Ozone layer hole, Antarctic

Ozone has received increased attention for its occurrence and function in the Earth s atmosphere.For example the decreasing ozone concentration in the stratospheric ozone layer, becoming most obvious with the Antarctic ozone hole. 

Events that take place on a grand scale often can be traced to the molecular level. An excellent example is the depletion of the ozone layer in the Earth s stratosphere. The so-called ozone hole was first observed above the Antarctic in the 1980s and is now being observed above both the Arctic and Antarctic poles. The destruction of ozone in the stratosphere is caused primarily by reactions between chlorine atoms and ozone molecules, as depicted in our molecular inset view. 

The chlorine atoms in the upper atmosphere come from the breakdown of CF2 CI2 and other similar chlorofluorocarbons (CFCs), known commercially as Freons. Production of these compounds was more than one million tons in 1988, largely for use in relrigerators and air conditioners. Once released into the atmosphere, CFCs diffuse slowly upward in the atmosphere until they reach the ozone layer. There, ultraviolet light Irom the sun splits off chlorine atoms. These react with ozone, with dramatic results. Annual ozone decreases have exceeded 50% above Antarctica. The background photo shows the Antarctic hole (red-violet) on September 24, 2003. 

The history of ozone depletion took a dramatic turn in 1985 when J. C. Farman at the BAS Halley Bay station announced that ozone levels over the Antarctic had decreased by more than 40 percent between 1977 and 1984. Farman explained that ozone levels had fallen so low that one could say that a "hole had formed in the ozone layer above the South Pole. In 1984, that "hole covered an area of more than 15 million square miles (40 million square kilometers), equal to the size of the continental United States. Clearly, ozone depletion was not a long-term problem about which scientists could debate for the next century or so. It was an issue that demanded quick attention and action. 

CFCs released to the atmosphere evenmally find their way up to the stratosphere where they destroy the ozone layer which protects the Earth s surface from harmful ultra-violet radiation. During the last decades, the ozone layer has been severely depleted, both over the Antarctic region where the ozone hole now appears annually, but also over the northern hemisphere. Ozone depletion up to 40% has been recorded in each of the last three years over Northern Europe. 

By 1974, millions of tonnes of CFCs had been produced (see Figure 12.22). At the University of California, chemists F. Sherwood Rowland and Mario Molina began to wonder where all of these CFCs ended up. They realized that CFCs are chemically very stable. However, they began to calculate what happens when CFCs are exposed to high levels of radiation far up in the atmosphere. As it turned out, their fears were well-founded. In 1985, British scientists in the Antarctic noticed a large decrease in the ozone layer above the Antarctic. A hole in the ozone layer was beginning to form. In 1995, Rowland and Molina, along with a third ozone scientist, won the Nobel Prize for their work with CFCs. 

Based on predictions of the effect of CFCs on the ozone layer, in 1987 a previously unprecedented step was taken when many countries signed the UN Montreal protocol specifying the control and phase-out of these ozone-depleting chemicals. Since that time the protocol has been modified in order to speed up the schedule and extend the range of chemical covered to further lessen the effect of these chemicals (see Figure 27). One of the factors that lead to more rapid world action on CFCs was the discovery of the so-called Antarctic ozone hole. 

FIGURE 20.29 This false color image shows total stratospheric ozone amounts over the southern hemisphere for September 24, 2006, as recorded by the Ozone Monitoring Instrument (OMI) mounted on the Aura spacecraft. The dramatic depletion of the ozone layer over Antarctica is revealed with the help of the false color scale at the bottom of the figure. Ozone amounts are commonly expressed in Dobson units 300 Dobson units is a typical global average over the course of a year. The size of the Antarctic ozone hole was near a record high and the levels of ozone near a record low on this date. 

Fig. 3.6 Mean October levels of total ozone above Halley Bay (76°S), Antarctica, since 1957. The 1986 value is anomalous due to deformation of the ozone hole, which left Halley Bay temporarily outside the circumpolar vortex (a tight, self-contained wind system). Dobson units represent the thickness of the ozone layer at sealevel temperature and pressure (where 1 Dobson unit is equivalent to 0.01 mm). Data courtesy of the British Antarctic Survey. Inset shows seasonally averaged (Sep.-Nov.) ozone partial pressure at about 17 km at 70°S. Data courtesy of G. Konig-Langlo.

The UVR band has been identified as consisting of three regions, namely, UVA from 320 nm to 400 nm, UVB region from 290 nm to 320 nm and UVC from 200 nm to 290 nm. The regions of utmost concern for skin cancer are the UVB and UVA regions [75]. UVC, UVB and UVA radition are usually absorbed by the ozone layer, with no UVC and only half of the UVB reaching the surface of the earth. The size and depth of the Antarctic ozone hole has caused concern particularly in Australia, where it has been suggested that a decrease of 1% in ozone would lead to increase in the ultraviolet radiation at the earth s surface and may eventually lead to a 2-3% increase in skin cancer [76]. 

Shortly thereafter, the effect on stratospheric ozone of chlorine released from human-made (anthropogenic) chlorofluorocarbons was predicted by Mario Molina and F. Sherwood Rowland. For their pioneering studies of atmospheric ozone chemistry, Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry. It was not until 1985, with the discovery of the Antarctic ozone hole by a team led by the British scientist Joseph Farman, that definitive evidence of the depletion of the stratospheric ozone layer emerged. 

In 1985, a British team at Halley Bay Station, Antarctica, discovered the existence of a hole in the ozone layer above that continent. This totally unexpected phenomenon needed an explanation, and Susan Solomon—a young National Oceanic and Atmospheric Administration (NOAA) scientist—first proposed a good theory for it. While attending a lecture on polar stratospheric clouds, she realized that ice crystals in the clouds might do more than just scatter light over the Antarctic. Her chemist s intuition told her that the ice crystals could provide a surface on which chemical reactions of CFG compounds could take place. 

The most prominent instance of ozone layer destruction is the so-called Antarctic ozone hole that was first firmly established in 1985 by the British Antarctic Survey and observed with great alarm in subsequent years. This phenomenon is manifested by the appearance during the Antarctic s late winter and early spring months of September and October of severely depleted stratospheric ozone (up to 50%) over the polar region. The reasons why this occurs are related to the normal effect of NO2 in limiting Cl-atom-catalyzed destruction of ozone by combining with CIO ... 

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