The Ozone Layer

1. Many scientists prefer to regard the stratosphere and mesosphere as being different parts of the same layer, because apart from temperature their properties are similar. Sounding balloons filled with hydrogen or helium routinely penetrate the stratosphere with instruments of various kinds and send down data by radio to ground stations; some aircraft, too, are capable of exploring the stratosphere. The still higher elevations of the mesosphere require rocket-borne apparatus if direct measurements are to be made, but a number of experimental methods have been devised that enable observatories on the ground to determine some of the physical properties of this part of the atmosphere.

2. The most striking feature of the stratosphere and the lower mesosphere is the presence of ozone, a form of oxygen whose molecules contain three oxygen atoms instead of the usual two. The chemical symbol for ozone is accordingly O₃. Ozone is an excellent absorber of ultraviolet radiation; so excellent, in fact, that the relatively small amount of ozone in the upper atmosphere completely filters out the dangerous short-wavelength ultraviolet radiation emitted by the sun. Living things on earth would certainly have evolved very differently without the protection of the ozone layer, since few of today's organisms could survive exposure to solar ultraviolet rays at their full strength.

3. The ozone layer lies between 15 and 55 km. At its maximum density less than 1 molecule in 4 million is О₃ - hardly an impressive concentration for so efficient a filter. At sea-level temperature and pressure, all the ozone in the atmosphere would form a layer less than an inch thick. The elevated temperatures of the upper stratosphere and lower mesosphere are due to the heating effect of the solar ultraviolet energy absorbed at the top of the ozone layer.

4. Why is the ozone content of the atmosphere concentrated in a particular region instead of being more or less uniformly distributed? The first step in the formation of an ozone molecule is the breaking up of an O₂ molecule into two О atoms by solar ultraviolet light. The second step is the attachment of an О atom to an O₂ molecule to form O₃. The rate of ozone production thus depends upon both the O₂ concentration and the intensity of solar ultraviolet light. At extremely high altitudes there are not enough O₂ molecules for an appreciable amount of О₃ to be formed. Between 15 and 55 km above the ground, however, the atmosphere is dense enough for the production of О₃ but not so dense that the unstable ozone molecules break up into O₂ + О too often in collisions with other molecules. At lower altitudes the ultraviolet light has already been absorbed so no ozone can come into being there except as a result of lightning strokes in the lower atmosphere.

Ex. 1. Put the sentences in the logical order.

1. The second step in the formation of an ozone molecule is the attachment of an О atom to an O₂ molecule.

2. Ozone is an excellent absorber of ultraviolet radiation.

3. Apart from temperature their properties are similar.

4. The ozone layer lies between 15 and 55 km.

5. The first step in the formation of an ozone molecule is the breaking up of an O₂ molecule into two О atoms by ultraviolet light.

6. Many scientists regard the stratosphere and mesosphere as being different parts of the same layer.

7. The rate of ozone production depends upon both the O₂ concentration and the intensity of solar ultraviolet concentration.

8. The most striking feature of the stratosphere and the lower mesosphere is the presence of ozone

Ex. 2. Check your understanding.

1. Look at the first paragraph and say which of the following statements is correct:

• The stratosphere and mesosphere

1. are exactly the same

2. differ in their temperatures

3. have the same temperature

4. have different properties except for temperature

• Information about the stratosphere is usually gathered

1. only by rockets

2. only by ground stations

3. by balloons

4. only by special aeroplanes

2. Now look at paragraph 2 and say which words have the same meaning as:

• very good

• attracting interest/attention

• comparatively

3. Now look at paragraph 3. Is this statement correct or incorrect?

• The ozone layer in the upper atmosphere is less than an inch thick.

4. Now look at paragraph 4 and say which of the following statements is correct:

• The formation of О₃ requires

1. only the presence of O₂ molecules

2. only the existence of ultraviolet light

3. neither O₂ molecules nor ultraviolet light

4. both O₂ molecules and ultraviolet light

• At a height of 15 to 55 km above the earth, О₃ molecules

1. do not exist

2. always break up

3. sometimes break up

4. never break up

Ex. 3. Fill in the logical structural chart of the text.

Ex. 4. Using your own words and the chart write a short description of the ozone layer. Use the information in the text.

Text D

Task. Read the text, study Fig. 1 and say what role the ionized layers high up in the atmosphere play.