Ozone is a compound of oxygen that contains three atoms instead of the two found in the oxygen gas that sustains life. It was discovered in 1839 by a Swiss chemist, Christian Friedrich Schonbein. In high concentration ozone is a bluish green gas, with very strong oxidising properties. It is a toxic, irritating gas, often encountered in surface air pollution episodes, when it can trigger asthma and irritate mucous membranes. Dry air consists of 78% nitrogen and 21% oxygen and there are normally trace amounts of other gases, principally argon, water and carbon dioxide, present. The concentration of ozone is usually only a few parts per million and even in the ozone layer it is only one part in 100,000.
Ozone concentrations at the surface were first measured reliably by Robert Strutt (later 4th Lord Rayleigh) in 1918 using spectra of a hydrogen lamp recorded through five kilometres of air. These measurements showed that ozone concentration could not be uniform throughout the atmosphere as a higher concentration was required to explain the sharp cut off at around 300 nanometres (nm) seen in stellar spectra. Three years later Fabry and Buisson used spectrographic techniques to demonstrate that its principal atmospheric location is in the stratosphere, though it was not until the 1930s that the actual vertical distribution was first measured. It was soon recognised that measuring the variation in the total ozone column was of meteorological interest and Professor G M B Dobson developed a prototype ozone spectrophotometer in the 1920s. His instrument is still the standard today and around 120 Dobson spectrophotometers have been built.
The Sun emits radiation in all parts of the electromagnetic spectrum, roughly 7% in the ultraviolet between 200 and 400 nm, 41% in the visible between 400 and 760 nm and 52% in the infra red. The ultraviolet part of the spectrum is further divided into UV-A, UV-B and UV-C. UV-A lies between 315 and 400 nm and gives rise to a suntan and ageing of the skin. UV-B lies between 280 and 315 nm and is the damaging part of the spectrum. UV-C, which is totally absorbed by the atmosphere before it can reach the ground, lies between 200 and 280 nm.
Dobson’s instrument measures ozone by comparing the intensities of two wavelengths of ultra-violet light from the Sun, one of which is absorbed quite strongly by ozone, whilst the other is only weakly absorbed. The ratio of the intensities varies with the amount of ozone present in the atmosphere, and a well-calibrated instrument can measure ozone amounts to within a few percent. The instrument uses wavelengths between 305 and 340 nm and these are selected by means of prisms and a series of slits. It was initially a photographic instrument, but photocells were introduced in the mid 1930s and a photomultiplier in 1946.
Ozone is created in the upper stratosphere by the photo-dissociation of an oxygen molecule, which liberates a free oxygen atom and this can then combine with another oxygen molecule to create ozone. The dissociation of the oxygen molecule requires ultraviolet light of wavelength shorter than 240 nm. Ozone itself can be dissociated by light of wavelength shorter than 1100 nm. The free oxygen atom thus created quickly finds another oxygen molecule and the ozone is reformed with the net result of absorbing the solar radiation and inputting the energy into the atmosphere as thermal energy. The process is very efficient and virtually all radiation between 200 and 310 nm is absorbed, despite the relatively low concentration of ozone. The main ozone absorption bands in the ultraviolet are the Hartley (around 200 – 300 nm) and Huggins (around 300 – 350 nm), and there is the weak Chappuis band in the visible (440 – 740 nm). In the lower stratosphere, below about 30 km, ozone has a long lifetime, and the ozone mixing ratio can be used to trace atmospheric motions.
In the normal state of affairs the creation and dissociation processes run in balance and a typical value for the total amount of ozone in a vertical column of our atmosphere is around 300 Dobson Units (DU), or 300 milli-atmosphere-centimetres, which corresponds to a layer of ozone 3 mm thick at the Earth's surface. This 3 mm is in reality spread through the column, with the bulk of it lying between the tropopause, at 10 to 12 km altitude, and 40 km, with a maximum at around 17 to 25 km altitude depending on location. This is the ozone layer.
Over the last 50 years we have introduced chemicals into the atmosphere that are capable of destroying ozone through photochemical processes. Chloro-fluoro-carbons (CFCs) are widely known, but there are also other ozone depleting substances such as halons (bromo-fluoro-carbons) and methyl bromide. In certain circumstances the chlorine or bromine from these substances can react with ozone to turn it back into oxygen. In most parts of the world the reactions are very slow and there is little damage to the ozone layer, however over the Antarctic a dramatic hole opens in the ozone layer every spring and fills in again by mid-summer. This is created by the unusual atmospheric conditions that exist during the Antarctic winter.
