The Ozone Layer and CFCs


How does the ozone layer normally work?

At all times, the concentration of oxygen (O2) far exceeds that of ozone (O3) in the atmosphere. Ultraviolet (UV) light with wavelengths between 240 and 320 nm is absorbed by O3, which then falls apart to give an O atom and an O2 molecule. The O atom soon encounters another O2 molecule, however, and recreates O3 as shown here:


O3 + UV -> O2 + O

O + O2 -> O3


Thus ozone absorbs UV radiation without itself being consumed; the net result is to convert damaging UV light into heat. Indeed, this is what causes the temperature of the stratosphere to increase with altitude, giving rise to the inversion layer that traps molecules in the troposphere. The ozone layer isn't just in the stratosphere; the ozone layer actually determines the form of the stratosphere. Ozone is destroyed if an O atom and an O3 molecule meet; this is called recombination.


O + O3 -> 2 O2


This reaction is slow, however, and if it were the only mechanism for ozone loss, the ozone layer would be about twice as thick as it is. Certain trace species, such as the oxides of Nitrogen (NO and NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2) can catalyze the recombination.

How do CFCs affect ozone depletion?

Chloro-fluorocarbons (CFC's) themselves do not destroy ozone; certain of their decay products do. After CFC's are photolyzed (split into pieces after being struck by light), most of the chlorine eventually ends up as Hydrogen Chloride (HCl) , or Chlorine Nitrate, ClONO2. These are called "reservoir species" - they do not themselves react with ozone. However, they do decompose to some extent, giving, among other things, a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO, which can catalyze the destruction of ozone by a number of mechanisms. The simplest is:


2Cl + 2O3 -> 2ClO + 2O2

2ClO + 2O2 -> 2Cl + O2 + 2O2

Net effect: 2 O3 -> 3 O2


Note that the Cl atom is a catalyst - it is not consumed by the reaction. Each Cl atom introduced into the stratosphere can destroy thousands of ozone molecules before it is removed. The process is even more dramatic for Bromine; it has no stable "reservoirs", so the Br atom is always available to destroy ozone. On a per-atom basis, Br is 10-100 times as destructive as Cl.

Activity to demonstrate the chemical reaction that destroys the ozone layer:

Two students will be chlorine atoms. Chlorine requires one bond to be stable, which we will represent by one empty hand. The other hand will be held down to the side. All of the other students in the room will be oxygen atoms. Oxygen atoms requires two bonds (which we will represent by two empty hands) in order to be stable, so O by itself is reactive because both hands are empty. O2 is stable because each "hand" of one atom is held by the hand of a second atom. O3 is stable because it forms a ring, again with all hands being held. The intermediate molecule ClO, is somewhat reactive, because the O has only one of its bonds filled. Therefore, ClO will break if it comes across a free O. Start the simulation by making all of the O atoms form ozone (rings of 3). The Cl atoms can break the O3 rings, thus starting a chain reaction that ends when all of the ozone is converted into oxygen.

Further Investigation:

The global ban on chlorofluorocarbons has largely healed the ozone hole, and like the DDT ban before it, shows that when science raises the alarm, and the world community comes together, we can fix it. Here are two articles that talk about the ozone hole today.

Check with NOAA on the status of the south polar ozone hole.