There are now many measurements and observations of the changes in ozone that occur over Antarctica. Such measurements come from ground based instruments at the Antarctica research stations, from aircraft during scientific missions and from satellites.
Ozone loss was first detected in the stratosphere over the Antarctic (see Part I). Although mid-latitude and Arctic depletion has also been observed, the loss is most dramatic in the lower stratosphere over the Antarctica continent, where nearly all the ozone is destroyed over an area the size of Antarctica within a layer in the lower stratosphere that's many km thick.
The graph to the right shows the measured total ozone above the Halley Bay station in Antarctica. Each point represents the average total ozone for the month of October. Note the sudden change in the curve after about 1975. By 1994, the total ozone in October was less than half its value during the 1970s, 20 years previous. This dramatic fall in ozone was caused by the use of man-made chemicals known as 'halocarbons' which include the well-known CFCs commonly used in fridges and so on. These CFCs had made their way into the upper atmosphere where the much stronger UV radiation from the Sun had broken them down into their component molecules, releasing the potentially damaging chlorine (and bromine) atoms, which, given the right conditions, could destroy ozone. We'll learn more about the chemistry behind the loss of ozone in Part III of this tour.
Regular ozone measurement have been made from the Halley Bay Research Station for many years. Ozone depletion is most marked in the Antarctic Spring, around October.
|Inline movie of TOMS ozone measurements
from Nov 1978 to Jan 1992|
|MPEG movie of TOMS ozone measurements
from Nov 1978 to Jan 1992|
The TOMS instrument measures ozone levels from the back-scattered sunlight, specifically in the ultra-violet range. It measures wavelength bands centred at 312.5, 317.5, 331.3, 339.9, 360.0 and 380.0 nanometres. The first four wavelengths are absorbed to greater or lesser extents by ozone; the final two are used to assess the reflectivity. The ozone levels computed are 'column ozone' (i.e. Dobson Units or DU for short).
During the Antarctic winter (May - July), data is unavailable near the pole, which is in total darkness.
For more information, do visit the TOMS Home Page.
It is important to appreciate that the atmosphere behaves differently from year to year. Even though the same processes that lead to ozone depletion occur every year, the effect they have on the ozone is altered by the meteorology of the atmosphere above Antarctica. This is known as the 'variability' of the atmosphere. This variability leads to changes in the amount of ozone depleted and the dates when the depletion starts and finishes. To illustrate this, the monthly averages for October, from 1980 to 1991, are shown below.
You can obtain a larger image of a particular year by clicking on the appropriate globe.
Part III. The Science of the Ozone Hole
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© 1998. Centre for Atmospheric Science, Cambridge University, UK. No text or graphics can be used or reproduced without explicit written permission. This version designed and maintained by Dr. Glenn Carver. Original concept and design Owen Garrett.