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Stratospheric Ozone Layer & Ozone Depletion

Ultraviolet Filtration and the Ozone Layer

Let us look in detail at the first protective mechanism afforded by the fact that O3 and O2 both absorb ultraviolet but at slightly different wavelengths. O3 absorbs in a region from 240 - 280 nm and O2 absorbs wavelengths shorter than 175 nm. The energy absorbed in both cases is used to effect chemical change rather than re-emitted.

The UV radiation absorbed by O2 in the stratosphere actually splits the O2 into oxygen atoms. Each of these oxygen atoms combine with other oxygen atoms to form O2 or with O2 to form O3. O3 absorbs UV at the higher wavelengths (240-280 nm) to split into O and O2. The O released by O3 may recombine with an O to form O2 or with water to form 2OH radicals. These changes may be outlined in terms of the following equations:

1. O2 + UV radiation (< 175 nm) O + O
2. O + O2 O3
or
O + O O2
3. O3 + UV radiation (240-280 nm) O* + O2 gas
4. O* + O O2
or
O* + H2O 2OH
  and so on...

This cycle repeats and, over millions of years, has reached an equilibrium state. The net result of the above reactions is that O2 and O3 are constantly changing into each other, and each cycle takes up energy in the form of ultraviolet radiation, resulting in a large reduction of the amount of ultraviolet radiation reaching the troposphere. These reactions also result in there being a higher concentration of ozone gas in the lower region of the stratosphere with a maximum of O3 occurring between 20 and 26 km above the Earth's surface. This area is called the "ozone layer."

In general, ultraviolet radiation of the smaller wavelength damages the skin, and can initiate the process of skin cancer. The stratospheric ozone layer forms a protective shield protecting us from receiving large amounts of UV. Note however that some ultraviolet does get through and is responsible for sunburn, and skin cancer with excessive exposure.

The ultraviolet A absorbed by the skin can actually damage our DNA. Most of us have repair genes that can repair this damage, however when we are exposed to large amounts of UV, the repair is not enough to keep up with the damage and this damage can result in skin cancer. People who can not produce skin pigment (referred to as "albino") have a genetic condition known as xeroderma pigmentosum, which is accompanied by a lack of the UV repair gene. These people are therefore several hundred times as likely as the average person to contract skin cancer.

Ozone-Depleting Substances

Humans have introduced many compounds into the atmosphere that are capable of disrupting the cycle of creation and destruction of ozone molecules in the stratosphere. A family of compounds known as chlorofluorocarbons (CFC's) have had the most significant effect on the ozone layer by far. This discussion will focus primarily on CFC's, although the basic process of ozone depletion is very similar for any of the ozone-depleting substances (ODS).

CFC's have varying compositions, but all of them contain different proportions of three elements: carbon (C), Chlorine (Cl) and fluorine (F). Two of the CFC's that were in common use are: CFC-11 (CFCl3) and CFC-12 (CF2Cl2)

CFC's were produced and used extensively as refrigerants starting in the early 1930's. They were discovered by a scientist named Medgley who was searching for a more ideal cooling compound to replace the unsafe chemicals that were being used at that time, including ammonia and sulfur dioxide. Ammonia was most widely used, but was undesirable because it is a strong eye and respiratory irritant.

Chlorofluorocarbons were seen then as the ideal compounds because they are extremely non-reactive, and were therefore thought to be harmless. They are chemical inert, non-toxic, and insoluble in water. For close to fifty years, they were hailed as miracle substances, and were used extensively in aerosols, refrigerants, and foams.

What we did not know then was that because of their non-reactive nature, CFC's are able to rise undisturbed into the atmosphere. They are not destroyed by reactions or removed by precipitation in the tropospheric layer of the atmosphere, and migrate over several years, eventually reaching as high as the stratosphere.

Disruption of Ozone Cycle

When CFC's migrate high enough and are hit by enough ultraviolet radiation, they are broken down and release chlorine atoms. The chlorine atoms react with O3 gas and the following chain of reactions results:

Cl + O3 ClO + O2

ClO + O Cl + O2

These reactions make ozone molecules unavailable for the vital reactions that absorb incoming ultraviolet, and are the main source of ozone depletion. One chlorine atom can destroy over 100,000 molecules of ozone, and the result of this disruption is a markedly lower than expected concentration of stratospheric ozone at various points around the world.

Results

The possibility of ozone depletion in the stratosphere was predicted in the 1970s by two scientists named Roland and Molina. They based their prediction on the action of CFC's on the atmosphere. Although stratospheric ozone depletion is often referred to as the "ozone hole," that term is misleading. What we call a hole is actually a sharp reduction in expected ozone concentrations. Scientists have defined an ozone hole as an area having less than 220 dobson units (DU) of ozone in the overhead column (i.e., between the ground and space).

Lower ozone concentration means that less incoming ultraviolet radiation is absorbed by the reactions described earlier, and more reaches the troposphere and the Earth's surface. Humans and other forms of life are exposed to higher levels of ultraviolet, which can cause more damage to skin cells and sensitive tissues of the eye than they are capable of repairing.

Ozone depletion, or the concentration of stratospheric ozone, varies seasonally and latitudinally. There tends to be more ozone depletion in the winter with more depletion at the polar regions. The science behind this is somewhat uncertain but is related to the reaction surfaces that are caused by cold cloud formations near the poles.

Possible impacts from ozone depletion are related to the effects on ecosystems by ultraviolet radiation. The exact cause and effect relationship for many of these impacts is uncertain. The impacts are:

  • Malignant skin cancer
  • Non-malignant skin lesions
  • Lower crop productivity
  • Cataracts
  • Ecosystem abnormalities

Policy Efforts

In 1987, the first substantial international environmental treaty was passed. It is known as the Montreal Protocol and includes agreements to reduce the worldwide production of CFCs. The Protocol was precedent-setting in that it included funds to the developing countries to compensate for the higher costs of using alternate technologies.

The Montreal Protocol has been effective in lowering the production of CFCs in the U.S., although many developing countries have a longer time period for compliance. However, the CFC molecule is so stable (lasting 1700 years or more in the atmosphere) that previously produced CFC's will be entering the stratosphere continuously and we will feel their impacts for many years to come.

Several substitutes for CFC's are being developed. The desirable property of CFC--its chemical inertness--is also the reason it is able to reach the stratosphere. To engineer a a substitute, one must design a compound that has the desirable properties but will not contribute to stratospheric ozone depletion. The new compounds being considered have less chlorine and fluorine. The general replacements are HCFCs in which one chlorine is replaced by hydrogen, and HFCs in which chlorine is altogether replaced by hydrogen. Examples are CHClF2 and CH2F2. The lowered chlorine compounds are also banned in the U.S. after 2000 by the Clean Air Act.

 

 

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  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.