The balanced atmosphere

Stratospheric Ozone and the Ozone Hole

Ozone is very rare in our atmosphere, averaging about three molecules of ozone for every 10 million air molecules. In spite of this small amount, ozone plays a vital role in the atmosphere.

Ozone is a form of oxygen that comprises three atoms (O3) rather than the two atoms (O2) found in ordinary molecular oxygen.

Ozone is mainly found in two regions of the Earth's atmosphere. Most ozone (about 90%) resides in a layer that begins between 6 and 10 miles (10 and 17 kilometers) above the Earth's surface and extends up to about 30 miles (50 kilometers). This region of the atmosphere is called the stratosphere. The ozone in this region is commonly known as the ozone layer . The remaining ozone is in the lower region of the atmosphere, which is commonly called the troposphere. The figure (above) shows an example of how ozone is distributed in the atmosphere.

The level of maximum concentration is at about 25 km (15 mi) and approximately 10 ppm (parts per million); that is, in million molecules 10 are ozone molecules.

The ozone molecules in the upper atmosphere (stratosphere) and the lower atmosphere (troposphere) are chemically identical, because they all consist of three oxygen atoms and have the chemical formula O3. However, they have very different roles in the atmosphere and very different effects on humans and other living beings. Stratospheric ozone (sometimes referred to as "good ozone") plays a beneficial role by absorbing most of the biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the Earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually forms the stratosphere itself (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the Earth's atmosphere. Without the filtering action of the ozone layer, more of the Sun's UV-B radiation would penetrate the atmosphere and would reach the Earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown the harmful effects of excessive exposure to UV-B radiation.

Ultraviolet radiation and its biological effects

Classification of UV Radiation

Wavelength range (nm)	Name	Biological effect
320-400	UVA	Relatively harmless; causes tanning but not burning
290-320	UVB	Harmful; causes sunburn, skin cancer, and other disorders
200-290	UVC	Extremely harmful but almost completely absorbed by ozone

Ultraviolet (UV) radiation in the 200-400 nm region is usually subdivided into three distinct spectral regions shown in the table on the right.

If the concentration of stratospheric ozone decreases, the following might occur:

• An increase in the number of cases of skin cancer.
• A sharp increase in eye cataracts and sun burning.
• Suppression of the human immune system.
• An adverse impact on crops and animals due to an increase in ultraviolet radiation.
• A reduction in the growth of ocean phytoplankton.
• A cooling of the stratosphere that could alter stratospheric wind patterns, possibly affecting the production (and destruction of ozone).

When UV radiation hits the skin, it can cause the cell to "lock up" and scramble or delete DNA information. This action causes confusion in the DNA, and the body loses control of the growth and division of the cell. If the conditions are right, the cell may become cancerous. It is important to note that not all affected cells turn into skin cancer, for many can repair themselves. However, continual exposure to UV radiation increases the risk of skin cancer due to cumulative damage of the DNA.

Skin cancer can be divided into two categories: melanoma and non-melanoma. The melanoma form of skin cancer is the more dangerous of the two. This type of cancer has the ability to spread quickly throughout the body and invade other cells. On the other hand, non-melanoma skin cancer is not to be taken lightly either, but is a less serious form of the disease. Non-melanoma skin cancers are not usually life threatening, and removal is relatively routine. However, treatment does include radiation therapy or surgery. The concern of many is that sunburn may lead to increased risk of acquiring skin cancer. Some forms of cancer are associated with sunburn, while other forms are not. Melanoma skin cancer is a form that sunburns may play a leading role in. Jan van der Leun, a Dutch scientist, explains that, light hitting the outer layer of the skin, the epidermis, triggers the production of some substances which diffuse into the dermis below. The dermis is filled with blood vessels, and the chemical substances cause them to dilate, making the skin red and warm to the touch.

