This page contains QuickTime^¨ movies. Click the QuickTime^¨ button above if you need the plug in. There are two sections to this page. The first describes how ozone is generated in the upper atmosphere. The second describes Mario Molina's theory of how CFCs destroy ozone.

How Ozone Is Born

One of the most beautiful and elegant aspects of our earth is that the ozone layer is in fact generated by the same ultraviolet radiation from which it protects us. High energy ultraviolet photons in the upper atmosphere react with a normal oxygen molecule, breaking it into two oxygen atoms. Those atoms, in turn, react with other oxygen molecules to make ozone.

To view a QuickTime^¨ movie of the creation of ozone using molecular models, click on the image below.

Click to watch the movie.

How CFCs Catalyze Ozone Destruction

According to Molina's theory, the same photons that break oxygen molecules apart to produce oxygen atoms can also break CFCs apart to release chlorine atoms, among other things. Since the chlorine molecule has an unpaired electron, it is a radical. Radicals are very reactive, so chlorine atoms react easily with ozone molecules. When a chlorine atom encounters an ozone molecule, it takes one of the oxygen atoms away, leaving O₂ and chlorine oxide (ClO). Chlorine oxide is also a reactive radical and reacts quickly with another ozone molecule, converting it to two O₂ molecules, and freeing the Cl to do still more damage. Since the radical Cl atom starts this whole reaction, but isn't consumed in the process, we say it is a catalyst for the ozone destruction reaction.

Click to watch the movie.

CFCs are catalysts for ozone destruction. This not only means that they aren't consumed in the reactions that destroy ozone, but it also means they speed up the reaction rate of the destructive reaction. Naturally occurring reactions destroy ozone, but normally ozone is generated fast enough to replace that which is lost. When CFCs catalyze the reactions that destroy ozone, they speed up the destruction and the reactions that create ozone aren't fast enough to keep pace.

Given that this happens miles above the earth's surface, higher than most airplanes fly, how do we know any of this? When Mario Molina wanted to understand what happened to CFCs in the atmosphere he had to simulate stratospheric conditions in the laboratory. This is often how scientists study systems that are difficult to study directly. He had to subject gaseous mixtures of CFCs, ozone, and other compounds to the same conditions of temperature and pressure as found in the stratosphere, and expose them to the same amount of UV radiation that the stratosphere receives. He observed that ozone-destruction taking place at uncomfortably fast rates.

But this just suggests that ozone destruction could happen. This is not the same as showing that it really happens. And it is another thing still to show that ozone destruction, if happening, is caused by CFCs. Unfortunately, it took a near disaster for this kind of hard evidence in support of Molina's model to come to light. What was that evidence? That's what we'll explore in the next three readings.

Next: A Slap on the Head

Nobel Poster in Chemistry 1995 — an online presentation, adapted from the Nobel Foundation's poster dealing with the prize-winning science of Paul Crutzen, Mario Molina and F. Sherwood Rowland, from the Nobel e-Museum.