stratosphere

stratosphere The stratosphere is the atmospheric layer between the troposphere and the mesosphere and extends from about 12 km altitude to about 50 km. Within this layer the temperature rises with altitude to a maximum of about 0 °C at 50 km. A region in the atmosphere in which temperature increases with height is very stable in relation to vertical movement. Even large thunderstorms, which can produce updraughts of several metres per second, cannot penetrate far into the stratosphere. Stratosphere literally means ‘layered sphere’, and air in one layer remains in that layer without significant upward or downward motion. This contrasts strongly with the troposphere, where vertical overturning of air is the norm. In the troposphere, any small dust particles in the air are swept around rapidly and can fall back to the surface in a matter of days or weeks. In contrast, dust particles injected by very large volcanic eruptions can reside in the stratosphere for a year or more before slowly falling back to the troposphere. Gases that reach the stratosphere can remain there for many years, particularly if they are chemically inert.

Although 90 per cent of the mass of the atmosphere lies below the stratosphere, and the chemical composition of the stratosphere is almost exactly the same as the troposphere, the stratosphere has enormous significance because it contains minute amounts of ozone. Even at 25 km altitude, where the maximum concentrations of ozone are to be found, only about ten molecules in every million are ozone. However, the temperature structure of this part of the atmosphere, the radiation received at the Earth's surface, and, ultimately, the fate of many living things depend crucially on these ozone molecules.

Ozone absorbs ultraviolet (UV) light from the Sun. Ultraviolet light has a wavelength of less than 0.39 μm (micrometres). What happens to UV light that falls on the top of the atmosphere depends on its wavelength. If the wavelength of the UV light is less than 0.246 μm, it is capable of splitting oxygen molecules (O₂) into their two component oxygen atoms (O + O). Nearly all of this very short UV light is absorbed in the thermosphere at altitudes above 80 km, but a very small amount reaches the stratosphere and separates oxygen molecules there (Fig. 1). The odd oxygen atoms (O) can then combine with some of the many oxygen molecules (O₂) to form a molecule with three oxygen atoms: ozone (O₃). Some of the oxygen atoms can combine with other single oxygen atoms to form O₂ molecules, but, because oxygen molecules are much more common, ozone is more likely to be formed. These reactions in which oxygen combines to form either O₂ or O₃ cannot occur without another molecule also being involved. The role of this extra molecule, which can be of any kind but is most likely to be a nitrogen molecule, is to carry away excess energy produced during the combination. UV light at wavelengths less than 0.31 μm can break up ozone molecules into a single oxygen atom and an oxygen molecule (O + O₂). The net effect of all these processes is that ozone is being formed and destroyed continuously. A fine balance has been achieved during the evolution of the atmosphere, resulting in a layer in which a small amount of ozone is concentrated (if such a word can be used for concentrations of ten parts per million) in a layer between 20 and 30 km above the Earth's surface. UV light is absorbed in the process that leads both to the formation and destruction of ozone. The energy carried by that light is left in the stratosphere where the light is absorbed, and it is this energy that is responsible for the increase in temperature with height above the troposphere that is observed.

Why should the ozone layer exist at exactly the height it does? The same delicate balance that maintains the reactions which produce and destroy ozone is the reason why the ozone layer peaks at 25 km altitude. At greater altitudes there is more UV light, and more single oxygen atoms are produced. However, the air is less dense and the probability of three molecules colliding to form ozone is less than the probability that an ozone molecule will combine with a single oxygen atom to produce two oxygen molecules (O + O₃ Æ 20₂). This both destroys ozone and removes some of the single oxygen atoms needed to form ozone. At altitudes below the ozone peak very little UV radiation is found because it is absorbed by the ozone above. As a result, both creation and destruction of ozone below the peak in the ozone layer occur more slowly than above, and the observed distribution of ozone at these levels is determined largely by atmospheric motions.

The reactions which maintain the equilibrium of the ozone layer are more complicated than this relatively simple explanation because more minor constituents in the air are involved in reactions. However, before the late 1970s, all observations of the ozone layer suggested that it was in a stable equilibrium state in which the ozone production processes were balanced exactly by ozone destruction.

The ozone hole

Human activities have disturbed the delicate balance which maintains the ozone layer. CFCs (chlorofluorocarbons) are still used in refrigerators and as propellants for blowing plastic foam insulation and fast-food containers. They were in the past widely used as propellants for spray cans. Although their use in spray cans has largely ceased, the CFCs in the stratosphere have a very long residence time, and much of the CFC currently in the stratosphere came from spray cans.

In the early 1970s it was suggested that CFCs could destroy ozone, but it was not until the late 1970s that ozone depletion was actually observed by scientists at the British Antarctic Survey. For many years they had routinely made measurements of the amount of ozone at different heights over Antarctica. A detailed study of their records showed that in September and October (spring in Antarctica) the total amount of ozone is depleted by as much as 40 per cent. At some levels it is almost completely destroyed (Fig. 2). Satellite observations have now established that the ozone depletion is mainly limited to an area slightly larger than the Antarctic land mass (Fig. 3), but depletion on such a large scale is not observed elsewhere. Why should this happen over Antarctica and why in spring? The atmospheric circulation in the Antarctic stratosphere is marked by a belt of high winds at about 50 °S which defines a region within which air is relatively isolated from the rest of the atmosphere. During the Antarctic winter this region is in darkness and cools to temperatures as low as −85 °C. Under these conditions diffuse ice clouds, known as polar stratospheric clouds (PSCs), are formed. Chemical reactions occurring on the surfaces of the ice particles in these clouds allow chlorine molecules (from CFCs) to become fixed and stored in the clouds. This large reservoir of chlorine is released in the spring when sunlight returns, temperatures rise, and the clouds evaporate. The sudden release of the stored chlorine allows rapid destruction of ozone to occur and the ozone hole is formed. Over a period of a few months the usual equilibrium is regained and the hole is removed.

Although the ozone hole exists for only part of the year and over an inaccessible part of the world, it is natural to ask if the same process could occur elsewhere. Less dramatic, but nonetheless significant, ozone depletion has been measured in the southern hemisphere latitudes outside the belts of high winds. This could lead to increased ultraviolet radiation reaching the surface in places such as Australasia and South America. In the northern hemisphere most research has concentrated on the Arctic. A belt of high winds similar to that in the southern hemisphere also exists in the northern hemisphere, but it is not a complete ring; the effect of oceans and continents is greater in the northern hemisphere. Breaks in the belt of winds allow air from outside the polar regions to mix with polar air. This results in higher temperatures than are observed over Antarctica and only a few, relatively isolated, polar stratospheric clouds form. Some depletion of ozone is observed in the northern hemisphere, though not on the same scale as the Antarctic ozone hole.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co.