By the end of this lesson the student will:

- use data from tables to draw graphs

- interpret the graphed data

- understand how CFCs damage the ozone layer.

High above our heads in a layer of the atmosphere known as the stratosphere is a small amount of ozone gas. This gas is made up of molecules each containing three atoms of oxygen; its chemical formula is 03. (The formula of oxygen gas is 02.) The 'ozone layer' absorbs harmful ultraviolet radiation from the sun.

Scientist measure ozone layer thickness by measuring how much ultraviolet radiation reaches the ground, using a Dobson ozone spectrophotometer. Ozone layer thickness is measured in Dobson units. The higher the number the thicker the ozone layer.

Ozone is produced and destroyed naturally. Since the 1970s, gases produced for commercial purposes have been destroying the ozone layer, upsetting the natural equilibrium that existed. It is planned that by 2005 in developed countries and 2015 in developing countries, the use of ozone depleting gases, such as CFCs, will be phased out.

CFCs (chlorofluorocarbons) are used in spray cans, in the plastics industry, as refrigerants and for cleaning electronic circuit boards. Halons, used as fire extinguishing chemicals, also destroy ozone.

See the following animated model to understand how ozone is destroyed by CFCs.

A damaged ozone layer means that more of the sun's harmful UV radiation is able to reach the earth's surface. This radiation can cause sunburn, skin cancer and eye diseases. Go to the following link for more information about UV radiation.

Ozone depletion occurs over much of the planet. The depletion over Antarctica (the Antarctic ozone hole) peaks in spring, when sunlight activates chemical reactions in which chlorine released from the CFCs rapidly destroys ozone molecules. These reactions happen on ice crystals in the very cold, high-altitude clouds above Antarctica.

Students will need access to the internet to view the ozone animation showing - How ozone is destroyed by CFCs. Print off the student's worksheet and photocopy one for each student.

Ask the students to carry out the activity from the worksheet.

Solutions

1. Students to draw graph. The ozone layer is getting thinner.
   
   
2. Students to draw graph. In recent years the layer is thinner particularly around spring time.
   
   
3. It occurs around the same time - during spring.
   
   
4. When ultraviolet light waves (UV) strike CFC* (CFCl3) molecules in the upper atmosphere, a carbon-chlorine bond breaks, producing a chlorine (Cl) atom. The chlorine atom then reacts with an ozone (O3) molecule breaking it apart and so destroying the ozone. This forms an ordinary oxygen molecule(O2) and a chlorine monoxide (ClO) molecule. Then a free oxygen atom breaks up the chlorine monoxide. The chlorine is free to repeat the process of destroying more ozone molecules. A single CFC molecule can destroy 100,000 ozone molecules.
   
   * CFC - chlorofluorocarbon: it contains chlorine, fluorine and carbon atoms.
5. Stop using chlorofluorocarbons and halons. Dispose of chlorofluorocarbons and halons correctly i.e. do not let them escape into the atmosphere.
   
   
6. It absorbs harmful ultraviolet radiation from the sun. This radiation can cause sunburn, skin cancer and eye disease.