The creation of ozone in the stratosphere

1. Ultra-violet light can cause oxygen molecules to dissociate O2 + UV light > O + O The creation of ozone in the stratosphere Oxygen radical Oxygen radical Click mouse or arrow keys to work through this slideshow >>>.

2. Ozone (O3) Ozone (O3) The creation of ozone Oxygen radical Oxygen radical The atmosphere has many oxygen molecules floating around. Ultraviolet radiation hits an oxygen molecule (O2). The energy of the collision splits the oxygen molecule into two radicals (O+). These float off freely until they find a convenient new oxygen molecule to join. When the oxygen radical meets a new oxygen molecule, they join to create a new compound - ozone (O3)

3. This is the reverse of the previous reaction.. The energy is supplied by ultraviolet light O3 + ultraviolet light --> O2 + O The frequency of ultra-violet light absorbed = frequency of biologically damaging ultra-violet radiation - an extremely useful coincidence for life on earth! The ozone breaks down again into oxygen and a free oxygen radical - which can then seek a recombination with another molecule. The destruction of ozone 1

4. 0xygen molecule O2 Oxygen radical The destruction of ozone 2 The balance between the creation and destruction rates provides a steady state - a dependable region of ozone in the stratosphere known as the ozone layer. Whilst this “layer” is present from perhaps 20 - 50 km present height it is a very tenuous and fragile layer. If it was condensed down to sea level pressure the ozone layer would only be a few centimetres thick.

5. rate of ozone formation = rate of ozone removal amount of ozone in stratosphere remains constant - ie “steady state” These processes absorb ultra-violet light and release energy in the form of heat so the stratosphere heats up Steady state

6. The production of chlorofluorocarbons (CFCs) in the 20th century added a new dimension to the ozone balance. CFC’s are also broken down by sunlight, releasing chlorine (green in diagram). Chlorine attacks ozone (O3) to combine with the free oxygen radical - to form chlorine monoxide. This is unstable and breaks down quickly, re-releasing the chlorine to break down further ozone molecules. In the diagrams (right) one free chlorine has already destroyed 2 ozones and can easily destroy thousands more. CFC O3 O3 Human impacts O3

7. The threat • The finely tuned ozone balance is tipped by CFCs releasing chlorine to attack ozone. • Natural atmospheric processes combine to create “holes” in the ozone layer near the poles at certain times of the year. • The size of these holes is increasing (they now reach down to the temperate regions). • Ozone absorbs UV radiation from the sun. The absence of ozone leads to increased UV levels. • UV light damages cells. Over exposure triggers skin cancer and can cause sight problems. • The food chain could be affected because plants do not like UV light. The blue and purple areas in the image show the “ozone hole”. This regularly covers Australia in the South and Iceland/ Norway in the North