Chlorofluorocarbons and ozone

Brasseur, G., and C. Granier, Mount Pinatubo aerosols, chlorofluorocarbons, and ozone depletion. Science 257, 1239, 1992. 

Recognition of the threat of stratospheric ozone depletion posed by chlorofluorocarbons and chloro-fltiorohydrocarbons led 131 countries to sign the Montreal Protocol in 1987. Production of chlorofluorocarbons was banned as of January 1, 1996, because of their potential to further deplete stratospheric ozone. Chlorofluorohydrocarboiis will be... 

Global Chlorofluorocarbons and their effect on ozone in the upper atmosphere... 

Over the next two years, Midgley and his group discovered and patented other chlorofluorocarbons and the halons, a class of bromofluoro-carbon compounds that are the world s best Are fighters. At the time, their remarkable stability seemed like an advantage. In the 1970s scientists were able to determine that CFCs and halons—which are so stable that they remain in the atmosphere for long periods of time—deplete the ozone layer 15 miles above Earth. 

Methane is removed continually from the atmosphere by reaction with OH radicals (Section 8.3). In contrast, chlorofluorocarbons and related volatile compounds are inert under the conditions of the lower atmosphere (troposphere), so atmospheric concentrations of these refrigerants and solvents will tend to increase as long as releases continue. The chief concern over chlorofluorocarbons is that they are a major factor in destruction of the stratospheric ozone layer (Section 8.3). They have been banned under the Montreal Protocol of 1988, but it is important that whatever substitutes (inevitably greenhouse active) are introduced to replace them degrade relatively quickly in the troposphere to minimize any contribution they may be capable of making to greenhouse warming. 

F. S. Rowland, Chlorofluorocarbons and the depletion of stratospheric ozone Am. Sci. 77, 36-45 (1989) T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride and ice release of active chlorine. Science 238, 1253-1260 (1987) J. G- Anderson, D. W. Toohey, and W. H. Brune, Free radicals within the Antarctic vortex the role of CFCs in Antarctic ozone loss. Science 251, 39-46 (1991) P. S. Zurer, Complexities of ozone loss continue to challenge scientists. Chem. Eng. News June 12, 20-23 (1995). 

Many people hear about atmospheric ozone depletion and wonder why we don t simply replace that which has been destroyed. Knowing about chlorofluorocarbons and knowing how catalysts work, explain how this would not be a lasting solution. 

All of the chlorofluorocarbons and halons discussed above have been implicated in the halogen atom-catalyzed destruction of atmospheric ozone. As a result of U.S. Environmental Protection Agency regulations imposed in accordance with the 1986 Montreal Protocol on Substances that... 

Another example is that of chlorofluorocarbons (CFC). All chlorofluorocarbons show absorption in the range 160-280 nm. This UV radiation is available near ozone layer of earth atmosphere. Light spectrum available on the surface of earth is perfectly transparent to chlorofluorocarbons and thus behaves as photochemically inert. These do not dissolve in water and thus are not removed by rain. Slowly these molecules rise and reach the ozone layer. Their photons of 160-280 nm are available absorption of which lead to primary photochemical reaction... 

Safety of chemicals. Scientific notation, conversions Periodic Table of Elements Naming simple compounds Atomic structure and periodicity Balancing equations Drawing molecules Interaction of light with molecules Chlorofluorocarbons (CFC s) and ozone Development of green pesticides... 

Catalysis are classified into two types homogeneous and heterogeneous. In homogeneous catalysis the catalyst is present in the same phase as the reactants, as when a gas-phase catalyst speeds up a gas-phase reaction, or a species dissolved in solution speeds up a reaction in solution. Chlorofluorocarbons and oxides of nitrogen are homogeneous catalysts responsible for the destruction of ozone in the stratosphere. These reactions are examined in more detail in Section 20.5. A second example is the catalysis of the oxidation-reduction reaction... 

The most significant developments in metered-dose inhaler technology to occur since the early 1990s have been the introduction of hydrofluoroalkane (HFA) systems as alternatives to chlorofluorocarbon (CFC) systems [174]. This has largely been caused by the link between the use of CFC systems and ozone depletion in the upper atmosphere [152,175]. Albuterol and beclomethasone have been reformulated in HFA products, but as yet the CFC products are still subject to an annually renewable medical exemption. The Food and Drug Administration has recently published its position on alternative propellant formulations, which should initiate the phase-out of CFCs [176]. In the meantime, a number of generic CFC products of albuterol have been manufactured. The opportunity for reformulation of products as they come of patent is likely to increase research and development in this area in the near future. New formulation opportunities will also arise from these developments, including solutions [177], micellar [178,179], and microemulsion [180]. 

A carbon atom can be bonded to as many as four halogen atoms, so an enormous number of organic halides can exist. Completely fluorinated compounds are known as fluorocarbons or sometimes perfluorocarbons. The fluorocarbons are even less reactive than hydrocarbons. Saturated compounds in which all H atoms have been replaced by some combination of Cl and F atoms are called chlorofluorocarbons or sometimes freons. These compounds were widely used as refrigerants and as propellants in aerosol cans. However, the release of chlorofluorocarbons into the atmosphere has been shown to be quite damaging to the earth s ozone layer. Since January 1978, the use of chlorofluorocarbons in aerosol cans in the United States has been banned, and efforts to develop both controls for existing chlorofluorocarbons and suitable replacements continue. The production and sale of freons have been banned in many countries. 

Halocarbons have the further advantage of reducing the viscosity of the reaction mixture and, where used as the main blowing agent instead of the carbon dioxide produced by the isocyanate-water reaction, cheaper foams are obtained since less isocyanate is used. The reader should, however, note the comments made about the use of chlorofluorocarbons and their effect on the ozone layer made in Section 27.5.4. 

Special Topic 7-1 Green Chemistry—Substitutes for Chlorofluorocarbons describes substitutes for chlorofluorocarbons, and Special Topic 7.2 Other Ozone-Depleting Chemicals describes other ozone-depleting chemicals. 

Progress has been made in some areas since 1970. Emissions of pollutants from point sources into air and water have decreased. Toxic releases are decreasing. Some Superfund sites have been cleaned up. Businesses would no longer think of dumping a barrel of waste solvent on the ground at the landfill site so that the barrel could be used again for the same purpose. Control of pollutants from nonpoint sources is still a problem. There is now more international cooperation and discussion of global problems, such as ozone depletion by chlorofluorocarbons and the effect of... 

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