The Ozone Layer

Ozone is one of several gases that make up the earth’s atmosphere. The diatomic molecule of oxygen (O₂), the form of oxygen we breathe, makes up approximately 20 per cent of the atmosphere. Ozone (O₃), the triatomic molecule of oxygen, is much less abundant, making up only one part in three million of all gases in the atmosphere.

Importance Of Ozone

Ozone is concentrated in the lower stratosphere between 15 and 30 km above the earth’s surface – the so-called ‘ozone layer’. Ozone can be produced by numerous chemical reactions, but the main mechanism in the atmosphere for its production and removal is absorption of ultra-violet (UV) radiant energy from the sun.

Ozone is produced when O₂ absorbs UV radiation at wavelengths of less than 242 nanometers, and is removed by photo-dissociation from sunlight for wavelengths greater than 290nm. O₃ is also the major absorber of UV sunlight between 200 and 330nm. The combination of these processes is effective in maintaining a relatively constant amount of ozone in the layer, and in absorbing 90% of UV sunlight.

UV is linked to the formation of skin cancer and genetic damage. Increased levels of UV are known to have adverse effects on animal immune systems, aquatic organisms at the bottom of the food chain, terrestrial plants and food crops.  

The maintenance of enough stratospheric ozone to absorb harmful UV sunlight is therefore vitally important to all life forms on earth.

Ozone Balance

The amount of ozone in the atmosphere varies according to geographic location and season. Ozone is scaled in Dobson units (Du), where, for example, 300 Du would be equivalent to a 3 mm thick pure layer of ozone if compressed to sea level pressure.

Stratospheric ozone is mainly produced in the tropics and transported to higher latitudes by large-scale atmospheric circulation during the winter to spring months. The tropics generally possess lower ozone.

Threat from CFCs

A known threat to this ozone balance is the introduction of man-made chlorofluorocarbons (CFCs) which increase the rate of ozone removal, resulting in a gradual decline in the global ozone levels.

CFCs are used by modern society in a myriad of ways, in refrigerants, as propellants in spray cans, in foam manufacture and in solvents, particularly for the electronics industry.

The long lifetimes of CFCs means that a single molecule released today can exist 50 to 100 years in the atmosphere before it is eliminated.

Over periods of approximately five years, CFCs move slowly up into the stratosphere (10-50 km). Above the main body of the ozone layer, centred in the height range 20 –25 km, less UV is absorbed by ozone. The CFC molecules break down after reaction with UV, and release free chlorine atoms. These chlorine atoms are then able to destroy ozone.

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