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The Ozone Hole from Space

How does ozone absorb UV light?

When ultra violet light hits diatomic (normal) oxygen, it causes a photochemical reaction that splits the molecule into the two atoms:

+ UV light —> O + O

If there is a non-reacting species (M) present, it can allow another reaction to take place:

O + + M —> + M

So from two diatomic oxygen molecules we can create one triatomic ozone molecule.

Unfortunately ozone is also destroyed by photochemical reactions:

= UV light —> O +

So from two triatomicoxygen molecules we can create three diatomic ozone moleculse.

The total amount of ozone depends on a delicate balancing act between these two reactions.

Because it needs short wavelength ultraviolet light and much of this is scattered by the Earth's atmosphere, ozone cannot be formed below an altitude of about 20 km. Above an altitude of about 50 km the density of the atmosphere is too little to allow ozone to be formed.

So the ozone layer is between 20 km and 50 km above the Earth - the stratosphere.

Why are CFCs dangerous?
Ozone can also be destroyed by catalytic photochemical reactions using things like nitric oxide (NO), chlorine (Cl), Hydroyl (OH) and bromine (Br). This is where CFCs come in. CFCs drift up into the stratosphere and break down into Cl and ClO. These then accelerate the breakdown of ozone.

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