THE OZONE LAYER
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, with roughly 7% of its energy output 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 ultraviolet 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 per cent. 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 mankind has 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.
Frequently asked questions:
Selected web sites with background and related material