The Ozone layer

Ozone, O3, is an alloptrope of oxygen. It has a bent structure like that of water however it only has 1 ½ bonds between the oxygen atoms. This makes it much more reactive than normal molecular oxygen. The diagram opposite shows the structure of ozone. The dashed line represents the bond between the atoms which is neither a single bond or a double bond:-

Ozone is a pale blue gas that is very poisonous and has a strong odour. So how can it protect us from the sun?

The majority of the ozone in the atmosphere is found in the ozone layer which is found between 19 and 48km above the earth's surface (in the stratosphere). Ozone is naturally created and destroyed by the action of ultra-violet light creating a dynamic equilibrium. By absorbing ultra-violet light ozone protects life on earth from the full health threatening effects of the sun's radiation. The U.V radiation causes an increased chance of skin cancer, eye cataracts and a reduction in immunity in humans. It also damages the microscopic life in the surface oceans which is the basis of the world's food chain. However, ozone in the troposphere is a pollutant as it is toxic to animals and plants. In the ozone layer the maximum concentration of ozone is 10ppm (parts per million). The actual ozone molecules are being constantly made and destroyed maintaining this concentration for centuries. Although the concentration of ozone is very low it has successfully protected the planet for millenia.

Ozone production (photolysis)

Ozone is produced in the stratosphere by the action of ultra-violet radiation on oxygen molecules. A photon of U.V radiation is absorbed by an oxygen molecule which splits the oxygen molecule into two oxygen atoms (equation 1). These oxygen atoms then collide with another oxygen molecule to produce an ozone molecule. However, a lot of energy is released in this reaction which is enough to split the ozone molecule. Therefore to enable the ozone molecule to stay together another molecule is required. This is normally a nitrogen or oxygen molecule which absorbs the excess energy produced allowing the ozone molecule to be produced (equation 2). This reaction occurs two times per oxygen molecule split resulting in the net formation of two ozone molecules from three oxygen molecules (equation 3).

1. O₂ + U.V photon → O + O
2. O + O₂ + neighbouring molecule → O₃ + neighbouring molecule (plus energy)
3. 3O₂ → 2O₃

The diagram below summarises the process of ozone production in the strosphere.

Ozone destruction is described on the next page