EMSC 100
Freshman Seminar in the College of Earth and Mineral Sciences

The Ozone Cycle


The presentation below shows the process of ozone depletion. Ozone depletion is caused by chlorofluorocarbons (CFCs) and other ozone-depleting substances. Please watch the following 1:16 video.

Ozone Depletion
Click Here for Transcript of Ozone Depletion video

In this picture we are seeing how the ozone layer is destroyed and the effects of that destruction. Certain compounds such as chlorofluorocarbons, CFC’s, are released into the atmosphere by human activities, and these CFC’s particularly, they are very, very unreactive at the atmospherical ground level and they go all the way up to the stratosphere and then the CFC’s dissociate or give up the chlorine. Each chlorine atom is capable of destroying or basically turning ozone into oxygen and which again doesn’t have the same capability as ozone does in shielding us from the UV radiation. And once that ozone layer or the number of ozone molecules in that layer goes down, more and more UV rays can pass through the atmosphere and reach the surface and can cause more skin cancer and cataracts in older people.

Production and Destruction of Ozone

Ozone is constantly produced and destroyed in a natural cycle, as shown in the figure below. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream's depth at a particular location. Although individual water molecules are moving past the observer, the total depth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades. Please watch the following 1:32 video about ozone destruction.

Natural production and destruction of ozone in the stratosphere
Click Here for Transcript of ozone production and destruction video

Here we are looking at the ozone science. How the ozone is produced and how the ozone is destroyed. In the first step, the oxygen molecules are photolyzed or converted by the UV rays that are coming from the sun into two oxygen atoms; nascent oxygen atoms. Oxygen atoms are very, very reactive and they react with another oxygen molecule and form ozone, O3. Ozone and oxygen atoms are continually being interconverted as rays break the ozone and turns into nascent oxygen and oxygen molecules. And the oxygen atom again reacts with the oxygen molecules forms ozone. Our activities, which are producing the CFC’s and liberating into the atmosphere, they are going and these chlorine atoms are destroying the ozone molecules in addition to the natural process of formation and destruction. That is what is causing the reduction in the concentration of ozone in the stratosphere and when the concentration goes down below certain levels, like 220 Dobson units we call that ozone hole. Ozone hole does not mean that there is a big hole up there but what it means is that the concentration is below a certain level.

Large increases in stratospheric chlorine and bromine, however, have upset the balance of the Ozone. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up. Therefore, ozone levels fall.

Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more the ozone is depleted, the larger will be the increase in incoming UVB radiation. UVB has been linked to:

  • skin cancer;
  • cataracts;
  • damage to materials like plastics;
  • harm to certain crops and marine organisms.

Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem.

Ozone-Depleting Substance(s) (ODS) are:

  • CFCs;
  • HCFCs (used in the energy related to refrigeration and air conditioning in homes, commercial buildings, and cars, and manufacture of foam products);
  • Halons (used in fire extinguishers);
  • Methyl bromide, carbon tetrachloride;
  • Methyl chloroform (used as solvents in chemical industries).