Ozone shield

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

The buildup of greenhouse gases in the atmosphere, and the rupture of the earth s stratospheric ozone shield by chlorofluorocarbons (CFCs), indicate that the human economy has already exceeded the assimilative and regenerative capacities of the biosphere to absorb some human wastes. There is considerable evidence to support this general proposition. Eor example, persistent synthetic toxicants are now measurable from the peaks of the highest mountains to the floors of the deepest oceans and everywhere in between. It is... 

Stratospheric ozone is produced at maximum rates in equatorial regions, where solar radiation is most intense. Ozone does not really occur as a layer, but instead as a broadly distributed gas whose peak concentration occurs in midstratosphere. The total amount of ozone present in the atmosphere is small, typically between 200 and 400 Dobson units. A Dobson unit is the amount of ozone that, if gathered together in a thin layer covering Earth s surface at a pressure of 1 atm, would occupy a thickness of 1/100 of a millimeter (10 gm). The entire ozone shield, which protects life on Earth from damage by the UV-B radiation of the Sun (ultraviolet radiation in the 280-320 nm range), is equivalent to a layer of ozone only 2 to 4 mm thick at sea level pressure. 

The carbon-chlorine bond in most chlorofluorocarbons can be homolytically cleaved under photolytic conditions (185-225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Walker (1972) it is not excluded that the biologically protective ozone shield was formed at this altitude at an earlier time. 

The study of the possible modification of the stratospheric ozone shield seems to be particularly important since ozone absorbs radiation dangerous to living species. Furthermore, this substance plays an important role in the control of the heat balance of the stratosphere. 

Ozone (O3), a major constituent of smog, is a health hazard at ground level. In the stratosphere, however, a layer of ozone shields the Earth from harmful solar radiation. The greatest concentration of ozone occurs between 12 and 15 miles above the Earth s surface. The ozone layer is thinnest at the equator and densest towards the poles. Ozone is formed in the atmosphere from the interaction of molecular oxygen with very short wavelength ultraviolet light. 

The Chlorofluorocarbon-Ozone Question.— For information on atmospheric chloro-fluorocarbons(CFCl3 and CF2CI2 derived from aerosol dispensers) and the possibility that they damage the Earth s ozone shield via stratospheric photo-dissociation ... 

The reactions of the ozone cycle account for some, but not all, of the facts about the ozone layer. Many chemical reactions occur that involve substances other than oxygen. We must also consider the effects of turbulence and winds that mix up the stratosphere. A complicated picture results. The overall result of ozone formation and removal reactions, coupled with atmospheric turbulence and other factors, is to produce the upper-atmosphere ozone profile shown in FIGURE 18.4, with a maximum ozone concentration occurring at an altitude of about 25 km. This band of relatively high ozone concentration is referred to as the ozone layer or the ozone shield. ... 

What type of radiation does ozone shield us from What wavelengths of this radiation are absorbed by ozone ... 

The ozone layer and its thinning have been extensively covered both in the daily press and the popular scientific hterature. The words ozone shield and ozone hole have joined the vocabulary of maity literati. These phrases are easy to remember and quite easily visualized. Unfortunately, they are also quite deceiving if their meaning is taken literally. 

One fate of the ozone formed is that It may absorb a different UV photon. Doing so results in the dissociation of the ozone. This reaction is the reason ozone shields us from excess UV radiation. 

Measurements from satellite instraments have allowed one to monitor the global ozone shield since the late 1970s. 

Ozone is formed in the stratosphere by radiation from the sun and helps to shield life on Earth from some of the sun s potentially destructive ultraviolet radiation. In the early 1970s, scientists suspected that the ozone layer was being depleted. By the 1980s, it became clear that the ozone shield is indeed thinning in some places and at times even has a seasonal hole in it, notably over Antarctica. The exact causes and actual extent of the depletion are not yet fully known, but most scientists believe that various chemicals in the air are responsible. 

FIGURE 6.3 The ozone shield Atmospheric ozone shields life on Earth from harmful ultraviolet light. 

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