendment (2016):

  • when CFCs were phased out by the Montreal Protocol, HFCs were manufactured to replace them
  • HFCs do not have the same ozone-depleting effects of CFCs - but can lead to global warming