Ozone depletion is also suggested to cause immuno-suppression. This theory was first explored in the 1960s when guinea-pigs, who were exposed to an allergen, showed a lowered immune system response after they had been irradiated with UV. In addition, another study showed that UV radiation had the same effect on animals as X-ray treatment and chemical immuno-suppression. Logically, all three factors suppressed the immune system.

Like immuno-suppression and skin cancer, science is not able to provide society with a confident answer to the question: Will the depletion of the ozone layer cause an increased number of cataract cases? Cataracts are a condition that begin with blurry vision and in some cases, develop into blindness. It has been proven that UV light can damage the DNA, membranes, and proteins in the eye, and in animal studies, this damage has resulted in scattered light and the formation of opaque areas in the eye. It was estimated by the Environmental Effects Panel of the United Nations Environment Programme that for each percent decrease in ozone, the number of people developing blindness would increase by approximately 100,000 to 150,000 people. However, this estimation was contradicted by a team of Dutch scientists, who stated, "it is not scientifically justifiable to quantify the effects of UV radiation on the eye, if such effects are present under normal circumstances". The UNEP then published an updated statement and included information that poor diet and diseases, such as diabetes, also contribute to cataract development. Thus, it must be recognized that cataracts can result from poor nutrition, poor hygiene, and diabetes, and not solely from increased UV radiation.

Another common, yet, highly debated concern with regards to the depleting ozone layer is crop and plant damage. As it has been well stressed, depletion of the ozone layer results in higher UV-B radiation on the earth?s surface. Ironically, while plants use light as their main fuel for growth, a delicate balance must be achieved in order for the plant to survive. If a plant is exposed to too much UV radiation, the DNA of the plant may become damaged due to penetration of harmful UV radiation into sensitive areas of the plant. UV radiation also causes problems in the photosynthetic machinery by hampering the photosynthesis process, the cell membrane by altering the transportation of essential potassium, and the cell's skeleton by affecting cell growth and morphology. With this information taken into account, it would seem logical that increased UV radiation from the depleting ozone layer would lead to plant damage. However, it is not that simple. Some plants actually employ a mechanism that allows them to protect themselves from UV damage. Thus, research suggests that if ozone depletion became serious enough, the plants without the protective mechanisms would die out, but the plants with these mechanisms would be able to replace the extinct plants, and not affect the level of productivity in the ecosystem. Thus, much depends on which plants survive.

Formation of the ozone layer

The ozone concentration in the stratosphere is maintained by a delicate balance between production and destruction. Therefore, the ozone layer is highly susceptible to alterations by anthropogenic processes.

Several natural factors influence these chemical reactions:

• volcanic eruptions - sulfate particles are released into the stratosphere and result in breakdown of ozone
• solar activity
• changes in atmospheric circulation

In the early 1970s atmospheric chemists Sherwood Rowland and Mario Molina suggested that human made CFCs (chlorofluorocarbons) could destroy ozone in the stratosphere.

CFCs are produced in great quantities for refrigeration (air conditioners), propellants, cleaning solvents, etc. They were considered ``Ideal'' for this purpose because of their inertness; which turned out to be true only in troposphere where they are protected by the ozone layer! CFCs have a lifetime of 40-150 years in the stratosphere.

Due to scientific evidence that CFCs and other chemicals destroy ozone in the upper atmosphere, the United States, the country which has traditionally been the largest emitter of CFCs worldwide, is rapidly scaling back the use of these chemicals and phasing out their production.

In 1985 - Scientists from the British Antarctic survey reported a 50% drop in total ozone present in the Antarctic stratosphere during the Antarctic Spring. The media labeled this phenomena the Ozone Hole which has been used ever since.

In order for scientists to evaluate how much ozone is in the layer, a unit of measurement called the Dobson Unit is employed. A Dobson Unit is a measurement of how thick a specific portion of the ozone layer would be if it were compressed into a single layer at zero degrees Celsius with one unit of atmospheric pressure acting on it (standard temperature and pressure - STP). Thus, one Dobson Unit (DU) is defined as .01 mm thickness at standard temperature and pressure. The figure on the right shows a column of air over Labrador, Canada. Since the ozone layer over this area would form a 3 mm thick slab, the measurement of the ozone over Labrador is 300 DU.

