Although aviation-related activities currently account for only 0.5 percent of total air pollution in the United States, the types of pollutants emitted by these activities are among the most prevalent and harmful in the atmosphere, and are expected to grow over time. The major sources of aviation-related emissions are aircraft, which emit pollutants at ground level as well as over a range of altitudes; the equipment (such as vehicles that transport baggage) that services them on the ground at airports; and vehicles transporting passengers to and from the airport. The amount of emissions attributable to each source varies by airport. A 1997 study of mobile source emissions at four airports found that ground access vehicles were the most significant source (accounting for 27 to 63 percent of total mobile source emissions), followed by aircraft (15 to 38 percent of the total) and ground service equipment (12 to 13 percent of the total). The emissions produced by these sources include carbon monoxide; sulfur dioxide; particulate matter; toxic substances (such as benzene and formaldehyde); and nitrogen oxides and volatile organic compounds, which contribute to the formation of ozone, a major pollutant in many metropolitan areas. In addition, aircraft emit carbon dioxide and other gases that have been found to contribute to climate change due to warming. According to the United Nations’ Intergovernmental Panel on Climate Change, global aircraft emissions accounted for approximately 3.5 percent of the warming generated by human activities. (The types, amounts, and impact of emissions from aviation-related sources are described in detail in appendix II.) Although only limited research has been done on the impact of projected growth in air travel on emissions, indications are that emissions are likely to continue increasing. FAA reported in June 2001 that the number of commercial flights is expected to increase about 23 percent by 2010 and about 60 percent by 2025. Each flight represents a takeoff and landing cycle during which most aircraft emissions enter the local atmosphere. In addition, an EPA study of 19 airports projected that the proportion of mobile-source emissions of nitrogen oxides attributable to aircraft in the areas adjacent to these airports will triple from a range of 0.6 to 3.6 percent in 1990 to a range of 1.9 to 10.4 percent in 2010. Such projections, however, do not consider recent industry changes, such as airlines’ increased use of smaller aircraft and the financial uncertainties in the aviation industry. A recent report by the Department of Transportation indicated that the September 11, 2001, terrorist attacks, combined with a cut-back in business travel, had a major and perhaps long-lasting impact on air traffic demand. A number of federal, state, and international agencies are involved in controlling aviation-related emissions. The Clean Air Act mandates standards for mobile sources of emissions such as aircraft, ground service equipment, and automobiles. As mandated by the act, EPA promulgates emission standards for aircraft, and has chosen to adopt international emission standards for aircraft set by ICAO, which was chartered by the United Nations to regulate international aviation and includes the United States and 188 other nations. As the United States’ representative to ICAO, FAA, in consultation with EPA, works with representatives from other member countries to formulate the standards. EPA and FAA work to ensure that the effective date of emissions standards permit the development and application of needed technology and give appropriate consideration to the cost of compliance, according to FAA officials. The officials also noted that EPA is responsible for consulting with FAA concerning aircraft safety and noise before promulgating emission standards. In addition to issuing aircraft emission standards, ICAO has studied aviation-related emission issues and issued guidance to its members on ways to reduce these emissions. States can address airport emissions in plans, known as state implementation plans, that they are required to submit to EPA for reducing emissions in areas that fail to meet the National Ambient Air Quality Standards set by the EPA under the Clean Air Act for common air pollutants with health and environmental effects (known as criteria pollutants). Geographic areas that have levels of a criteria pollutant above those allowed by the standard are called nonattainment areas. Areas that did not meet the standard for a criteria pollutant in the past but have reached attainment and met certain procedural requirements are known as maintenance areas. The options available to states for controlling pollution from airports are limited because most emissions come from mobile sources, such as automobiles, which are already regulated by EPA, and states are generally preempted from issuing regulations on aircraft emissions because of EPA’s federal responsibility in this area. FAA is responsible for enforcing the emission standards and for ensuring that emissions resulting from airport construction projects under their authority comply with the National Environmental Policy Act, which requires an environmental review of such projects, and the Clean Air Act’s requirement that the projects comply with state implementation plans for attaining air quality standards. (See appendix III for additional information on federal, state, and international responsibilities concerning aviation- related emissions.) Many of the nation’s busiest airports and airlines that serve them have initiated voluntary emission reduction measures, such as converting shuttle buses and other vehicles from diesel or gasoline fuels to cleaner alternative fuels. While the