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Name: Elizabeth L.
Status: Student 
Age: 12
Location: N/A
Country: N/A
Date: October 16, 2004

Dear Mr. Scientist,
I was wondering if you could answer this question for my classmates, teachers and me. We were all wondering if you knew if the ozone layer grows back or comes back at all. We got on the subject in health and we would love an answer!

Ozone (O3) is made by shining ultraviolet light of around 240 nm wavelength on oxygen molecules (O2), which split into single oxygen atoms, which each join an O2 molecule to make an O3 molecule. There is no shortage of ultraviolet radiation or O2 high up in the atmosphere, so O3 is being made continuously. It is also continually being broken up into O2 + O1 by ultraviolet light of wavelength in the range 240-320 nm, and recombining into O3. (This is why we want the stuff around in the first place, so it will absorb the UV radiation that ordinary oxygen molecules do not absorb.)

The problem in ozone depletion is not that the stuff is actually lost, but rather that oxygen atoms are spending more time as members of O2 molecules, and consequently less time as members of O3 molecules, because other chemical reactions are converting 2 O3 molecules into 3 O2 molecules. It is a dynamic balance, and the balance point is shifted by the presence of chemicals that catalyze the conversion of O3 to O2.

Tim Mooney


The answer is yes, ozone is continually produced and destroyed in the upper atmosphere. Ozone is a compound with a chemical formula of O3. When light from the sun strikes oxygen molecules, O2, it breaks up into two oxygen atoms, 2 O. Each of these oxygen atoms may find other oxygen molecules and combine to give ozone. O + O2 = O3.

The importance of ozone is that it can interact with harmful UV light. When it does this, it breaks down to oxygen gas, O2, and an oxygen atom. O3 = O2 + O. So in effect, there is a cycle of making and destroying ozone in the atmosphere.

Since it is the presence of O3 that allows the great absorption of harmful UV-light, substance that speed up the destruction of O3 is not good for the ozone layer. Chlorine, Cl, in the atmosphere can combine with ozone to give different compound and oxygen. For example: Cl + O3 = ClO + O2. Thus, if chlorine is in the atmosphere, ozone is depleted without the absorption of UV light - and that's not good for us.

Greg (Roberto Gregorius)

Dear Elizabeth, I am not an expert in this field and I hope someone who is responds to you fully. My general understanding of the process is that the ozone layer does not grow back. Although this layer varies over time in size, the overall trend has been downward. That is why some measures are in place to restrict the discharge to the atmosphere of components that destroy the ozone layer.

Dr. Ali Khounsary
Argonne National Laboratory

Dear Elizabeth,

Thanks for your great question! According to the research I have read, the maximum ozone depletion will probably occur within the next 10 years; thereafter, the ozone layer is expected to slowly recover over the next several decades.

Here is an EPA web site that describes the history of the discovery of the depletion of stratospheric ozone and its recovery:


Todd Clark, Office of Science
U.S. Department of Energy

Yes, Elizabeth, the ozone can grow back. Ozone is always slowly forming, from sunlight trying to sun-burn the top of earth's air. Unfortunately it seems we have put impurities in the air which make ozone turn back into air sooner than it used to, and so we have maybe 20% less ozone than we used to.

These ozone-eating impurities float in the upper atmosphere for many years, because the air up there is kind of quiet and empty, and rather disconnected from to all the rain and weather we have down here. But in fifty years or less the impurities can finally settle to the ground, and then the ozone can build up to its old-time levels.

Some measurements look like it is coming back faster than fifty years. That is good news. We have learned; we have laws so we will not make those same impurities any more. But there is always a chance we will start making more impurities of a different kind, before we know they are bad too. That is the bad news. We may even outlaw many useful substances which really do not eat much ozone. Is that being too careful and stifling people, or is it just right? For this you will have to keep reading your newspaper and voting.

Some permanent environmental disasters are possible. Ozone is no longer one of them. This ongoing problem is likely to be "socially managed" for the whole time you are growing up. I think we are going to be OK, and even pretty good some day.

Jim Swenson

Dear Elizabeth,

The ozone layer is always here. What changes is the size of the layer and the biggest changes happen at the South Pole. The size of the layer changes because ozone is being made at the same time it is being destroyed. If more ozone is destroyed than is made, then the layer gets smaller.

If you have a pool of water and add one bucket of water, the water level will rise. If you add a bucket of water and take out a bucket of water, the water level will remain the same. If you add a bucket of water, and you take out two buckets of water, the water level will drop. The ozone layer is like the last example. Ozone is made, but at the South Pole during September, more ozone is being destroyed than is made.

Dr. Jim Tokuhisa
Max Planck Institut fuer Chemische Oekologie

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