How ozone is destroyed by CFCs

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Ozone animation

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Key to atoms:
chlorine atom Chlorine (Cl)
Oxygen atom Oxygen (O)
Fluorine atom Fluorine (F)
Carbon atom Carbon (C)

  Explanation

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.
** UV radiation breaks oxygen molecules (O2) into single oxygen atoms.

Chemical equation

CFCl3 + UV Light ==> CFCl2 + Cl
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

The free chlorine atom is then free to attack another ozone molecule

Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

and again ...

Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

and again... for thousands of times.

 

Fact File

Ozone is a pale blue gas irritating to the nose and throat; it is explosive and toxic. It is produced and destroyed naturally in the atmosphere and until recently, this resulted in a nicely balanced equilibrium. It is formed when oxygen molecules absorb ultraviolet radiation with wavelengths less than 240 nanometres and is destroyed when it absorbs ultraviolet radiation with wavelengths greater than 290 nanometres.

Ozone graphic

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. 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.

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.