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Acknowledgments US Department of Transportation - BTS

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In general, lead emissions are estimated by multiplying an activity level by an emissions factor that represents therate at which lead is emitted for the given source category. This estimate is then adjusted by a factor that representsthe assumed effectiveness <strong>of</strong> control technologies. For lead released during combustion, a top-down approach isused to share national estimates <strong>of</strong> fuel consumption by fuel type to each consumption category (e.g., motor fuel,electric utility, etc.) and, subsequently, each source (e.g., passenger cars, light-duty trucks, etc.).TABLE 4-51. Air Pollution Trends in Selected Metropolitan Statistical Areas (MSAs)TABLE 4-52. Areas in Nonattainment <strong>of</strong> National Ambient Air Quality Standards for Criteria PollutantsThe U.S. Environmental Protection Agency measures concentrations <strong>of</strong> pollutants in the ambient air at its air qualitymonitoring sites, which are operated by state and local agencies. These sites conform to uniform criteria for monitorsiting, instrumentation, and quality assurance, and each site is weighted equally in calculating the composite averagetrend statistics. Furthermore, trend sites must have complete data for 8 <strong>of</strong> the 10 years in the trend time period to beincluded. However, monitoring devices are placed in areas most likely to observe significant concentrations <strong>of</strong> airpollutants rather than a random sampling <strong>of</strong> sites throughout the nation.TABLE 4-53. U.S. Carbon Dioxide Emissions from Energy Use by SectorThe combustion <strong>of</strong> fossil fuels, such as coal, petroleum, and natural gas, is the principal anthropogenic (humancaused) source <strong>of</strong> carbon dioxide (CO2) emissions. Since fossil fuels are typically 75 percent to 90 percent carbon byweight, emissions from the combustion <strong>of</strong> these fuels can be easily measured in carbon units, as is shown in thetable.CO2 emissions data are derived from estimates. The U.S. <strong>Department</strong> <strong>of</strong> Energy, Energy Information Administration(EIA), estimates CO2emissions by multiplying energy consumption for each fuel type by its carbon emissionscoefficient, then subtracting carbon that is sequestered by nonfuel use <strong>of</strong> fossil fuels. Carbon emissions coefficientsare values used for scaling emissions to specific activities (e.g., pounds <strong>of</strong> CO2emitted per barrel <strong>of</strong> oil consumed).Emissions estimates are based on energy consumption data collected and published by EIA Several smalladjustments are made to its energy consumption data to eliminate double counting or miscounting <strong>of</strong> emissions. Forexample, EIA subtracts the carbon in ethanol from transportation gasoline consumption because <strong>of</strong> its biologicalorigin.Emissions coefficients are based on the density, carbon content, and heat content <strong>of</strong> petroleum products. For manyfuels, except liquefied petroleum gas (LPG), jet fuel, and crude oil, EIA assumed coefficients to be constant over time.For LPG, jet fuel, and crude oil, EIA annualized carbon emissions coefficients to reflect changes in chemicalcomposition or product mix.Since the combustion <strong>of</strong> fossil fuels is a major producer <strong>of</strong> CO2emissions, sources <strong>of</strong> uncertainty are related to: 1)volumes <strong>of</strong> fuel consumed; 2) characteristics <strong>of</strong> fuel consumed; 3) emissions coefficients; and 4) coverage. EIA notesthat volumetric fuel data are fairly reliable in the 3 percent to 5 percent range <strong>of</strong> uncertainty. The density and energycontent <strong>of</strong> fuels are usually estimated. According to EIA, the reliability <strong>of</strong> these estimates vary. For example,estimates <strong>of</strong> the energy content <strong>of</strong> natural gas are reliable to 0.5 percent, while estimates for coal and petroleumproducts are lower because they are more heterogeneous fuels. The reliability <strong>of</strong> emissions coefficients depends onwhether the characteristics <strong>of</strong> a fuel are difficult to measure accurately. Finally, uncertainties may result because datamay be excluded or unknown sources <strong>of</strong> emissions not included.EIA's estimation methods, emissions coefficients, and the reliability <strong>of</strong> emissions estimates are discussed in detail inU.S. <strong>Department</strong> <strong>of</strong> Energy, Energy Information Administration, Emissions <strong>of</strong> Greenhouse Gases in the UnitedStates, 1998 available on: www.eia.doe.gov/oiaf/1605/ggrpt/index.html.TABLE 4-54. Petroleum Oil Spills Impacting U.S. WaterwaysThe U. S. Coast Guard's (<strong>US</strong>CG) Marine Safety Information System (MSIS) is the source <strong>of</strong> these data. It includesdata on all oil spills impacting U.S. navigable waters and the Coastal Zone. The <strong>US</strong>CG learns <strong>of</strong> spills through directobservation, reports from responsible parties and third parties. Responsible parties are required by law to report spillsto the National Response Center (NRC).Reports may be made to the <strong>US</strong>CG or Environmental Protection Agency pre-

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