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--> 1 Context For This Review Introduction Aviation is an integral part of the global transportation network. The number of commercial aircraft flights has grown steadily for decades and is expected to increase even more rapidly in the years to come, particularly in developing countries. The impacts of this activity on atmospheric composition and climate are not yet entirely clear. Researchers are only now beginning to quantify the likely effects of aircraft exhaust, which includes carbon dioxide (CO2), water (H2O), oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HCs), sulfur (S), and soot. Assessing the atmospheric effects of aviation requires understanding processes with scales that range over orders of magnitude—from microphysical details of particle production in the engine to global climatic changes. Over the last decade, most attention has been focused on the questions of how aircraft emissions (principally NOx) could interact with species present in the atmosphere to alter the concentration and distribution of ozone, and how these changes, as well as the addition of CO2 and water vapor, could influence climate. The focus of concern has recently shifted to other impacts that are less direct but may ultimately be more important. For instance, contrails, which are formed by water vapor and particulate matter in aircraft exhaust, may increase the amount of cirrus clouds in the atmosphere, thereby influencing radiative forcing. Likewise, ozone concentrations could be affected through the heterogeneous chemical reactions that take place on the surface of the particles emitted or formed in aircraft wakes and on naturally occurring clouds. All these concerns take on added importance because of the ongoing inter-
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--> national climate change treaty negotiations. The Kyoto Protocol to the United Nation's Framework Convention on Climate Change specifically mentions the need to limit emissions of greenhouse gases from aviation. A determination that aviation is a significant contributor to anthropogenic effects on climate could have far-reaching regulatory and economic implications for the airline industry. The NRC Panel on Atmospheric Effects of Aviation (PAEAN) is charged with evaluating the direction and progress of the Subsonic Assessment (SASS) component of NASA's Atmospheric Effects of Aviation Project (AEAP). PAEAN was asked to evaluate the appropriateness of the research plan, to appraise the project-sponsored results relative to the current state of scientific knowledge, to identify key scientific uncertainties, and to suggest research activities likely to reduce those uncertainties. In doing so, the panel has focused principally on SASS's most recent self-assessment, Atmospheric Effects of Subsonic Aircraft: Interim Assessment Report of the Advanced Subsonic Technology Program (Friedl, 1997), and on the SASS strategic plan that governs the next few years' research. The complementary progress described in the European Scientific Assessment of the Atmospheric Effects of Aircraft Emissions (Brasseur et al., 1997), as well as other appropriate literature, were also taken into consideration. The panel had the benefit of briefings by two of SASS's project scientists at its November 1997 meeting, followed by discussions with them about SASS's objectives and factors that affect the project's ability to achieve those objectives. The panel then decided that the appropriateness of the SASS research plan needed to be examined in the context of all related NASA research activities; thus, at the panel's January 1998 meeting, representatives of programs in NASA's Office of Earth Science (formerly Mission to Planet Earth) briefed the panel on other ongoing and planned research activities. These discussions of NASA's Tropospheric Chemistry Program, Upper Atmosphere Research Program, Radiation Science Program and atmospheric chemical modeling activities were extremely useful to the panel's deliberations, and influenced the recommendations given in this report. The SASS Strategic Plan To implement a project like SASS, it is essential to have an operational framework for characterizing objectives, establishing goals, and prioritizing activities. In its earlier review of SASS (NRC, 1997a), the panel requested that AEAP staff provide an explicit plan that would establish a context for reviewing and interpreting the project's activities. A strategic plan was drawn up and presented to the panel at its November 1997 meeting. It has since been modified somewhat. This plan provides both a vision and an approach to which the panel can compare SASS's specific efforts. The key elements of SASS's strategic plan are: Goal: Develop a scientific basis for assessment of the atmospheric impact of subsonic aviation, particularly commercial aircraft cruise emissions.
