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| CFCs | |
| Chlorofluorocarbons (CFCs) are compounds consisting of chlorine, fluorine, and carbon atoms which are very stable in the troposphere. They are degraded only in the stratosphere by the sun's radiation where released chlorine may contribute to ozone depletion. They can persist in the troposphere for 100 years or longer. | |
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| Fluorocarbons | |
| Fluorocarbons are chemical compounds which include CFCs, hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). For many years, CFCs have served vital functions in society. They are used in a variety of applications including refrigeration, air conditioning, energy-efficient insulation, medical products, and cleaning of electronic and precision engineering components. HCFCs and HFCs retain many of the desirable properties of CFCs but, because they exist for a shorter time in the atmosphere, concerns about ozone depletion and global warming are significantly reduced. | |
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| Global Warming | |
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Global warming, which is an increase in the natural greenhouse effect, refers to
the physical phenomenon that may lead to heating of the earth. Most of the sun's
energy reaches the earth as visible light. After passing through the atmosphere,
part of this energy is absorbed by the earth's surface and in the process is
converted into heat energy. The earth, now warmed by the sun, radiates heat
energy back into the atmosphere toward space.
Naturally occurring gases, such as carbon dioxide, water vapor, and ozone, absorb and thus retain some of the outgoing heat energy. This process slows the heat loss, making the earth's surface warmer than it would be if this heat energy had passed unobstructed through the atmosphere into space. The warmer earth's surface, in turn, radiates more heat until a balance is established between incoming and outgoing energy. This warming process --- caused by the atmosphere's absorption of the heat energy radiated from the earth's surface --- is called the greenhouse effect. Increasing concentrations of gases from man-made sources (e.g., carbon dioxide, methane, and CFCs) that absorb the heat radiation could lead to a slow warming of the earth. This phenomenon is commonly referred to as global warming. | |
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| Global Warming Potential (GWP) | |
| An index developed to provide a simplified means of describing the relative ability of each greenhouse gas emission to affect radiative forcing and thereby the global climate. GWPs are defined on a mass basis, relative either to CFC-11 (known as the Halocarbon GWP, or HGWP) or to carbon dioxide. Because CFC-11 has a finite lifetime in the atmosphere, the HGWP can be calculated explicitly and is a single number. Because carbon dioxide does not have a finite lifetime in the atmosphere, GWPs relative to it have to be calculated up to a particular time horizon, for example, 20, 100, or 500 years. | |
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| Greenhouse Gases | |
| Greenhouse gases are present in relatively small quantities in the atmosphere and strongly absorb infrared radiation or "heat" emitted by the earth. The primary greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, ozone, and some of the chlorofluorocarbons. Concentrations of several greenhouse gases are increasing, primarily as a result of human activities. | |
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| HCFCs | |
| Hydrochlorofluorocarbons (HCFCs) are compounds comprised of hydrogen, chlorine, fluorine, and carbon atoms. These compounds have many of the useful properties of CFCs, but are destroyed naturally in the lower atmosphere and do not persist to the same extent as CFCs. Only a fraction of HCFCs emitted can be transported to the ozone layer in the stratosphere where their chlorine could deplete ozone. HCFCs typically have an ozone depletion potential 2% to 11% that of CFCs. | |
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| HFCs | |
| Hydrofluorocarbons (HFCs) are compounds consisting of hydrogen, fluorine, and carbon atoms which, like the HCFCs, are destroyed naturally in the lower atmosphere. They have many of the useful properties of the CFCs. Because they do not contain chlorine, they are not involved in ozone depletion. | |
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| Not-in-Kind (NIK) Technologies | |
| Not-in-kind technologies do not rely on the use of fluorocarbons. | |
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| Ozone | |
| Ozone, formed in the stratosphere by the action of sunlight on oxygen, is also an airborne pollutant near ground level. Low altitude (tropospheric) ozone is formed by reactions between hydrocarbons and oxides of nitrogen in sunlight. | |
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| Ozone Depletion | |
| Ozone is continually being formed and destroyed by chemical reactions occurring in the stratosphere. There are large natural changes in ozone concentration in the stratosphere; for example, between summer and winter there is a change of about 25% at mid-latitudes. Ozone depletion occurs if the rate of ozone destruction is increased due to human activities. | |
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| Ozone Depletion Potential (ODP) | |
| The ozone depletion potential is a quantity describing the extent of ozone depletion calculated to arise from the release to the atmosphere of 1 kilogram of a compound relative to the ozone depletion calculated to arise from a similar release of CFC-11. The calculation is an integration of all potential effects on ozone over the whole time that traces of the compounds could remain in the atmosphere; it shows the total potential. | |
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| Ozone Hole | |
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Each winter, over the Antarctic, the atmosphere is isolated from the rest of the world by a natural
circulation of wind, called the polar vortex. It is dark and becomes very cold, resulting in the
formation of clouds in the ozone layer of the stratosphere. On these clouds inorganic forms of
chlorine undergo chemical reactions to give species which, when the sun shines again on
Antarctica each spring, are further converted by sunlight into forms that can deplete ozone.
