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Atmospheric Ozone


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Wrapped around the Earth is the atmosphere, which has evolved over the course of billions of years. Without this protective blanket, life as we know it would be impossible. The thickness of the atmosphere, though, is in proportion to the thickness of the skin on an apple, and it is more fragile than we would like to imagine.

Figure 1. The ozone hole as seen by satellite. (Courtesy of Dr Arlin Krueger. NASA)

Atmospheric chlorine and the Antarctic ozone hole

Over the past century, scientists have begun to realise that human beings are playing a significant role in modifying the atmosphere's composition. In 1985 they discovered that a large amount of ozone--more than half--was being lost from the Antarctic stratosphere each spring following the return of the sun to the polar cap. The scientific community was astounded, and many theories were advanced as to why this might be.

For 3 years scientists in several countries conducted intensive research in an attempt to identify the cause of the problem. They now know that chlorine, used by industry in chlorofluorocarbons (CFCs), is responsible. Many solvents, refrigerants, and spray-can propellants contain CFCs. It has been estimated that more than half a million tonnes of chlorine is released into the atmosphere each year. In the lower atmosphere, CFCs are very stable--dangerously stable, as it turns out, because once there, they may be lost only by climbing to higher levels where they are broken down by sunlight, which might take a hundred years. Once in the stratosphere, each chlorine molecule can destroy thousands of ozone molecules.

Scientists have found that chlorine from CFCs is stored on stratospheric cloud surfaces during the long polar night and then released to attack ozone when the sun returns in spring. The process involves a series of unusual chemical reactions taking place at temperatures as low as -85 degrees C (188K). Normal ozone levels are restored in the summer when the Antarctic atmosphere, isolated during the winter inside a cold "polar vortex", becomes mixed with ozone-rich air from other latitudes.

Satellite data shows that between 1978 and 1985--at least between latitudes 53 degrees North and 53 degrees South--the total amount of ozone around the world declined by about 2.5%. It is too early to say; though, whether this was due to damage by chlorine or whether the normal "solar cycle" decline in the output of the sun contributed to it. Even when natural causes are allowed for, however, data from ground stations in the northern hemisphere show a decrease there of 1.7% to 3.0% in total ozone between 1969 and 1986. Very recent research shows that an ozone hole may also be developing in the Arctic.

But why the fuss? Top

Ozone is a trace gas. Although the concentration of trace gases in the atmosphere may be only a few parts in a million million (parts in 10^12), their presence can have a profound effect on the environment. They do this either by their direct chemical effects or by absorbing radiation in important regions of the electro-magnetic spectrum.

Figure 2. (a) Energy received and transmitted by the earth. (b) Absorption by various gases (after Houghton).

Ozone absorbs harmful radiation from incoming sunlight in the ultraviolet (UV) part of the spectrum. Scientists have calculated that a 1% loss in ozone will result in a 2% increase in damaging W. A reduction in ozone, then, would allow more W to penetrate to the Earth's surface, with the following possible consequences--

Ozone also absorbs infrared radiation travelling from the Earth back to space. Along with carbon dioxide, methane, and water vapour, ozone is known as a "radiatively active" or "greenhouse" gas because some of the energy it absorbs is re-radiated to warm the earth even further. A change in the level of ozone, therefore, will change the temperature balance of the atmosphere which would have largely unpredictable effects on our climate. Chlorofluoro- carbons absorb infrared radiation too. Their build up may also increase global warming and contribute to the phenomenon known as the "greenhouse effect", although the part they and ozone play here is small compared to that of gases like CO2.

Ozone production and loss Top

If all the ozone in a column above the earth was subjected to normal temperature and pressure, it would form a layer only 3 mm thick. About 90% of ozone in the atmosphere is in the stratosphere--the region between about 15 km and 50 km above the earth.

Figure 3. Temperature and ozone concentration changes with height.

It is important to understand how ozone is created and destroyed to maintain a delicate balance.

Sunlight breaks down oxygen molecules to form atoms--

O2 + hv = O + O

which can combine with O2 to create ozone, O3--

O + O2 + M = O3 + M + 100 kJ **

Ozone can be destroyed like this--

O + O3 = 2O2 + 390 kJ **

or through a very important set of reactions involving oxides of hydrogen, nitrogen, and chlorine. These gases take part in the "catalytic" destruction of ozone; they destroy it without themselves being lost.

Figure 4. How ozone is destroyed by chlorofluorocarbons.

It can be seen that the reactions ** liberate a large amount of heat, and because more heat is generated higher up (more O atoms) the temperature of the ozone layer increases with height (see Figure 3). This is a very stable situation which restricts mixing of gases by convection, so that the stratosphere effectively "puts a lid " on the atmosphere.

The amount of ozone in the column above the earth is not just dependent on photochemistry (i.e., the interaction between light or "radiant energy" and gas molecules), however. Ozone is redistributed vertically and horizontally by both small-scale and global-scale winds and atmospheric tides. An image from the Total Ozone Measuring System (TOMS) on the NIMBUS satellite shows in Figure 1, for instance, a large "high" in ozone in the vicinity of New Zealand that is caused by a redistribution of ozone by atmospheric waves.

So what can we do? Top

It is clear that, because chlorine is potentially dangerous to the stratosphere, any unnecessary use of chlorofluorocarbons should be stopped. You can help by checking labels on spray cans to see that chlorine compounds are not involved.

Because ultraviolet radiation is damaging to DNA--a vital component of living material--you should avoid unnecessary exposure to high levels of it. Take precautions in summer: wear a sunhat and use protective creams, especially if you have fair skin.

As scientists have identified the ozone hole, the greenhouse effect, and acid rain, we have become more aware of the extent of the large-scale damage people have done to the Earth. For the first time, nations of the world have become conscious of the need to identify~ and reduce these common threats. In New Zealand, local government legislation has been introduced on pollutants and the Government has signed an international agreement designed to protect the ozone layer. A greater awareness of our environment will certainly improve our well-being, and that of generations to follow.

How do scientists measure trace gases? Top

Absorption of light by gases provides us with a way of measuring their amounts in the atmosphere. Each gas has a characteristic absorption "fingerprint" which tells us that it is there:

Figure 5. Absorption of visible light by NO2.

We are able, then, to set up a spectrograph on the ground and, by comparing the recorded atmospheric spectrum with those of laboratory controls, calculate how much of each gas is in the light path of the column above us. Instruments designed to measure trace gases in the stratosphere in this way have been installed by NIWA staff in New Zealand and at several bases in Antarctica. As well, infrared spectrometers are operated in cooperation with overseas groups to measure compounds in the stratosphere such as nitric and hydrochloric acids.

Chemically sampling "sondes" may also be flown up through the atmosphere on balloons, and information on pressure, temperature, and ozone concentration is transmitted back to the ground station, to reveal the ozone "profile" with height.

It is known from such measurements, and also those from satellites, that the amount of ozone above a particular location can change from day to night, with the season, and with the passage of weather frontal systems. Variations like this tend to mask any changes due to human interference, such as those predicted to result from the release of chlorine compounds into the atmosphere.

Gordon Keys,

National Institute of Atmosphere and Water

Links to Ozone related webpages Top