More than 99.9% of the Earth's atmosphere is made up of the gases nitrogen, oxygen and argon. The trace gas ozone only occurs in very small quantities. While nitrogen and oxygen are vertically (at right angles to the Earth's surface) equally distributed, the distribution of ozone varies with height and reaches its maximum amount (concentration) at around 20 km, in the stratosphere. The normal atmosphere reaches up to about 120km above the Earth's surface, and pressure reduces exponentially with height. If one were to take the whole air-mass over a particular area on the Earth's surface and press it so that was all under the same pressure as the air at ground level, then that air would take up a space 8km high. Of these 8km, ozone would make up only 3mm! It is in this way that the amount of ozone in the atmosphere is described. The units are called Dobson units (DU): 300 DU mean 3mm of the air column is made up of ozone. Ozone is a relation of oxygen. It is made of three oxygen atoms (O3
,unlike the oxygen we breathe and which plays an important role in the burning process, which is made of two oxygen atoms. Ozone is produced when oxygen (O2
) is broken up into its atoms (O) by high-energy solar radiation, of wavelengths below 240 nanometers. (1000000000 nanometer (nm) is the same as one meter.) O3
protects the Earth's surface from harmful sun rays. Ozone absorbs above all radiation from the sun which lies in the UV-B region: (280-315 nm wavelength) and through this absorption is split up again into an oxygen atom and an oxygen molecule.
Illustration: The distribution of atmospheric ozone with height
Bildquelle: IUP Bremen
The picture shows two ways of looking at the vertical distribution of ozone. The solid line represents the ratio of ozone as a volume. One takes for example one cubic metre of air and compares the number of all the particles within that volume with the number of ozone particles, or molecules. So 1 ppmv (parts per million volume) means 1 ozone molecule in one million air molecules. But, the total number of molecules in that volume reduces with increasing height because of the reduction in air pressure. The volume mixing ratio (VMR) is therefore a relative term, always related to the air. If one wants to talk about the total number of particles at a particular height, one uses the "particle number density", or concentration. This tells you how many particles of one sort (like ozone) are in a certain volume, e.g. 1 cubic centimetre. This is shown by the dotted line. The units
are not shown here though because ozone concentration is only to serve as an illustration. The most ozone (the highest concentration) is found about 20km above the Earth, and between about 15 and 35km lies the ozone layer, with high ozone concentrations. Under and over it are much lower ozone concentrations. The mixing ratio is important to judge where the most ozone is to be found. Because ozone increases compared to air until about 40km height, one can assume that the most ozone is produced at 40km. There, there are 345 DU of ozone. This is a normal value for the ozone layer.
Illustration: The formation of ozone from oxygen in the stratosphere.
Illustration Source: IUP Bremen
Oxygen atoms are shown in the illustration as blue balls. When atoms are bonded to form moleclues, they are shown overlying one another. Sunlight is symbolised by yellow waves.
Ozone is produced in small quantities from oxygen (O2) when the sun shines. The illustration shows in a simplified form what happens in the stratosphere. When very short-wavelength ultra-violet light (< 240 nm) hits an oxygen (O2) molecule, it is split into two single oxygen atoms (O), in a process called photolysis. These very reactive oxygen atoms can react with other oxygen molecules to form ozone (O3). The air is warmed up by this process.
Destruction: The destructive reactions of ozone in the stratosphere
|Here, the reaction in the air is presented in a more lively manner. The UV-light is symbolised by violet flashes, which split the oxygen molecule (O2) into two O atoms. The free O atoms each pair themselves up with another oxygen molecule, to form tri-atomic ozone (O3) molecules. These ozone molecules can also be split up again by the ultra-violet light.
Animation courtesy of:
Michael Weigend - Uni Hagen
Illustration Source: IUP Bremen
Ozone itself can be photolysed with weak ultra-violet light and even with light from the range of wavelengths which can be seen by the human eye. From the splitting of ozone, oxygen molecules (O2
) and atoms (O) are produced again. The atomic oxygen can collide with another ozone molecule and split it up, to produce two oxygen molecules.
The reactions of ozone synthesis and photolysis occur very quickly. This means ozone is produced quickly and then destroyed quickly, and that the sunlight is absorbed quickly, and therefore effectively. During this process, a lot of heat is produced so the stratosphere warms up. Because the air is very thin at high altitudes, it is easier to warm up than air from lower layers. At higher altitudes there is not so much oxygen available for this reaction to take place. As for lower layers, the photolysis of oxygen is dependent on the availability of very short-wave sunlight and in lower layers of the stratosphere there is a shortage of the necessary short wavelength light because much of it is absorbed higher up in the atmosphere. These are the reasons for the existence of the ozone layer and the increase of temperature with height.
Are there other ways to destroy ozone?
The mechanisms described above are the natural ways of producing and destroying ozone. But, in the stratosphere there are also other gases such as water vapour, nitrogen oxide and chlorine-containing compounds, which can destroy ozone.
Another mechanism is responsible for the famous Antarctic ozone hole. It involves reactions which occur on stratospheric ice crystals.
Illustration: The ozone hole over the Antarctic in 1979 and 2000.
Illustration Source: IUP Bremen
Ozone is a tri-atomic form of normally two-atomic oxygen. Both the formation and the natural destruction of ozone occur through the splitting (photolysis) of oxygen or ozone molecules by high-energy UV sunlight. Ozone makes up about 3mm (300 Dobson units, or DU) of the total air column. The main part of this is found in the stratosphere between 20 and 35 km (ozone layer).
Text: Kai Uwe Eichmann - Institut for Environmental Physics / University of Bremen
Translation: Fiona Roos - University of Berne