问答题 1 The atmosphere consists of a number of gases, the most important of which from the point of view of life on Earth is oxygen. It is a by-product of plant metabolism. It is essential to the metabolism of animals, and it also takes part in an important reaction in the stratosphere.
The oxygen molecule consists of two atoms of oxygen-written as O ₂ . In the stratosphere the ultraviolet rays of the sun can break the molecule down into individual atoms—O and O. 2 These atoms are unstable and can attach themselves to other oxygen molecules to form molecules of ozone oxygen with three atoms of oxygen—O₃. Under the influence of radiation from another part of the ultraviolet spectrum the molecules of ozone can break down and unite with more free oxygen atoms to form oxygen molecules again—O ₃ unites with O to produce O ₂ and O ₂ .
As a result there is a layer in the atmosphere where ozone is continually produced and destroyed. 3 This ozone layer lies between 15 and 50 km above the ground—with the greatest concentration at about 23km—and is estimated to contain about 5 billion tonnes of ozone at one time. All this activity absorbs the energy of the ultraviolet light and prevents it from reaching the ground, which is a good thing as certain wavebands of ultraviolet light can cause cancers, cataracts, and decreases in crop yields.
4 The presence of certain other molecules, particularly of chlorine, influences the speed of these reactions—usually increasing the speed of the breakdown of the ozone. This is very noticeable in springtime in the Antarctic when chorine from pollutants in released from the icy winter clouds. 5 The ozone concentration is reduced here, and the still conditions cannot bring it in from elsewhere, resulting in the so-called ozone hole. It is believed that much of the chlorine responsible comes from industrial gases such as CFCs, as well as from natural sources such as volcanoes. This is becoming a worldwide problem as a thinning of the ozone layer over Europe was detected early in 1993.