The rapid destruction of the majority of the ozone in the region of the Antarctic stratosphere that contains clouds leads to a marked temporary local depletion called the ozone hole. The hole disappears when the Antarctic atmosphere warms enough to break up the circulation which isolates it from the rest of the world. Ozone-rich air then flows in to replenish the ozone layer over the Antarctic. The enhanced springtime loss of ozone in the Antarctic started to occur late in the 1970s when the inorganic chlorine loading of the stratosphere was about 1.5 parts per billion. Hence, this value has been suggested as a target for reduction in chlorine loading. Although springtime ozone depletion occurs in the Arctic, there is no ozone "hole" as such. Clouds can form in the stratosphere and similar chlorine chemistry to that in the Antarctic has been observed; the degree of isolation of the air mass is much less than over the Antarctic. | |
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| Radiative Forcing | |
| There are several factors that can change the balance between the energy absorbed by the earth and that emitted by it in the form of longwave infrared radiation. These factors, which include seasonal changes in solar radiation, changes in reflectivity of the earth's surface, and presence of aerosols, cause the radiative forcing on climate. | |
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| Stratosphere | |
| The stratosphere is the layer above the troposphere, ranging in altitude from 8-15 kilometers at the lower boundary to about 50 kilometers at the top, and accounting for 10% of the mass of the Earth's atmosphere. Temperature increases with altitude and vertical mixing is relatively slow. Between the height of about 15 and 35 kilometers, the air contains relatively high concentrations of naturally occurring ozone (up to 10 ppm); this is known as the ozone layer. | |
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| Time Dependent Ozone Depletion Potential (TDODP) | |
| Time dependent, or transient or short-term ODP, like the full lifetime or steady state ODP, is calculated for 1 kilogram of the compound relative to 1 kilogram of CFC-11, but the calculation is truncated at a specific time (between 5 and 50 years). Unlike the full lifetime ODP, no account is taken of any effect of the compound (or CFC-11) after the specified time. | |
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| Total Equivalent Warming Impact (TEWI) | |
| TEWI is the sum of the direct (chemical) and indirect (energy) emissions of greenhouse gases from the operation or use of a CFC or fluorocarbon system for its useful life. In the Global Warming and Energy Efficiency Study, co-funded by AFEAS and the U.S. Department of Energy, the TEWI was quantified for each alternative, including non-chemical alternatives, for each major application area: refrigeration, air conditioning, insulation, and solvent cleaning. | |
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| Transitional Substances | |
| Because they retain many of the desirable properties of CFCs and have much lower ODPs, HCFCs can be used to meet some of the societal needs and demands currently served by CFCs, while reducing the risk of ozone depletion. However, because they contribute some chlorine to the ozone layer, they may need to be used for only a transitional period until non-ozone depleting solutions are developed. Thus, HCFCs are recognized as transitional substances. | |
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| Troposphere | |
| The troposphere is the lowest layer of the atmosphere extending from the earth's surface to a height of 8 to 15 kilometers. The troposphere contains 90% of the atmosphere and is well mixed by weather systems. The temperature of the troposphere decreases with height. | |
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| UV-B Radiation | |
| The wavelengths of radiation in the range of 280 to 320 nanometers. UV-B radiation represents the portion of solar radiation reaching the earth's surface that is most efficiently filtered and controlled by atmospheric ozone. | |