An international treaty, the Montreal Protocol, has been drawn up to control the release of ozone depleting chemicals into the atmosphere. This treaty is clearly working, and the amount of these chemicals in air near the surface is beginning to decline. The chemicals are however so stable that it will take a long time before they drop to the levels that existed 50 years ago and it is likely that we will see an annual ozone hole over Antarctica for many decades to come.
Why does the ozone hole form over Antarctica ? The answer is essentially 'because of the weather in the ozone layer'. In order for rapid ozone destruction to happen, clouds (known as PSCs, Stratospheric Clouds Mother of Pearl or Nacreous Clouds) have to form in the ozone layer. In these clouds surface chemistry takes place. This converts chlorine or bromine (from CFCs and other ozone depleting chemicals) into an active form, so that when there is sunlight, ozone is rapidly destroyed. Without the clouds, there is little or no ozone destruction. Only during the Antarctic winter does the atmosphere get cold enough for these clouds to form widely through the centre of the ozone layer. Elsewhere the atmosphere is just too warm and no clouds form. The northern and southern hemispheres have different 'weather' in the ozone layer, and the net result is that the temperature of the Arctic ozone layer during winter is normally some ten degrees warmer than that of the Antarctic. This means that such clouds are rare, but sometimes the 'weather' is colder than normal and they do form. Under these circumstances significant ozone depletion can take place over the Arctic, but it is usually for a much shorter period of time and covers a smaller area than in the Antarctic.
Is the ozone hole recovering ? Some reports in the media suggest that the ozone layer over Antarctica is now recovering. This message is a little confused. Recent measurements at surface monitoring stations show that the loading of ozone destroying chemicals at the surface has been dropping since about 1994 and is now about 6% down on that peak. The stratosphere lags behind the surface by several years and the loading of ozone depleting chemicals in the ozone layer is at or near the peak. Satellite measurements show that the rate of decline in ozone amount in the upper stratosphere is slowing, however the total ozone amount is still declining. The small size of the 2002 ozone hole was nothing to do with any reduction in ozone depleting chemicals and it will be a decade or more before we can unambiguously say that the ozone hole is recovering. This assumes that the decline in ozone depleting chemicals continues and that there are no other perturbations to the ozone layer, such as might be caused by a massive volcanic eruption or Tunguska like event. It will be the middle of this century or beyond before the ozone hole ceases to appear over Antarctica. What we saw in 2002 is just one extreme in the natural range of variation in the polar stratosphere and is the equivalent of an extreme in 'stratospheric weather'. By contrast the 'weather' in 2003 moved to the opposite extreme and we saw one of the largest ozone holes on record.
Global warming and the ozone hole. The ozone hole is a completely different phenomenon to global warming, however there are links between them. The ozone hole is caused by ozone depleting chemicals in the atmosphere, which have been produced by industry, for example CFCs. One link is that CFCs are also 'greenhouse gasses'. Enhanced global warming is a probable consequence of increasing amounts of 'greenhouse gasses', such as carbon dioxide and methane, in the atmosphere. Although the surface of the earth warms, higher up the atmosphere cools, thus increasing the area where stratospheric clouds can form. This makes a larger area susceptible to ozone depletion and provides another link between the two issues.
Nacreous clouds or mother of pearl clouds. Occasionally stratospheric clouds can be seen from the UK, normally during the late winter and just after sunset or before sunrise. A display was widely seen across the UK on the evening of February 16 1996. These clouds form in the stratosphere, at heights of between 10 and 30 km, when the temperature there falls below -80°C and are probably composed of ice particles with a liquid coating of nitric acid tri-hydrate. They appear bright because they are high enough to be illuminated by the sun long after local sunset and the pastel colours arise through diffraction or interference effects in much the same way that colours appear in a film of oil on a puddle of water. Occasionally seen from Scotland during the winter months, they are a once in a lifetime sighting from southern England. They are more frequently seen from the southern hemisphere, particularly from locations along the Antarctic Peninsula where the mountains create lee-waves in the upper atmosphere.