One of the primary objectives of the SCISAT-1, ACE (Atmospheric Chemistry Experiment) mission is to monitor the Earth’s ozone layer, especially at high latitudes. This is of special importance to Canadians.

The Earth is surrounded by a thin layer of gases made up of a mixture of molecules. These are mostly molecules of nitrogen, oxygen, argon and carbon dioxide. This thin layer we call the Earth’s atmosphere.

The Ozone Layer


The Ozone Layer. The scale in this diagram is greatly exaggerated for purposes of illustration

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In the upper atmosphere ultraviolet light (short wavelength light) from the Sun provides enough energy for some of these molecules to undergo chemical reactions with other molecules, synthesizing new compounds whose abundance is quite variable. Ozone is a synthesized molecule. The abundance of these synthesized compounds (such as ozone) depends upon the amount of sunlight available and the availability of “intruder” molecules. “Intruder” molecules can either speed up the formation of new molecules or speed up the destruction of atmospheric molecules.

Chlorine atoms and molecules are considered to be intruder atoms and molecules because their abundance has been increased by the use of man-made chorine-based compounds which have been carelessly released into the atmosphere. Even in very small quantities, chlorine atoms can dramatically shift the equilibrium in the chemical composition of the Earth’s upper atmosphere.

One very important molecule, which occurs at high altitudes in the Earth’s atmosphere is ozone. The region of the Earth’s atmosphere containing ozone is often referred to as the ozone layer even though it comprises a very small portion of the molecular composition in that region.

The diagram above illustrates how the ozone layer surrounds the Earth in a zone high in the Earth’s atmosphere. The abundance of ozone gradually increases as one moves upwards through the Earth’s atmosphere, reaching a maximum abundance at an altitude of about 30km, and then gradually decreasing as one moves further upwards in through the atmosphere.

The Formation of Ozone

uv Absorption by Oxygen


The basic mechanism for the synthesis of ozone from the photo-dissociation of oxygen molecules

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Ozone is created when “free” oxygen atoms (O) unite with oxygen molecules(O2) to produce (O3).

The chain of events begins with the absorption of a high energy ultraviolet photon by a molecule of oxygen. The oxygen molecule absorbs sufficient energy to break its molecular bond, producing two free oxygen atoms.

These free oxygen atoms either re-combine or react with other oxygen molecules to produce oxygen and ozone molecules.

Oxygen molecules are very efficient absorbers of high energy ultraviolet radiation (uv-C).

The Importance of Ozone

uv Absorption by Ozone


Ozone is an efficient absorber of lower energy solar ultraviolet A and B radiation which oxygen does not effectively absorb.

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Ozone is an efficient absorber of the middle and lower energy parts of the ultraviolet spectrum (uv-A and uv-B). In the absorption process the ozone molecule undergoes photo-dissociation releasing free oxygen atoms, which re-combine to produce oxygen molecules.

Both the synthesis and photo-dissociation of ozone convert the energy of the ultraviolet radiation into thermal energy which heats the Earth’s stratosphere.