actual impact of these measures is unknown, some measures (such as shifting to new cleaner gas or diesel engines or alternative fuels) have the potential to significantly reduce emissions, such as nitrogen oxides, volatile organic compounds, particulate matter, and carbon monoxide. The airports and airlines have undertaken these efforts for a variety of reasons, including requirements by states imposed as part of their plans to ensure that severely polluted areas (i.e., nonattainment areas) achieve the air quality standards established by the Clean Air Act and to gain federal approval for airport construction projects. In late 2003, EPA will begin implementing stricter standards for ozone, which could make it more difficult for areas to achieve or maintain attainment status. Representatives from the aviation industry as well as federal and state officials told us that the new air quality standards, combined with the boost in emissions expected from increases in air travel, could cause airports to be subject to more emission control requirements in the future. In addition, according to FAA officials, approval of some projects in these areas may be less likely because of several factors, including increased focus on air quality by communities that oppose airport development. Many of the nation’s busiest airports, in conjunction with the air carriers that serve them, have implemented voluntary control measures to reduce emissions from major sources, including aircraft, ground support equipment, and passenger vehicles entering and exiting the airport, according to our review of FAA documents and interviews with airport and state environmental officials. Specific guidelines or regulations for airports to reduce emissions from these sources do not exist, but some airports have been proactive in developing programs and practices that reduce emissions. Although the actual impact of these measures is unknown, some initiatives have the potential to significantly reduce emissions from certain sources. For example, a number of carriers at Dallas/Fort Worth International and Houston airports have agreed to voluntarily reduce emissions associated with ground service equipment by up to 75 percent. Figure 1 provides examples of activities to reduce emissions that have been implemented at U.S. airports. Appendix V provides more information on some airports’ voluntary efforts to reduce emissions. Only 3 of the 13 states with major commercial airports in nonattainment areas—California, Texas, and Massachusetts—have targeted airports for emission reductions. The remaining states have not included emission reductions at airports as part of their strategies for bringing nonattainment areas into compliance with the Clean Air Act’s ambient air quality standards because they have attempted to achieve sufficient reductions from other pollution sources. Officials from these states noted that EPA has the authority to set emission standards for aircraft and nonroad vehicles, including ground support equipment at airports, which preempts the states’ regulation of these sources. California and Texas face major ozone nonattainment problems— California in the Los Angeles metropolitan area and Texas in the Dallas- Fort Worth and Houston metropolitan areas. According to air quality officials from both states, even after imposing all of the traditional emission control measures available, such as vehicle emission inspections, the three metropolitan areas still may not be able to reach attainment status for ozone by the 2010 deadline for Los Angeles and by the 2005 and 2007 deadlines for Dallas-Fort Worth and Houston, respectively. Despite potential legal challenges from airlines, both California and Texas turned to airports for additional emission control measures. Texas has negotiated an agreement with the Dallas/Fort Worth International and Houston airports and the airlines that serve them to reduce emissions attributable to ground support equipment by 90 percent. California has reached a similar agreement with the major airlines serving the five commercial airports in the Los Angeles nonattainment area to reduce emissions from ground support equipment. California’s efforts to cut ground support equipment emissions in the Los Angeles area are part of a statewide campaign to reduce airport pollution. In addition to using its limited authority under the Clean Air Act to implement airport-related emission reductions, the state has also employed a certification process provided for in federal law. Under this provision, before FAA can approve a grant for any new airport, new runway, or major runway extension project, the governor must certify that the project complies with applicable air and water quality standards. California has developed criteria for determining whether a proposed airport expansion project would have an impact on the environment, including air quality. Unlike other states, California uses the criteria as a mandatory condition for project certification. If the project exceeds one of the criteria—by increasing the number of passengers, aircraft operations, or parking spaces and thereby producing an impact on the environment— the airport is required to implement emission mitigation measures in order to attain certification. Thus far, three airports—Sacramento International, San Jose International, and Ontario International—have initiated expansion projects that were required to comply with the certification standards. However, in a legal opinion issued in August 2000, FAA’s Office of Chief Counsel stated that California has no legal authority to impose operational limitations on airports through the certification process. According to FAA, California has not publicly responded to the opinion.