ozone layer

ozone layer Ozone is a blue-green poisonous gas that is composed of three oxygen atoms (O₃). It occurs in trace amounts throughout most of the atmosphere but it is most abundant in the stratosphere. Here, there is a thin layer between 15 and 40km altitude with a maximum concentration at around 25km (Fig.1). This ozone layer is believed to be most important to life on Earth because ozone absorbs the most carcinogenic part of the solar spectrum. This fraction of the solar spectrum has the wavelength range 280 to 315 nm and is referred to as ultraviolet-B (UV-B). Indeed, ozone is so efficient that, with the small amounts of oxygen that also occur in the stratosphere, no radiation with wavelengths less than 298 nm manages to reach the troposphere. It is believed that if the ozone layer is depleted, then more UV-B radiation will penetrate into the lower atmosphere and so be detrimental to life on Earth. Some of the many health risks associated with increased UV-B are higher rates of skin cancers and cataracts, and also possible suppression of the human immuno-defence system. In the plant world, growth, yield, and photosynthesis may all be reduced.

Stratospheric ozone distribution

The abundance of ozone is usually measured as the amount of it that exists in an entire atmospheric column rather than the amount at a particular altitude. The unit of measure is a Dobson unit, which is equivalent to 1 milli-centimetre (0.01mm) atmosphere of column ozone. The amount of stratospheric ozone varies naturally in both space and time. Production of ozone takes place mostly above 25km altitude by photochemical reactions. From here it moves to the lower stratosphere, where it is stored and redistributed by atmospheric winds. Measurements tell us that ozone is most abundant at the poles and least abundant at the Equator, even though creation of ozone takes place at the Equator and is succeeded by transport to the poles, where it is destroyed. Seasonal change in the distribution of ozone is greatest at the poles; little seasonal change occurs at the tropics. The maximum amount of ozone at the poles occurs during late winter and early spring. There are, however, distinct hemispheric differences: the maximum occurs nearer the pole and later in the season in the northern hemisphere than in the southern hemisphere. This is probably due to differences in the amount of ozone below 20km altitude as a consequence of hemispheric differences in the way that lower atmosphere weather systems affect the stratosphere. Short-term changes in stratospheric ozone are linked to weather phenomena that occur in the troposphere. There are also longer-term changes that are of importance. Approximately every 28 months the zonal flow in the tropical lower stratosphere reverses direction (the quasi-biennial oscillation or ‘QBO’). There are variations in the amount of ozone linked to the QBO, the solar sunspot cycle, and solar flares. The Junge layer, an aerosol layer of sulphate that exists in the stratosphere, is believed to help in the destruction of ozone. Volcanic eruptions will enhance this layer. This short discussion shows that there are several interlinked processes that affect the amount and distribution of ozone, namely chemistry, dynamics, and radiation.

Photochemical reactions in the upper stratosphere are common because of the influx of short-wave radiation. The lifetime of a chemical is determined by the reaction rate, which itself is temperature-dependent. If the lifetime of the chemical is short compared to the time needed for the atmospheric winds to move the chemical (the transport time), then photochemical equilibrium can occur; this is the case for ozone above 25km. There is a constant cycle of creation and destruction of ozone, and in the 1930s the first attempt at describing these photochemical reactions was made. It was recognized at the time that a catalyst was needed in the creation and destruction cycles. Since then, further cycles of destruction resulting from catalysts have been identified, most notably those due to hydrogen oxides, nitrogen oxides, and chlorine oxides.

Although the vertical distribution of ozone can be principally explained by photochemical reactions, its geographical distribution relies heavily on dynamical processes. This is because in the lower stratosphere the lifetime of ozone becomes comparable with that of the transport time, and so the atmospheric winds become important. In the stratosphere, because ozone absorbs ultraviolet radiation, the temperature increases with height. This temperature structure means that there are a few vertical motions. Aerosols and gases injected into the stratosphere remain for a long time. The extreme dryness of the stratosphere enhances this feature and there is little opportunity for material to be rained out. As a consequence, large accumulations can occur. The zonal flow is extremely strong, and in winter wavelike structures can occur at high latitudes.

