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*:/ : 11-1 h WA <br />reactions (e.g. manufacture of cement). Carbon dioxide is removed from the atmosphere (sequestered) <br />when it is absorbed by plants as part of the biological carbon cycle. <br />Methane (CHa) is emitted during the production and transport of coal, natural gas, and oil. Methane <br />emissions also result from livestock and other agricultural practices and from the decay of organic waste <br />in municipal landfills and water treatment facilities. <br />■ Nitrous oxide (N20) is emitted during agricultural and industrial activities as well as during combustion <br />of fossil fuels and solid waste. <br />■ Fluorinated gases are synthetic, strong GHGs that are emitted from a variety of industrial processes. <br />Fluorinated gases are sometimes used as substitutes for ozone -depleting substances. These gases are <br />typically emitted m smaller quantities, but because they are potent GHGs, they are sometimes referred to <br />as high global -warming- potential (GWP) gases. <br />• Chlorofluorocarbons (CFCs) are GHGs covered under the 1987 Montreal Protocol and used for <br />refrigeration, air conditioning, packaging, insulation, solvents, or aerosol propellants. Since they are <br />not destroyed in the lower atmosphere (troposphere, stratosphere), CFCs drift into the upper <br />atmosphere where, given suitable conditions, they break down ozone. These gases are also ozone - <br />depleting gases and are therefore being replaced by other compounds that are GHGs covered under <br />the Kyoto Protocol. <br />• Perfluorocarbons (PFCs) are a group of humanmadechemicals composed of carbon and fluorine <br />only. These chemicals (predominantly perfluoromethane [CF4] and perflumoethane [C2F6]) were <br />introduced as alternatives, along with HFCs, to the ozone -depleting substances. In addition, PFCs are <br />emitted as by-products of industrial processes and are used in manufacturing. PFCs do not harm the <br />stratospheric ozone layer, but they have a high global warming potential. <br />• Sulfur Hexafluon&e (SF6) is a colorless gas soluble in alcohol and ether, slightly soluble in water. <br />SF6 is a strong GHG used primarily in electrical transmission and distribution systems as an insulator. <br />HydrocAloroflucrocarhons (HCFCs) contain hydrogen, fluorine, chlorine, and carbon atoms. <br />Although ozone depleting substances, they are less potent at destroying stratospheric ozone than <br />CFCs. They have been introduced as temporary replacements for CFCs and are also GHGs. <br />• Hydrofluorocarbons (HFCs) contain only hydrogen, fluorine, and carbon atoms. They were <br />introduced as alternatives to ozone -depleting substances to serve many industrial, commercial, and <br />personal needs. HFCs are emitted as by-products of industrial processes and are also used in <br />manufacturing. They do not significantly deplete the stratospheric ozone layer, but they are strong <br />GHGs (IPCC 2001; USEPA 2018b). <br />GHGs are dependent on the lifetime or persistence of the gas molecule in the atmosphere. Some GHGs <br />have stronger greenhouse effects than others. These are referred to as high GWP gases. The GWP of GHG <br />emissions are shown in Table 7, GHG Emissions and Their Relative Global lVarming Potential CoVared to COZ. The <br />75C-155 <br />