1. Ozone Depletion
Synthetic chemicals are destroying the ozone layer
Man-made chemicals, called chloroflurocarbons, are consuming the protective ozone layer, eating holes in it and making it thinner. They are synthetic chemicals, entirely of human origin with no counterparts in nature.
The name of the chemicals is a giveaway to their composition. Each molecule has been constructed in company laboratories by linking atoms of chlorine, fluorine and carbon. The shorthand CFC notation abbreviates some of them. A number sometimes follows, providing a complex description of the number of atoms in each molecule, the most widely used being CFC-11 and CFC-12.
Beginning in 1930, the biggest producer of CFCs, the Du Pont Company, manufactured and marketed them under the name Freons. They have been widely used in refrigerators, plastic foams, spray-can propellants, automobile air-conditioning systems, and the cleaning of circuit boards used in televisions and computers.
The hardy chemicals don't interact chemically to form other substances. They are so inert and stable that once entering the atmosphere the CFC molecules can survive for more than a century, permitting them to drift and waft up into the ozone layer in the stratosphere. Although more than 20 million tons of CFCs have been released into the air, their combined concentration isnít very significant, only about one CFC molecule for every two billion molecules in the air. Yet, even these seemingly insignificant amounts can have enormous impact.
In 1974, Mario J. Molina (1943-) and F. Sherwood Rowland (1927-), two chemists who were then at the University of California at Irvine, showed that the chlorine in the CFCs can destroy enormous amounts of ozone. Once arriving in the stratosphere, the Sun's ultraviolet rays will split chlorine atoms out of the CFCs, and the liberated chlorine sets off a self-sustaining chain reaction that destroys the ozone. A single chlorine atom will react with an ozone molecule, taking one oxygen atom to form chlorine monoxide; the ozone is thereby returned to a normal oxygen molecule and its ultraviolet absorbing capability is largely removed. Moreover, when the chlorine monoxide encounters a free oxygen atom, the chlorine is set free to strike again. Each chlorine atom thus acts as a catalyst, destroying about 10,000 ozone molecules before it finally combines permanently with hydrogen.
Molina and Rowland were awarded the Nobel Prize in Chemistry in 1995 for their "contribution to our salvation from a global environmental problem that could have catastrophic consequences". They shared the prize with the German chemist, Paul Crutzen (1933-), who showed how the rate of ozone depletion could be accelerated by other chemical reactions in the atmosphere.
The ozone layer is itself invisible. But you can determine its ozone content by measuring the amount of solar ultraviolet radiation getting through the layer and reaching the ground. When there is more ozone, greater amounts of ultraviolet are absorbed in the stratosphere and less reaches the ground, and when the ozone layer is depleted, more of the Sunís ultraviolet rays strike the Earthís surface.
The British scientist G. M. B. Dobson (1889-1976) pioneered measurements of the airís ozone content about half a century ago. When his instrument was installed at Halley Bay, Antarctica, in 1957-58, Dobson found that the ozone abundance in polar spring (September-November) was noticeably less than that above other parts of the world.
Other British scientists continuously monitored the southern polar skies for 27 years; always detecting a springtime loss that became steadily larger as the years went on. By 1985 the ozone loss above Antarctica had nearly doubled when compared to the earlier measurements in the 1960s, and it extended all the way to the top of South America, where another British monitoring station detected it. A continent-sized hole had opened up in the sky Ė the ozone hole (Fig.1).
This unexpected discovery astounded space-age scientists who had not detected any ozone hole using satellites that had been monitoring the ozone layer from above. Their computers had been programmed to automatically reject large ozone depletion, apparently because their models did not predict such huge losses. So the now-famous ozone hole had been discarded as an anomaly, perhaps caused by an instrumental error. After reanalyzing the satellite data, the scientists confirmed the existence of an ozone hole in the local springtime above the South Pole.
We now know that whirling winds concentrate ozone-destroying chemicals, the CFCs, within a vast, towering vortex above Antarctica, resembling the eye of an immense hurricane. Each year the gaping hole opens up during Antarctic spring when the sunlight triggers ozone-destroying chemical reactions; the hole starts to close up in the early polar fall when the long sunless winter begins. Ozone-depleted air is dispersed globally, and the ozone is slowly restored, filling the hole until the cycle repeats in the following year.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University