The Role of Chlorine

The amount of chlorine in the Earth's atmosphere has remained very small, and more or less constant over geological time-scales. It's role in atmospheric chemistry has only become of importance to humans during the last century when it was discovered that chlorine has an affinity for ozone (0₃), and will react with ozone to produce molecular oxygen (0₂).

Dynamic Equilibrium (without chlorine)

The formation and depletion of ozone. Red arrows indicate the path of ozone molecules and blue arrows show the path of oxygen molecules. The air within the glass jar assumes the role of the earth's atmosphere.
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The diagram to the left illustrates the basic principles of ozone chemistry. The principle players are ultraviolet-C (uv-C) and ultraviolet-A and ultraviolet-B (uv-A&B)

For purposes of illustration we assume that the contents of the large jar represents the Earth's atmosphere.

The primary role of uv-C is to use ultraviolet light in the wavelength range 200-280nm to form ozone from oxygen molecules.

The primary role of uv-A&B is to use ultraviolet light in the wavelength range 200-320nm (B) and 320-400nm(A) to photo-dissociate ozone and form oxygen molecules.

The speed of the reaction is controlled by several factors such as the intensity and wavelength of the ultraviolet light, the density of the air and the presence of other atoms which may act as catalysts to accelerate or retard the reaction rate.

Generally the rate at which uv-C produces ozone is exactly balanced by the rate at which uv-A&B converts ozone back into molecular oxygen.

Key Idea

Now, here's the key idea... uv-A&B is a bit slower at destroying ozone than uv-C is at making ozone. As a result there is always a surplus of ozone in the atmosphere and uv-A&B is totally consumed trying to deplete the ozone in that atmosphere. This surplus ozone forms the Earth's ozone layer and it is responsible for absorbing almost all the uv-A and uv-B light from the Sun and protects plants and animals from this potentially harmful radiation.

The Effect of Chlorine

uv Absorption by Oxygen

The formation and depletion of ozone with the introduction of chlorine. Red arrows indicate the path of ozone molecules and blue arrows show the path of oxygen molecules. The air in within the glass jar represents the Earth's atmosphere.
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When chlorine is introduced into the picture things become very different. Chlorine cannot only speed up the conversion of ozone into molecular oxygen, but it can do this without requiring assistance from uv-A&B.

In effect, chlorine "steals" the ozone from the atmosphere. Moreover the chlorine reaction doesn't have same reaction rate (speed control) limitations as the uv-A&B process. Chlorine converts ozone into molecular oxygen at a rate faster than uv-C can produce ozone.

With no ozone left in the atmosphere (because it is efficiently depleted by reactions with chlorine), ultraviolet A&B has no absorbing ozone molecules available.

With no molecules available in the atmosphere to absorb uv-A&B radiation, the radiation reaches the Earth's surface.

How Chlorine Reacts

CFCs (chlorofluorocarbons) undergo a series of reactions which lead to the dispersal of ClO molecules in the stratosphere. These molecules contain a single chlorine atom and a single oxygen atom. It is this molecule (from its precursor CFCs) which reduces the amount of ozone in the upper atmosphere.

A simplified description of the process is shown below.

Transparency Master	The diagram to the left shows how a "free" chlorine atom attacks an ozone molecule to form ClO and O₂. The process however does not simply end with these molecules. ClO itself is a highly reactive substance. It very quickly reacts with any "free" oxygen molecules it encounters before the free oxygen can react with an oxygen molecule to form ozone.
Transparency Master	In other words, ClO reacts with the free oxygen atoms to turn them back into oxygen molecules thereby destroying their ability to form O₃ molecules of ozone. Equally serious is the fact that the Cl atom is released back into the atmosphere to destroy more ozone.