The Antarctic ozone hole

A new printed edition of this pamphlet has just been published. Copies can be obtained from the BAS publicity section.

Antarctica is a huge continent over 50 times the size of Great Britain. It rises from sea level at the coast to over 3000 metres inland. There are three British bases, which are at coastal sites, but only two of these make ozone measurements. Other nations carrying out research on the ozone hole in the Antarctic include Argentina, Finland, France, Germany, Italy, Japan, New Zealand, Russia, the Ukraine and the U.S.A. Ozone measurements in the Antarctic started in the mid 1950s and the longest continuous records are from Faraday and Halley.

Our atmosphere consists of nitrogen (78%) and oxygen (21%) and a small amount of other gases. Ozone (a molecule with three oxygen atoms) is quite a rare gas: even in the ozone layer there is less than one ozone molecule for every 100,000 molecules of air. Since ozone is a toxic, irritating gas, it is fortunate that its concentration at the Earth's surface is much lower than in the ozone layer. In the polluted air from large cities, ozone may be present in higher concentrations and this can cause severe health problems. In its proper place higher in the atmosphere, ozone provides a safety screen against harmful ultra-violet light from the Sun, which can cause sun burn, skin cancers and cataracts. The ozone layer also controls the temperature structure of the upper atmosphere because it can absorb radiant energy from the Sun.

Ozone has been measured from Halley and Vernadsky Research Stations (Vernadsky was the British station Faraday until 1996) for over 35 years using a Dobson spectrophotometer. This instrument compares the intensities of two wavelengths of ultra-violet light from the sun. One wavelength is strongly absorbed by ozone and the other is only weakly absorbed. Once the instrument has been calibrated, the ratio of the intensities tells us how much ozone there is in the atmosphere. Because the instrument normally uses the sun as a source of ultra-violet light it is not possible to make regular measurements of ozone during the dark Antarctic winter.

The amount of ozone in the atmosphere is measured in milli-atmo-centimetres (Dobson Units, DU) and a typical measurement is about 300 DU. This means that if you took all the ozone in a vertical column above the instrument and brought it down to sea level it would form a layer just three millimetres thick. However, the ozone is not actually confined to a narrow layer in the atmosphere, but is spread throughout the atmosphere, mostly between 10 and 50 km above the ground. Graphs show where it is to be found, both before and after the formation of the ozone hole.

The data up to the mid 1970s show that ozone at Halley was typically around 300 DU through the winter until late October (spring in the Antarctic). It then rose rapidly to about 400 DU by the beginning of December and slowly declined to reach 300 DU again by March. Since the 1980s, however, a seasonal decrease or "hole" has appeared in the Antarctic ozone layer each spring, with values at Halley now dropping below 100 DU. Data from Faraday show a similar general picture, but there is a much greater day-to-day variation than at Halley, with a periodic fluctuation in the ozone amount. The mean October ozone values at Halley show a rapid decline, which is just beginning to level out.

The ozone is being destroyed because of the release of chlorofluorocarbons (CFCs), mostly in the northern hemisphere. These spread throughout the world and diffuse into the stratosphere, where they are broken down to release chlorine. During the Antarctic winter a strong westerly circulation around the continent, known as the circumpolar vortex, builds up in the stratosphere. This effectively cuts off the interior and allows it to cool, with temperatures falling below -80° C at 17 km. Thin clouds form, which enable reactions with gases which contain chlorine to take place. When the sun returns in the spring, the chlorine is able to take part in complex catalytic chemical reactions which destroy ozone and create the ozone hole. When the stratosphere warms up again during the late spring and summer, these reactions cease, the circumpolar vortex breaks down and the ozone hole disappears as fresh ozone is brought in.

Unlike Antarctica, which is a continent surrounded by oceans, the Arctic is an ocean surrounded by mountainous continents. This means that the stratospheric circulation is much more irregular, and the temperature does not fall as low as it does in the Antarctic. Stratospheric clouds are therefore less common, which prevents the formation of a deep ozone hole over the Arctic.

Every day at Halley a balloon is launched, carrying meteorological instruments. The instrument package signals back the temperature, humidity and pressure to an altitude of over 20 km. The temperature near the height of the ozone maximum (about 17 km) is strongly linked with the total amount of ozone in the atmosphere, and this gives us additional information on the state of the ozone layer. The graph shows that temperatures at 17 km during the 1996 Antarctic spring remained cold for much longer than normal. That year had one of the deepest ozone holes yet observed.

The vertical distribution of ozone in the atmosphere is measured by launching a balloon which carries a chemical sensor, known as an ozone sonde. This uses the reaction between ozone and a solution of potassium iodide to generate an electric current. The strength of the current modulates a signal which is transmitted back to the ground. These measurements show that the normal maximum in the ozone layer at 17 km is replaced by a deep minimum during the spring ozone hole. Most of the ozone between 14 and 20 km altitude is destroyed, but when the circumpolar vortex breaks down the hole fills in, as the 1996 sonde data from the German Neumayer station show.

From space a more complete picture can be seen. Several satellites carry sensors which make global measurements of atmospheric ozone and other trace gases. Maps made from these measurements clearly show the "hole" as it forms in the spring and subsequently recovers in the summer. They also show why the ozone amount at Faraday varies in a regular manner: as the air rotates around the pole, the station is sometimes inside the vortex in ozone-poor air and sometimes outside in ozone-rich air.

A major international study of the Antarctic ozone hole took place during the southern spring of 1987. Many flights were made in specially equipped high-altitude research aircraft from Punta Arenas in southern Chile, along the Antarctic Peninsula and towards the South Pole. Lidar (laser radar) and microwave experiments from McMurdo and Amundsen-Scott stations also probed the ozone layer. The measurements showed conclusively that chlorine plays a major role in forming the ozone hole. In the northern spring of 1989 similar studies took place over the Arctic. More recent studies from the Space Shuttle and satellites specially designed to study the atmosphere show that CFCs are the source of the chlorine and the clear link between it and ozone depletion.

The US has set up a UV Irradiance Monitoring Network at unpolluted sites throughout the world and data from the network clearly shows that increases in UV are associated with decreases in ozone. Some recent studies have linked a decrease in phytoplankton production in the marginal ice zone of the Southern Ocean with the decrease in ozone associated with the Antarctic ozone hole. Other studies on the biological effects of ozone depletion are still in progress.

BAS has set up a special unit, the Meteorological and Ozone Monitoring Unit (MOMU) to conduct long term studies of the atmosphere. BAS scientists within MOMU continue to study the spring depletion of ozone using the Dobson spectrophotometer in the Antarctic. Work in collaboration with colleagues at British and overseas universities is helping to develop theoretical models of the structure and dynamics of the ozone hole. Laboratory experiments help to improve our knowledge of the chemistry that is taking place in the stratosphere. A novel star-tracking spectrometer, deployed at the Halley Research Station, studies the trace chemicals involved in the development of the ozone hole, stratospheric clouds, and ozone itself.