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A Hole in the Sky: Ozone Depletion - Evidence Of Ozone Depletion

scientists thinning layer chlorine

Many scientists believe that the introduction of certain chemicals into the stratosphere alters the natural ozone balance by depleting ozone molecules. Chlorine and bromine atoms are particularly destructive. They can bind to loose oxygen atoms and prevent them from reforming either oxygen or ozone. Chlorine and bromine are found in the sea salt from ocean spray. Chlorine is also present in the form of hydrochloric acid emitted with volcanic gases. These are natural sources of ozone-depleting chemicals.

In the mid-1970s scientists first began to speculate that the ozone layer was rapidly being destroyed by reactions involving industrial chemicals that contained chlorine and bromine. Two chemists, F. Sherwood Rowland and Mario Molina, discovered that chlorofluorocarbons (CFCs) could break down in the stratosphere, releasing chlorine atoms that could destroy thousands of ozone molecules. This discovery led to a ban on CFCs as a propellant in aerosols in the United States and other countries.

In 1984 British scientists at Halley Bay in Antarctica measured the ozone in the air column above them and discovered alarmingly low concentrations. Measurements indicated ozone levels about 50 percent lower than they had been in the 1960s.

Scientists report ozone concentrations in units called Dobson units. The unit is named after G.M.B. Dobson (1889–1976), a British scientist who invented an instrument for measuring ozone concentrations from the ground. One Dobson unit (DU) corresponds to a layer of atmospheric ozone that would be 0.001 millimeters thick if it was compressed into a layer at standard temperature and pressure at the Earth's surface. Atmospheric ozone is considered "thin" if its concentration falls below 220 DU. A thin spot in the ozone layer is commonly called an "ozone hole."

Since 1982 an ozone hole has appeared over Antarctica in the springtime (our autumn) for several months. (See Figure 3.3.) The hole varies in size throughout each of its appearances, as indicated by the vertical lines on the graph. The average area calculated for each year is indicated by the FIGURE 3.1
Ozone in Earth's atmosphere
heavy dots on the graph. As shown in Figure 3.3, the hole's average area has increased dramatically since the early 1980s. By the early 1990s the hole was consistently larger than the area of Antarctica. Throughout most of the early 2000s the hole has been larger in size than the continent of North America. In 2002 the hole size dropped dramatically due to unusually warm weather at the South Pole. The hole rebounded to its largest area yet in the autumn of 2003.

Ozone depletion has also been detected over other parts of the world. In 1993 satellite measurements indicated a 10 to 20 percent reduction in ozone levels over parts of Canada, Scandinavia, Russia, and Europe compared to 1992 levels. In 1995 scientists first detected thinning of the ozone over the North Pole. The extreme cold and unique climate conditions over the poles are thought to make the ozone layers there particularly susceptible to thinning. In colder areas of the planet, such as Antarctica, where cloud and ice particles are present, reactions that hasten ozone destruction also occur on the surface of ice particles.

In January 2002 the European Space Agency (ESA) announced that ozone thinning had been detected over Europe by the Global Ozone Monitoring Experiment instrument aboard the ESA's ERS-2 satellite. The thinning in the ozone layer lasted for only three days. Similar thinning was observed previously in November 1999 and November 2001. Scientists believe that unusual air currents in the stratosphere, rather than chemical depletion, may be responsible for the thinning.

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