The Ozone Hole

Although hypothermia, sunburn, frostbite, and snow blindness do occur in Antarctica, these conditions rarely affect experienced field scientists. The most common weather-related health problems arise because of dehydration whieh not only affects kidney function and blood chemistry but also causes uncomfortable eracking and irritation of the skin. The most dangerous weather-related hazard is fire which can cause painful and potentially life-threatening burns and can also result in the loss of shelter, such as a dormitory building in McMurdo Station or a Scott tent in the field. The fire hazard is enhanced by the low humidity of the air and by the scarcity of liquid water with whieh to douse the flames. 

The field manual that is issued by the Office of Polar Programs of the National Science Foundation to all groups that deploy through McMurdo Station contains the following admonition  

Anyone deploying to remote locations in Antarctica should have a strong background in cold-weather survival or, at the very least, [should] employ a safety-survival guide with previous Antarctic experience. Antarctica is not a place to learn cold-weather skills. (Anonymous 1994) 

The hazardous working conditions in Antarctica arise not only from the harsh climate and the rugged surface environment, but also from exposure to exeessive ultraviolet (UV) radiation caused by the destruetion of ozone in the stratosphere above the eontinent (Appendices 2.11.2-2.11.5). 

The seasonal loss of ozone from the stratosphere over Antarctica was first reported by Farman et al. (1985). Their data in Fig. 2.6 indicate that the amount of ozone in the atmosphere above the British Antarctic research station at Halley Bay on the east coast of the Weddell Sea (Appendix 1.10.2) declined from about 330 Dobson Units (DU) in October of 1957 to about 220 DU in October of 1984 (Appendix 2.11.4). In subsequent years, the average amount of atmospheric ozone in October over Antarctica continued to decline and approached 120 DU in 1989 followed by progressively lower values throughout the 1990s. The seasonal deficit in the amount of ozone over Antarctica is called the Ozone Hole. The lowest amount of ozone of 90 

In its most stable form, elemental oxygen exists as diatomic molecules (O2). Ozone is another form of oxygen that contains three oxygen atoms (O3) and is less stable than O2 (see Retouches section 10.R.1). At room temperature, ozone is a pale blue gas with a sharp odor, characteristic of the air after a thunderstorm or near an old Xerox machine (copying machine). Ozone is a very reactive gas, and, even at low concentrations, it is irritating and toxic. 

In the stratosphere, the presence of O3 is absolutely essential since this molecule, which is, otherwise toxic to humans when is present in the atmosphere, acts in 

SCHEME 10.2 The reaction of two molecules of O2 produces O3 and an atom of O. The two arrows mean that the reaction is in equilibrium, going to and fro with a constant ratio of 03 02 under the conditions existing in the stratosphere. Ball-and-stick stmctures of the molecules are shown underneath the equation. 

One of the causes of the destruction of the ozone layer is the presence of free radicals. For example, chlorofluorocarbons (CFCs), like CF2CI2 or CFCI3, which are very stable in the atmosphere, drift to the stratosphere, where they absorb UV light and break into free radicals. 

SCHEME 10.3 The three steps of the chain reaction by which CFCs, such as CF2CI2, destroy ozone. Initially, the CFC molecule absorbs UV radiation and generates two radicals. In step (b), the CT radical destroys an ozone molecule, and this reaction forms a new radical, CIO, which combines with O in step (c) and regenerates the Cl radical. The newly generated CT radical repeats step (b). And the chain process continues, in principle indefinitely. 

Spectrum of ozone, showing maximum absorption of ultraviolet radiation at a wavelength near 260 nm. At this wavelength, a layer of ozone is more opaque than a layer of gold of the same mass. [From R. P. Wayne, Chemistry of Atmospheres (Oxford Clarendon Press, 1991).] 

Spectroscopically measured concentrations of O5 and CIO (measured in ppb nL/L) in the stratosphere near the South Pole in 1987. Destruction of 0. and increased CIO above latitude 68 are consequences of Reaction (2). [From J. G. Anderson, 

October average atmospheric ozone at Halley in Antarctica. Dobson units are a measure of total ozone. 

In 1985, the British Antarctic Survey reported that ozone over Antarctica had decreased by 50% in early spring (October), relative to levels observed in the preceding 20 years. This ozone hole appeared only in early spring and deepened for four decades. 

Ozone destruction begins with chlorofluorocarbons such as Freon-12 (CCI2F2) from refrigerants. These long-lived compounds diffuse to the stratosphere, where they catalyze ozone decomposition  

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We have expanded some subject areas, which previously were of concern to only a few scientists, but which have been popularized by the media to the point where they are common discussion subjects. These include "Global Warming," "The Ozone Hole," "Energy Conservation," "Renewable Resources," and "Quality of Life."... 

J. C. Farman discovered the ozone hole (substantial seasonal depletion of ozone) over Halley Bay, Antarctica. 

The catalyzed decomposition of ozone is known to be responsible for the ozone hole (Figure A) that develops in Antarctica each year in September and October, at the end of winter in the Southern Hemisphere. No ozone is generated during the long, dark Antarctic winter. Meanwhile, a heterogeneous reaction occurring on clouds of ice... 

The ozone (03) layer aver the southern hemisphere stratosphere in August 2007. The bar at the bottom indicates the color-coding used to indicate the thickness of the layer. The thickness is measured in Dobsons (= 0.01 mm thick). The normal ozone layer for the stratosphere is 360 Dobsons (color coded as green). The ozone "hole" shown in pink is 200-220 Dobsons. The "hole" will increase when another reading is taken in September. 

The ozone hole has been recurring now for over a decade and we can expect this phenomenon to continue in varying magnitude as an annual event. 