Why there and then?

• over most of the world and most seasons stratosphere is too dry to form clouds over Antarctica in late winter temperatures fall to -90 degrees C;
• when it is this cold minute amounts of water vapor form ice clouds;
• chemical reactions occur on the surface of the ice crystals which convert chlorine into unstable forms that in the presence of sunlight destroy ozone
• these reactions contribute to an enhanced destruction of ozone
• another factor is the presence of a strong circumpolar circulation around Antarctica (``Polar Vortex'') which isolates the air in the center of the vortex from warmer, ozone-rich air outside the vortex.

More detailed information about the Antarctic ozone hole is provided by the Why is the Ozone Hole Over the Antarctic? EPA web page.

See also TheOzoneHole webpage

The current and past status of the Antarctic ozone hole in terms of area and minimum value is presented by the two figures on opposite sides of this text.

During 2001, early data show that the ozone hole that develops each year over Antarctica has reached about the same magnitude as those of the past several years, according to scientists from the Commerce Department's National Oceanic and Atmospheric Administration (NOAA) and NASA.

Last year, the geographic area covered by the ozone hole was one of the largest on record. By early October, additional data will provide a more complete picture of the extent and intensity of this year's ozone hole in Antarctica.

Earth Probe TOMS ozone hole animation for 1996

This movie shows a sequence of daily images of the southern hemisphere using an orthographic projection looking down on the Earth from 60 degrees south latitude. The ozone hole is indicated by ozone values less than 220 Dobson Units (the violet colors).

The black circle in the middle is due to polar night, when TOMS cannot measure ozone. Notice that it's size decreases and then it disappears as the apparent position of the sun crosses over the equator (signaling the start of spring in the southern hemisphere) and more and more of Antarctica becomes sunlit. The other black gaps are due to the fact that Earth Probe is in a 500 kilometer orbit and cannot, therefore, provide full daily global coverage for ozone measurements within 60 degrees of the equator.

The discontinuity that is sometimes seen at the international date line (180 degrees longitude) is due to the fact that data on the east side of the line (negative longitudes) are taken up to 24 hours later than the west side of the line (positive longitudes).

For further animations and pictures check out the TOMS Multimedia Files.

The Antarctic ozone hole' recent behavior

The ozone hole made the top news spot during 2002. Here are links to CCN's report "Antarctic ozone hole splits in two", and The New York Times "For the Ozone Layer, a New Look".
2004 results and overview

Collaborative Global Government Efforts

As a result of the many concerns that a thinning ozone layer poses to society and the environment, the U.S. government and many international agencies have been relatively active in attempting to monitor, regulate, and solve the problem. Perhaps the most well known acts to help control the depletion of the ozone layer were the Montreal Protocol, and the London Ozone amendment to the Montreal Protocol. On September 14, 1987, delegates from 43 countries met to discuss threats of the thinning ozone layer. After much discussion, the delegates agreed to halt production and consumption of CFCs at 1986 levels by the year 1990. In addition, nations also agreed to reduce CFCs 20 percent by January 1, 1994 and an additional 30 percent by January 1, 1999. This was known as the Montreal Protocol. Even though this protocol helped the state of the ozone layer, the results were not significant enough. Thus, shortly after the implementation of the protocol, in 1990, it was amended. This amendment recruited more countries, bringing the total number involved to almost 100. The new goals were to eliminate the use of all CFCs by the year 2000, and to help set up a fund so that developing countries may find alternates to using CFCs. The name of this amendment was the London Ozone Agreement. Thus, many nations recognized the need for rapid and dramatic action in fighting the war with CFC responsible ozone depletion.