The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth Shine is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

The Atmospheric Effects of Stratospheric Aircraft (AESA) project is the stratosphere-oriented half of NASA's Atmospheric Effects of Aviation Project (AEAP). A component of the High-Speed Research Program, AESA is a comprehensive effort to predict the atmospheric impacts of a future fleet of supersonic aircraft flying in the stratosphere. AESA, which began in FY 1990, completed its first phase and issued an assessment report in November 1995. NASA issued an interim assessment in 1993, which was evaluated by an NRC Panel on the Atmospheric Effects of Stratospheric Aircraft in a document issued in 1994. The project has now entered a second phase; its final assessment report is due in late 1998.

The present review of AESA is the product of the NRC Panel on the Atmospheric Effects of Aviation (PAEAN). PAEAN members were selected to provide expertise in relevant fields that include field observations, laboratory chemistry, atmospheric dynamics and modeling, aircraft engines, and climate. The charge from its NASA sponsor, AEAP, is to provide assessment of and guidance to AEAP by evaluating the appropriateness of the AEAP's research plan, appraising the project-sponsored results relative to the current state of scientific knowledge, identifying key scientific uncertainties, and suggesting research activities likely to reduce those uncertainties. In 1997 the panel published Interim Review of the Subsonic Assessment Project and An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements; this report completes PAEAN's first evaluation of AEAP's activities.

While the effects of the current subsonic fleet are perhaps of more urgent concern at the moment, PAEAN has been asked to assess how well the state of

scientific knowledge will allow an informed decision to be made at the end of 1998 regarding the possible atmospheric impact of a future fleet of high-speed civil transport aircraft. The purposes of this panel's report on AESA are two: to evaluate the progress of AESA-sponsored research since 1990, and to provide recommendations for future direction of the project.

When PAEAN was formed, it included sixteen people. To produce its first reports, it broke up into working groups—supersonic/stratospheric, subsonic/tropospheric, and emissions. The original stratospheric group put together the initial draft of this document, and we thank them for their efforts. We appreciate the skill and perseverance of our study director and editor, Ellen Rice; the knowledgeable assistance of staff officer Laurie Geller; and the administrative support of Doris Bouadjemi. Last, we are grateful to the many people, both those involved with AEAP and those outside it, who through briefings and reports have kept us apprised of the progress of AESA and the science.

This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the NRC's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The content of the final report is the responsibility of the NRC and the study committee, and not the responsibility of the reviewers. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals, who are neither officials nor employees of the NRC, for their participation in the review of this report:

Thomas E. Graedel, Yale University

Donald M. Hunten, University of Arizona

Robert D. Hudson, University of Maryland

Conway B. Leovy, University of Washington

Lynn M. Russell, Princeton University

Anne K. Smith, National Center for Atmospheric Research

Paul H. Wine, Georgia Institute of Technology

While the individuals listed above have provided many constructive comments and suggestions, it must be emphasized that responsibility for the final content of this report rests entirely with the authoring committee and the NRC.