The ozone hole would almost certainly be much worse if chemists had not studied the reactions of CFCs with atmospheric gases before ozone depletion was discovered. The 1995 Nobel Prize in chemistry was awarded to the three pioneers in this effort. A German chemist, Paul Crutzen, discovered how ozone concentration is regulated in a normal stratosphere, while two Americans, F. Sherwood Rowland and Mario Molina, showed that CFCs can destroy ozone. These studies of molecular reactions allowed quick determination that CFCs are a likely cause of ozone depletion and led to the international restrictions described above. 

The story of the ozone hole illustrates how important it is to learn the molecular details of chemical reactions. Some chemists use information about how reactions occur to design and synthesize useful new compounds. Others explore how to modify reaction conditions to minimize the cost of producing industrial chemicals. This chapter explores how chemical reactions occur at the molecular level. We show how to describe a reaction from the molecular perspective, introduce the basic principles that govern these processes, and describe some experimental methods used to study chemical reactions. 

The development of the ozone hole over Antarctica is accelerated by heterogeneous catalysis on microciystals of ice. These microcrystals form in abundance in the Antarctic spring, which is when the ozone hole appears. Ice microciystals are less common in the Arctic atmosphere, so ozone depletion has not been as extensive in the Northern Hemisphere. 

The formation of photochemically driven layers in the atmosphere, such as the ozone hole a natural consequence of atmospheric structure... 

Alarmingly, the ozone hole has turned mammalian food into food for thought. The ozone hole permits an increase of ultraviolet radiation in the range 290 to 320 nm, the same wavelengths that induce furocoumarin (also known as... 

El-Sayed, S. Z. (1988). Eragile life under the ozone hole. Natural History 97,72. 

But does that mean that the ozone hole has started to disappear That question is more difficult to answer. The concentration of ozone over the Antarctic, as measured at the Halley Bay station. 

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. 

Photochemical production and decomposition of ozone, and the ozone hole... 

The substantial concentration of ozone in the stratosphere can be significantly depleted by comparatively small amounts of other substances. The significantly depleted ozone level in polar regions (mostly over Antarctica) is referred to as the ozone hole. 

The discovery of the Ozone Hole in the Antarctic stratosphere has led to the realization that previously unsuspected heterogeneous chemical reactions occuring on the surface of ice and other stratospheric cloud particles play a critical role in atmospheric ozone depletion — not only in the Antarctic stratosphere,... 

Figure 12.17 shows the ozone profiles over the U.S. Amundsen-Scott Station at the South Pole in 1993 on August 23 prior to formation of the ozone hole and on October 12 after the ozone hole had developed. The total column ozone decreased from 276 DU on August 23 to only 91 DU on October 12, and, in addition, there was essentially no ozone in the region from 14 to 19 km (Hofmann et al., 1994a). During the same period at the McMurdo Station in Antarctica, the total column ozone decreased from 275 to 130 DU (B. J. Johnson et al., 1995). While similar profiles have been observed since the discovery of the ozone hole, these data show some of the most extensive ozone destruction ever observed, although 1994 and 1995 showed almost as much 03... 

FIGURE 12.17 Vertical O, profile before (August 23) and after (October 12) development of the ozone hole at the U.S. Amundsen-Scott Station, South Pole, in 1993 (adapted from Hofmann et al., 1994a). 

In summary, reactions (43a)-(45) have generally been taken to represent the chemistry occurring in the ozone hole. However, reduced efficiency of chlorine atom production in the photolysis of (C10)2, reaction (44), and hence ozone destruction, needs to be modeled and tested against the atmospheric observations. 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

Atkinson, R. J., and R. A. Plumb, Three-Dimensional Ozone Transport during the Ozone Hole Breakup in December 1987, J. Geophys. Res., 102, 1451-1466(1997). 

Hamill, P., and O. B. Toon, Polar Stratospheric Clouds and the Ozone Hole, Physics Today, 44, 34-42 (1991). 

As discussed in Chapter 12, trends in stratospheric ozone in the Antarctic spring during formation of the ozone hole are clear. However, as treated in detail in... 

Basher, R. E., X. Zheng, and S. Nichol, Ozone-Related Trends in Solar UV-B Series, Geophys. Res. Lett., 21, 2713-2716 (1994). Beaglehole, D., and G. G. Carter, Antarctic Skies. 1. Diurnal Variations of the Sky Irradiance, and UV Effects of the Ozone Hole, Spring 1990, J. Geophys. Res., 97, 2589-2596 (1992). Bednarek, G J. P. Kohlmann, H. Saathoff, and R. Zellner, Temperature Dependence and Product Distribution for the Reaction of CF30 Radicals with Methane, Int. J. Res. Phys. Chem.. Chem.. Phys., 188, 1-15 (1995). 

Ristaino, J. B., and W. Thomas, Agriculture, Methyl Bromide, and the Ozone Hole. Can We Fill the Gaps Plant Dis., September, 964-977 (1997). 

Stratospheric O3 depletion, commonly known as the ozone hole, is caused by the release into the atmosphere of certain manmade substances that destroy the ozone (the good ozone) at high altitude. Because of the thinning of the ozone layer,... 

The cycle repeats itself, so that one Cl atom can destroy thousands of O3 molecules (Seinfeld Pandis 1998). It is estimated that so far about 10% of the stratospheric ozone has been depleted. Because of the Montreal Convention of 1987 and its Amendment of 1992, fully haloge-nated CFCs are no longer manufactured legally in the world. Unfortunately, these CFCs are very long lived (in the order of hundreds of years), so the ozone hole will only be slowly filled in by natural production of O3 in the stratosphere (IPCC 2001). 

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