The polar vortex is not only an important feature of the stratospheric winds but also of considerable significance to ozone depletion, which is discussed below. As winter approaches, the long polar night begins and temperatures begin to fall. Centred close to the winter pole, a vortex forms which effectively isolates the stratospheric air over the polar region during the polar night. Temperatures in the vortex can reach below −78 °C in the lower stratosphere. These low temperatures allow the formation of polar stratospheric clouds. Those which form at −88 °C are composed of water-ice, but at −78 °C nitric acid trihydrate clouds form. The vortex that forms over the southern hemisphere pole is much more intense than that over the northern hemisphere. This is because of the position of Antarctica, isolated from surrounding land masses with a circumpolar ocean current that cuts it off from warmer water. In the atmosphere the large-scale planetary waves that occur in the upper troposphere rarely penetrate into the stratosphere over Antarctica, but they frequently disrupt the polar vortex over the Arctic. When the Sun returns in spring the polar vortex breaks down. This happens quite rapidly and is called the final stratospheric warming event. The polar vortex breaks into segments that drift equatorward. The final warming usually occurs in early spring in the northern hemisphere, but it can be very late in the season in the southern hemisphere. The intensity of the polar vortex responds to changes in the QBO. An easterly flow seems to weaken the polar vortex.

Ozone depletion

In the early 1970s concern began to mount that human activity might deplete the ozone layer. At first the chief concern was from the then proposed fleets of supersonic aircraft, such as Concorde. These supersonic planes would fly in the stratosphere, and combustion products such as water vapour and nitrogen oxides would then be deposited there. As mentioned above, the stability of the stratosphere would allow the nitrogen oxide to accumulate and then enhance the catalytic destruction of the ozone layer. The large number of supersonic planes envisaged were never built, but in 1974 came a new worry: that chlorofluorocarbons (or CFCs), a synthetic product, might also be capable of depleting the ozone layer. CFCs had been used commercially as refrigerants since the 1930s, but because they were inert and non-poisonous they were soon used for many other applications, most notably as propellants in aerosol spray cans. By the 1970s they were a measurable quantity in the troposphere worldwide, and because they were so inert in the troposphere they had a long enough lifetime to penetrate into the stratosphere. It was suggested by two scientists, M. J. Molina and F. S. Rowland, that in the stratosphere CFCs could be broken down by ultraviolet light. The chlorine released from the CFC would then be capable of destroying ozone.

Although there were many criticisms of the theory, it was quickly accepted. The level of ozone destruction predicted from models at the time was a 13 per cent decrease in the ozone column in 50 to 100 years. However, even though this initial figure rose to 19 per cent in 1979, successive model predictions down-rated the level of ozone destruction until by 1982 it was less than 5 per cent. CFC use, which had declined after the initial reports, subsequently began to increase again. Then in 1985 it was reported that surface observations taken by the British Antarctica Survey indicated a substantial thinning of the ozone layer over Antarctica in early spring. Satellite observations from NASA confirmed the report. It is now widely accepted that the ozone hole, as it became popularly known, is caused predominantly by the chlorine from CFCs and other anthropogenic sources. The depletion of the ozone layer first manifested itself over Antarctica because of the wintertime polar vortex. The polar stratospheric clouds that form in the extreme cold provide a surface on which the chlorine from CFCs converts from a stable to a reactive component. Once the sunlight arrives in spring, the chlorine is released and is free to attack the ozone layer. As spring progresses, the vortex breaks up and ozone-rich air can penetrate the region. The extent of the thinning was initially linked to the QBO, being greatest when the vortex was strongest. This, however, appears to be no longer the case, and it has been postulated that the change is due to increasing levels of chlorine. Measurements now indicate significant thinning in the northern hemisphere polar regions and at mid-latitudes.

Frances Drake

Bibliography

Fisher, M. (1992) The ozone layer. Chelsea House Publishers, New York.
Roan, S. L. (1989) Ozone crisis: the 15-year evolution of a sudden global emergency. John Wiley and Sons, New York.