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--> Objective: Provide a measure of the change in radiative forcing due to changes in ozone (O3), carbon dioxide (CO2), water vapor (H2O), and aerosols in the upper troposphere and lower stratosphere by emissions from both present and future subsonic aircraft. Minimum Success Requirement: a quantitative measurement of the changes in O3, CO2, H2O, and aerosols resulting from subsonic aircraft emissions, with quantified uncertainties. Key Scientific Issues: What are current and future emissions from aircraft? What chemical and physical processes in the atmosphere could be perturbed by aircraft emissions? Are atmospheric observations consistent with the current understanding of aircraft emissions-related chemistry and physics? What are the predicted ozone changes and climatic impact associated with aviation? What are the uncertainties in these predictions? Major Science Objectives for 1999 (in priority order): Reduce uncertainty in evaluation of present-day tropospheric O3 change due to aircraft NOx. Constrain estimates of indirect radiative impacts of soot and sulfur emissions. Quantify radiative forcing from contrails. Assess impact of subsonic aircraft emissions on stratospheric O3. Calculate changes in CO2 and O3 radiative forcing from future aircraft fleet (with IPCC). Provide robust limit on potential impacts of CO and non-methane hydrocarbon emissions on O3 chemistry. Major Science Objectives for 2001 (in priority order): Begin to quantify indirect radiative impact of soot and sulfur emissions. Calculate O3 changes and climate forcing from aircraft NOx for the current and future fleets, with a factor of two as the goal for uncertainty level. SASS's overall approach is designed to lead to formal assessment reports that can be used to guide decisions related to the development of a new generation of subsonic aircraft. The strategy taken by AEAP in general is to link the information gained from emissions characterization and laboratory studies to interpretation of near-field interactions and aircraft operational scenarios. This information is intended to be used as input to models, and as a means of interpret-
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--> ing atmospheric observations. The results will constitute a basis for the development of the assessment reports. PAEAN's assessment of SASS was also made in light of an advisory it received from Howard Wesoky, then with NASA's Office of Aeronautics and Space Transportation Technology (OASTT). He made the following points about his perception of the appropriate scope and responsibility of AEAP, particularly with regard to the subsonic assessment component: the impacts of subsonic aviation do not appear to be unique and are likely to be small relative to total anthropogenic influences on the atmosphere; obtaining a more accurate assessment of aviation's impacts will require many more years of research and considerably greater investment than is possible from the Advanced Subsonic Technology and OASTT programs; a prudent management approach to this assessment area may be to recognize that within NASA the basic research necessary to advance understanding is the responsibility of the Office of Earth Science. These issues clearly have a substantive bearing on the SASS program, and thus on the deliberations of the panel. The confluence of the strategic plan and the statements listed above can be perceived as addressing a key point that PAEAN made in its Interim Review of the Subsonic Assessment Project (NRC, 1997a): ''It is essential that AEAP apply its research funds where they are most likely to reduce the major uncertainties." The Goals and Organization of This Review PAEAN's evaluations, and the recommendations given in this report, are aimed primarily at helping SASS make the greatest possible progress before its final assessment report, slated for the year 2001. The panel raises issues that it feels deserve more attention than they are receiving at present, and it suggests where research performed outside SASS may help bound uncertainties. However, it will very likely take until well beyond 2001 to identify with reasonable certainty all the processes that may be important in quantifying the effects of the present or future subsonic fleets on the atmosphere. Some longer-term goals and strategies are therefore discussed as well. The panel feels that, in general, SASS has correctly identified the most important scientific issues, and the project's major objectives are all of considerable scientific interest. The SASS strategic plan represents a significant step forward in clarifying the goals and strategies of the program. The panel has some concerns, however, that the current emphases may not be properly balanced, and that the program's goals may not be most efficiently and rapidly achieved with the present approach. Chapter 2 contains a discussion of the different scientific issues that form the basis of SASS's investigations, and Chapter 3 continues this discussion, focusing
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--> on the issues directly related to modeling. It should be noted that these discussions are not meant as a comprehensive review of all issues involved with sub-sonic aviation's atmospheric impacts1; rather, they are limited to the particular issues that the panel feels present the greatest remaining sources of uncertainty and may present the most important opportunities for SASS to make progress in reducing uncertainties. Chapter 4 contains a list of the panel's highest priority concerns and recommendations, covering both scientific and management related issues. 1 A comprehensive assessment of the atmospheric impacts of aviation is being prepared by the Intergovernmental Panel on Climate Change (IPCC). Since this IPCC report is not yet final, the PAEAN report neither draws upon or refers to the IPCC effort.
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