The creation of ozone in the stratosphere

1
The creation of ozone in the stratosphere

• Ultra-violet light can cause oxygen molecules to
  dissociate
• O2 UV light gt O O

Oxygen radical
Oxygen radical
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2
The creation of ozone
When the oxygen radical meets a new oxygen
molecule, they join to create a new compound -
ozone (O3)
The atmosphere has many oxygen molecules floating
around. Ultraviolet radiation hits an oxygen
molecule (O2).
3
The destruction of ozone 1

• This is the reverse of the previous reaction..
• The energy is supplied by ultraviolet light
• O3 ultraviolet light --gt 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.

4
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
Steady state

• 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

6
Human impacts

• The production of chlorofluorocarbons (CFCs) in
  the 20th century added a new dimension to the
  ozone balance.
• CFCs 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.

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