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--> 1 Introduction and History Scientists and policy-makers alike are concerned that operation of a fleet of high-speed civil transports (HSCTs) could significantly alter important physical and chemical processes of Earth's atmosphere.* A recent NASA report (Stolarski et al., 1995) states that for every kilogram of current jet fuel burned, such future aircraft would typically emit the following mass amounts of these species of concern: 3155 g of carbon dioxide (CO2), 1237 g of water (H2O), 18 g of oxides of nitrogen (NOx), 3.5 g of carbon monoxide (CO), and less than 1 g each of sulfur dioxide (SO2), hydrocarbons, and soot or carbon (C). Each of these emission products is capable of directly affecting the atmosphere in one or more of the following ways: destruction of ozone, production of ozone, absorption and scattering of incoming solar radiation, absorption and emission of infrared (heat) radiation. Through various feedback processes these direct influences may lead to other, indirect effects on the chemistry of the atmosphere or on climate. To assess the impact of aircraft emissions, the aircraft component of each effect should be evaluated quantitatively and compared with the magnitude of the global natural component. NASA's Atmospheric Effects of Aviation Project (AEAP), in particular its Atmospheric Effects of Stratospheric Aircraft (AESA) portion, is largely devoted to these goals. * The inevitable increase in global demand for commercial air travel can be met either by a marked increase in the size of the subsonic-aircraft fleet, or by the introduction of supersonic aircraft. These options will have somewhat different environmental consequences. This report deals only with the projected atmospheric impacts of a supplemental fleet of 500 HSCTs.
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--> The history of the study of atmospheric impacts of high-speed commercial aircraft began about 25 years ago with the Climatic Impact Assessment Program (CIAP), which was sponsored by the U.S. Department of Transportation over the period 1972 to 1975. Research on stratospheric ozone begun under CIAP has continued in NASA's Upper Atmosphere Research Program (UARP) since 1976. Although not specifically directed at assessing the effects of aircraft on the stratosphere, UARP contributes to the general understanding of the stratosphere that is essential for realistic assessments. In 1991, the UARP sponsored the first reviews of the impact of aircraft emissions on the stratosphere (Johnston et al., 1991; Douglass et al., 1991) that had been made in 15 years. The most recent effort, AESA, was initiated as part of NASA's High-Speed Research Program in 1988. For proper evaluation, AESA must be examined in the context of the entire NASA program of stratospheric research. Since its inception, AESA has produced a number of significant reference publications. The Atmospheric Effects of Stratospheric Aircraft (Prather et al., 1992) reported on the three-day workshop in 1990 that defined seven critical needs for AESA, described the scope of the research being sponsored by AESA, and presented initial results. A second program report appeared early the following year (Stolarski and Wesoky, 1993a), together with a three-volume report on the 1992 Models and Measurements workshop (Prather and Remsberg, 1993). They were followed by the third program report (Stolarski and Wesoky, 1993b) and an Interim Assessment Report of the High-Speed Research Program (Albritton et al., 1993) that focused on AESA's research and the potential impact of HSCTs on the ozone layer. The 1993 assessment report was formally reviewed by the National Research Council's Panel on the Atmospheric Effects of Stratospheric Aircraft. Their evaluation is contained in the report Atmospheric Effects of Stratospheric Aircraft (NRC, 1994). The report focused on the key scientific uncertainties associated with the impact of stratospheric aircraft. The top three uncertainties, in the opinion of the panel, were: Dispersion characteristics of HSCT effluents in the lower stratosphere Physical and chemical properties of stratospheric aerosols and PSC particles The climate effects of the HSCT fleet. Also of importance, the panel thought, were issues related to the accuracy of flight and emissions scenarios, the modeling of plume and wake chemistry and dynamics, the adequacy of two-dimensional (2-D) models for the HSCT assessment, and model sensitivity studies. The 1994 NRC review closed with recommendations intended to guide NASA in their efforts to reduce uncertainties in predictions and to provide advice for better use of existing data. Highest priority was given to recommendations addressing the issues above; other recommenda-
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--> tions included development of instruments for particle measurements, evaluation of how the Pinatubo aerosol cloud evolved, more model-to-model comparisons, and utilization of UARS and other satellite data for model development and verification. In 1995 AESA commissioned an assessment of scientific predictions of the potential impact of a fleet of HSCTs. This second assessment, Stolarski et al. (1995), summarizes the topics of concern to AESA at the end of 1995 and offers strategies for reducing uncertainties about the effects of aviation. (It does not, unfortunately, include progress reports on microphysical models and aerosol instrumentation, evaluation of the lessons learned from the Pinatubo cloud, or discussion of the applications of UARS and other satellite datasets and their assimilation into models, as the AESA panel had requested.) The present NRC Panel on Atmospheric Effects of Aviation (PAEAN) has used Stolarski et al. as the focus of their evaluation of AESA's progress. To fulfill the promise of the word ''Science'' in the title of the present report, An Interim Review of AESA: Science and Progress, this review repeats some material in Stolarski et al. as needed background (Chapter 2), gives in some detail aspects of pertinent science that are not fully stated by Stolarski et al. (Chapter 3), and comments on some more recent issues (Chapter 4). Chapter 5 presents PAEAN's recommendations, some of which echo the strategies in Stolarski et al., and some of which are new. The "Progress" in this PAEAN report's title can be seen as it restates the situation that existed just before and just after AESA research started in 1990, and compares it with the project's status as reported in later AESA publications, especially Stolarski et al., and with certain articles published during 1996 and 1997. Thomas Graedel, Chairman of the NRC's AESA Panel, emphasized in his testimony (Graedel, 1994) to the Subcommittee on Technology, Environment, and Aviation of the U.S. House of Representatives that two years would not be enough to reduce the uncertainties associated with the effects of a fleet of HSCTs, and that "the assessment's overall reliability will not change significantly by 1995." Stolarski et al. confirms his statement; it remains to be seen how far those uncertainties will have been reduced by AESA's end.
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