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1 Introduction The inevitable increase in global demand for commercial air travel in the coming decades is likely to be met by an increase in the size of the subsonic aircraft fleet and also, perhaps, by the introduction of a supersonic aircraft fleet. However, the operation of a large fleet of supersonic aircraft could alter important physical and chemical processes of Earth's atmosphere, because these aircraft would emit CO2, H2O, NOx, SO2, CO, hydrocarbons, and sulfur and carbon aerosols directly into the stratosphere. A major focus of concern has been that the emissions could significantly alter the Earth's protective ozone layer, which is concentrated near the altitude range in which HSCTs are expected to fly. More recently, attention has been given to the question of whether aircraft emissions could also affect the radiative balance of the atmosphere and thereby contribute significantly to global climate change. The history of research on this issue goes back to the early 1970s. Originally, the U.S. Department of Transportation (DOT) was the lead agency in the development of an American supersonic transport (SST), in the face of competition from the Anglo-French Concorde and the former Soviet Union Tu-144. DOT commissioned the Climatic Impact Assessment Program (CIAP) in 1972. After the termination of the U.S. SST program, for both economic and environmental reasons, CIAP was completed in 1975 with a Report of Findings and a voluminous set of monographs (over 5,000 pages) that attempted to record all the scientific research on the effect of aviation on the atmosphere up to that date. In 1976, Congress assigned NASA the role of primary agency for stratospheric research, and its Upper Atmosphere Research Program (UARP) has been active since then in continuing much of the research begun under CIAP. In 1988,
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the Atmospheric Effects of Stratospheric Aircraft (AESA) project was formally initiated as a comprehensive effort to predict the atmospheric impacts of a future fleet of supersonic aircraft.2 Since then, many reports have been produced summarizing the results of this research. AESA's final assessment report (Kawa et al., 1999) formed the primary basis for this PAEAN evaluation. AESA has made praiseworthy progress toward its goal of providing a scientific basis for assessing the potential impact of a fleet of HSCTs on the atmosphere, through its investigations of aircraft engine combustion and emissions, atmospheric chemistry and transport, polar processing, microphysics in the plume wake, and climate studies. Because the proposed HSCTs would at times operate subsonically in both the stratosphere and troposphere, and subsonic aircraft sometimes fly in the lower stratosphere, there are common issues important for assessing the atmospheric effects of both the proposed HSCT fleet and the existing subsonic fleet. For example, to properly assess the atmospheric effects of both types of aircraft, one needs a quantitative determination of the amount of mixing between stratosphere and troposphere, and between low and middle latitudes. Many combustion/emission issues are also common to both types of aircraft. AEAP has been able to produce a degree of synergism between its supersonic component (AESA) and its subsonic component (SASS) by funding work that is relevant to both programs. Over the last decade, AESA has served as a focal point for the organization of large field programs featuring coordinated, in situ observations of numerous chemical species from aircraft and balloon platforms. The cooperative efforts of atmospheric modelers and a wide array of experimentalists in these field programs have contributed not only to assessment of the atmospheric effects of aviation but also to fundamental understanding of stratospheric chemistry and dynamics. For example, the first simultaneous measurements of all catalytically active radical species have demonstrated the dominance of HOx catalysis in lower stratospheric odd-oxygen destruction. (Wennberg et al., 1994). In addition, measurement of long-lived tracers such as SF6 and CO2 have been used to evaluate the 'mean age' of stratospheric air masses (see, for example, Elkins et al., 1996; Boering et al., 1996), which contributed to understanding of atmospheric transport processes. The results of NASA's studies are of more than just scientific interest; they have had, and will continue to have, a significant impact on national and international policy decisions. In the United States, the Environmental Protection Agency (EPA) is considering changing its regulations (40 CFR 87) covering aircraft engine emissions, potentially affecting both subsonic and supersonic air 2 AESA is operated under NASA's High-Speed Research program, but it is still closely coordinated with the UARP. AESA is managed together with NASA's subsonic aviation research program to form the Atmospheric Effects of Aviation Project (AEAP).
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craft. These regulatory decisions will be influenced by AEAP's estimates of the potential environmental impacts of aircraft emissions. Likewise, the International Civil Aviation Organization (ICAO) relies on NASA for input into its revisions to the International Standards and Recommended Practices on Environmental Protection (Annex 16, Volume II to the Convention on International Civil Aviation). NASA's research has also been incorporated into reports of the Intergovernmental Panel on Climate Change (IPCC), especially its upcoming report on Aviation and the Global Atmosphere (IPCC, 1999). These IPCC findings could influence international agreements such as the United Nations Climate Change Treaty, as well as national regulations. For instance, the European Community has closely followed the progress of the IPCC and may revise its aircraft emissions regulations independently of the United States. Some recent developments have greatly influenced the deliberations and focus of concern of the panel. In early 1999, it was reported that the Boeing Company had suspended its efforts to develop a fleet of HSCTs and that, consequently, the proposed NASA budget contains no future funding for the AESA project (or for AEAP in general) and no additional funding for any other project to assume the scientific research carried out under this program. Many scientists have expressed concern that with such an abrupt termination of this program, the United States' capability to resume research on this topic at a future date will be seriously degraded. Much scientific momentum may be lost as research efforts are turned to other issues, and this could greatly hinder NASA's ability to support both U.S. regulatory and international diplomatic requirements in regards to the effects of aviation on the atmosphere.
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