2.3 Ozone Depletion

Ozone (O₃) occupies only a very small fraction of the earth's atmosphere and yet the existence of the ozone layer is of vital importance to life on earth. Ozone is the only atmospheric gas which absorbs and reduces to reasonably safe levels the especially harmful portion of the UV spectrum known as UV-B. Without such UV protection most life forms, including plants and animals, can experience living cell damage with serious consequences including, for example, a decrease in photosynthesis activity in plants, and cancer in humans.

Generally, the destruction of ozone in the atmosphere results from a series of cycling chemical reactions between an O₃ compound and a catalyst such as chlorine, bromine, hydrogen or nitrogen. The catalyst breaks down the O₃ compound by stealing one oxygen molecule, creating a stable oxygen compound O₂ and a new catalyst/oxygen compound. In the case of some catalysts such as chlorine, the catalyst/oxygen compound can then easily break apart leaving a solitary oxygen molecule. The oxygen molecule can then combine with another single oxygen molecule, forming O₂. More importantly, the catalyst becomes available again to destroy other O₃ compounds. This chain reaction can result in the destruction of hundreds of thousands of O₃ compounds before the catalyst eventually forms a stable compound that is no longer available to destroy ozone.

Since the discovery In the mid-1980's of an ozone "hole" over Antarctlea and with subsequent discoveries of ozone depletion over other areas of the earth, most notably over the ArctIc, considerable research has been conducted to determine the specific forces behind ozone destruction. Evidence now suggests that ozone depletion is primarily caused by man-made chlorofluorocarbons (CFCs) which contain the all-important ozone-destroying catalyst, in this case, chlorine. CFCs have a particularly stable chemical structure which results in their being highly effective transporters of the chlorine. In fact, CFCs will not normally break down and release the chlorine molecule until they become exposed to the upper atmosphere's intense radiation, coinciding unfortunately with the very location of the highest concentrations of ozone.

CFCs have been used worldwide for over 60 years as refrigerants, solvents, foaming agents and spray can propellants. The level of free chlorine in the atmosphere, believed to be primarily attributable to the extensive use of CFCs, is estimated to be about six times higher now than at the turn of the century. Recently, substitute products having comparable performance to CFCs but imparting much less impact on the ozone layer have been developed. These include hydrofluorocarbons(HFCs)andhydrochlorofluorocarbons(HCFCs). HFCs contain no chlorine or other readily available catalyst thus they have an ozone depletion potential (ODP) of zero. HCFCs, although containing chlorine, break down in the lower atmosphere thus they do not provide a catalyst near the concentrated ozone layer (upper atmosphere). TheODPofHCFCs ranges from 10 to 50 times lower than that of CFCs.

Although the development of HFCs and HCFCs appears quite promising in terms of minimizing ozone depletion, the substitution of these refrigerants into the countless refrigeration systems currently using CFCs is not as easily accomplished as might first be assumed. The replacement refrigerants exhibit slightly different properties than CFCs resulting in reduced system efficiency and cooling capacity, and increased operation and maintenance costs. Losses in efficiency and capacity may require that additional equipment be purchased and put on line to meet the current loads, for which there may be no readily available space. Certain HCFCs are more corrosive than the CFC refrigerant, necessitating modifications to ensure equipment is suitable for operation on the new refrigerant. Depending on the class of equipment and the equipment's operating conditions, such as temperature and pressure, some replacement refrigerants may not be operationally suitable, requiring implementation of less desirable replacements. Indeed, factors may favour and result in selection of HCFCs over HFCs, even though HCFCs have a non-zero ODP and have recently been attributed with possible toxicity effects. Actually HCFCs are now intended to be phased out themselves, between the years 2020 and 2030, or possibly sooner. Major retrofitting efforts to accommodate an HCFC, only to have it phased out during the new equipments' lifetime, is forcing decision-makers to carefully examine their options.

Nitrogen, in the form of the relatively stable compound, nitrous oxide, is another ozone- destroying catalysts carried to the upper atmosphere. This compound is available in part as a result of fossil fuel combustion, thus both fuel combustion and refrigerant use aspects of thermal energy production schemes play a role in contributing to the depletion of the ozone layer problem.

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