Ozone is one of the several gases that make up the earth's atmosphere. It is the triatomic form of oxygen and makes up approximately one part in three million of all of the gases in the atmosphere. If all the ozone contained in the atmosphere from the ground level up to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 millimetres thick, and weigh some 3000 million tonnes. Ozone is toxic at high concentrations because it reacts strongly with other molecules.

Frequently Asked Questions:

• Where is ozone found?
• Why is ozone important?
• How is ozone measured?
• What is a Dobson unit?
• Is ozone being depleted?
• Is ozone the same as the greenhouse effect?
• What is causing the depletion?
• What is the ozone hole?
• When did the ozone hole first appear?
• When does the ozone hole occur?
• How large is the ozone hole?
• How deep is the ozone hole?
• Why does the ozone hole occur over Antarctica?
• Does the ozone hole ever come over Australia?
• How long have we been measuring the ozone hole?

About 90% of ozone is concentrated between 15 and 30 kilometres above the earth's surface (stratospheric ozone). It is also found at ground level in lower concentrations where it is a key component of smog over major cities (tropospheric ozone).

Ozone is the major absorber of UVB (Ultraviolet radiation in the wavelength range 280-320 nanometres) in sunlight, absorbing approximately 90% of it. Many experimental studies of plants and animals, and clinical studies of humans have shown the harmful effect of excessive exposure to UVB radiation.

Australia takes part in the Global Atmosphere Watch (GAW) international monitoring and research program coordinated by the World Meteorological Organization (WMO). The Bureau operates a network of ozone monitoring stations, where the total amount of ozone above the station (the total column ozone) is monitored each day. The total column ozone is measured with an instrument called a Dobson spectrophotometer, compares the amount of sunlight at two ultraviolet wavelengths. One wavelength is affected strongly by ozone, the other is not. The difference between the two is directly related to the total column ozone.

Polar orbiting satellites are now used now give a map of total column ozone over the entire globe. The Dobson spectrophotometers are used to check the results from the satellites, to extend existing long records of ozone monitoring (up to almost 40 years), and to provide a near simultaneous reference measurement for vertical profile measurements using ozonesondes. To get the vertical profile of ozone through the atmosphere an ozonesonde is generally used. An ozonesonde is an electrochemical cell and radio transmitter attached to a hydrogen filled balloon that can reach a height of about 35km. Air is drawn through the cell by a small pump. The solution in the cell reacts with the ozone, producing a small electric current that is directly proportional to the ozone amount. The signal from the cell is converted into a code and transmitted via radio to a receiving station. The Bureau conducts weekly ozonesonde flights from Laverton, near Melbourne. A pilot weekly program, funded by the Cooperative Research Centre for Southern Hemisphere Meteorology, is being conducted at Macquarie Island commencing in December 1996. Dobson spectrophotometers can also be used the derive vertical ozone profiles, the results of which do not have the detail of the ozonesonde. This technique is used at Brisbane, Darwin and Perth.

Ozone is measured in Dobson Units (DU). 100 DU is equivalent to a 1 millimetre thick layer of pure ozone at sea level temperature and pressure.

Ground based and satellite measurements show significant decreases in total column ozone in the middle to high latitudes of both hemispheres. The downward trends were larger in the 1980s than in the 1970s, and larger in the 1990s than the 1980s.

The greenhouse effect can be described as an altered balance between input and output radiation of the Earth. If the input is greater than the output, the difference causes the temperature of the Earth to increase. Ozone in the lower atmosphere (tropospheric ozone) is a minor greenhouse gas. In general the greenhouse effect and ozone depletion are separate issues, although they are connected in complex ways.

Overwhelming scientific evidence accumulated over more than two decades of study by the international research community has shown that human-made chemicals are responsible for the observed depletion in the ozone layer over Antarctica and likely play a major role in stratospheric ozone losses. The ozone depleting compounds contain combinations of the elements chlorine, fluorine, bromine, carbon and hydrogen. The general term for them is HALOCARBONS. Compounds that contain only carbon, chlorine and fluorine are called CHLOROFLUOROCARBONS (CFC). Compounds that carbon, bromine, fluorine (and sometimes chlorine) are called HALONS.

The commonly accepted definition of the ozone hole, is the region above Antarctica where the ozone level is below 220 DU.

The British Antarctic Survey first revealed the obvious decreases in ozone in the late 1970s to early 1980s. The Japanese station at Syowa also reported decreases in ozone.

The ozone hole starts to appear in September and breaks up in December.

The ozone hole grew in size through the 1980s and plateaued in the early 1990s. In the last three years the ozone hole has covered an area of up to 26 million square kilometres, nearly 3 times the area of Australia.

In 1993 and 1994, total column ozone over the South Pole fell below 100 DU and in 1995 and 1996 it was almost this deep.

Human emissions of CFCs occur mainly in the Northern Hemisphere. Gases such as CFCs which are insoluble in water and relatively unreactive are mixed within a year or two throughout the lower 10 kilometres of the atmosphere (the troposphere). The CFCs then rise from the lower atmosphere into the stratosphere, mainly in the tropics. Winds then move this air poleward - both North and South - from the tropics. The meteorologies of the two polar regions are very different. The South Pole is part of a very large land mass that is completely surrounded by ocean. These conditions produce a very cold stratosphere which leads to the formation of clouds. The clouds that form lead to chemical changes that promote rapid ozone depletion. The North Pole lacks the land/ocean symmetry of the South Pole. As a consequence the stratospheric air is much warmer and fewer clouds form. Therefore the ozone depletion in the Arctic is very much less than in the Antarctic.

After the ozone hole has broken up parcels of ozone depleted air mixed with mid latitude air move north. These parcels can move over the southern part of Australia, though their effect is far less pronounced than the effect of the ozone hole.

The CSIRO began ozone measurements in Melbourne in 1956 in cooperation with the Bureau of Meteorology and in 1982 total responsibility passed to the Bureau.

References

The above information was compiled by the Ozone Monitoring Unit, Bureau of Meteorology from the following references:

• Scientific Assesment of Ozone Depletion: 1994 (World Meteorological Organization, Global Ozone Research and Monitoring Project - Report No. 37)
• The Ozone Layer (Bureau of Meteorology Publication 1992)
• Depletion of the Ozone Layer (Bureau of Meteorology Ozone Science Unit Publication 1991)