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--> Introduction Like all fossil-fueled power plants, aircraft engines emit a variety of gases (among them carbon monoxide and dioxide; water vapor; methane and other hydrocarbons; and oxides of nitrogen, hydrogen, and sulfur), as well as particles such as soot. Each of these can affect Earth's climate, either directly or indirectly—for instance, by adding to the ''greenhouse gases", by reducing or increasing ozone concentrations, or by providing nuclei that influence cloud formation. Subsonic-aircraft exhaust components are of concern because they are emitted primarily in the upper troposphere and the lowermost stratosphere, a region where additions are thought to have a relatively large effect on global climate. Not all atmospheric processes at these altitudes are well understood, particularly those involving cloud physics and upper-tropospheric chemistry. Nor is there a large number of observations to provide "background levels" with which levels measured in high-traffic areas could be compared, or by means of which natural variability could be defined. Recent work has suggested that climatic impacts of upper-tropospheric ozone changes caused by the current subsonic aircraft fleet should be relatively minor, but impacts of aircraft-related particles on climate and lower-stratospheric ozone are largely unknown. Furthermore, with air traffic a rapidly growing part of the transport sector, especially in developing countries, this particular type of fossil-fuel pollution is likely to increase more rapidly than others. NASA's Atmospheric Effects of Aviation Project (AEAP) has as its goal the development of a scientific basis for assessment of the atmospheric impact of the exhaust constituents discharged during cruise operations by fleets of subsonic and supersonic civil aircraft. AEAP has two subprojects. The Atmospheric
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--> Effects of Stratospheric Aircraft project (AESA, a subelement of NASA's High-Speed Research Program) is designed to develop the body of scientific knowledge necessary for evaluating the impact of stratospheric aircraft on the atmosphere, whereas the Subsonic Assessment project (SASS, an element of the Advanced Subsonic Technology Program) is designed to develop an assessment that can say how aircraft emissions affect radiative forcing and climate, as well as how they affect ozone. The October 1996 colloquium organized by ONERA (the French Office National d'Etudes et de Recherches Aérospatiales) and the Comité Avion-Ozone showcased an impressive amount of research into aviation emissions and their behavior, and it was clear that investigators sponsored by AEAP were making major contributions. The NRC's Panel on Atmospheric Effects of Aviation (PAEAN) judges that AEAP has made significant progress, but that some shift in priorities could help reduce uncertainties more rapidly and, in a time of increased budget pressures, increase return on the taxpayers' investment. The SASS component of AEAP (a relatively new undertaking) is designed to assess the effects of emissions from both the current world fleet of subsonic aircraft and a likely larger future fleet. The next chapter of this PAEAN report discusses two of the major issues related to the current subsonic air fleet's emissions: the emittants' potential effect on ozone, principally through NOx, and the possible radiative forcing of climate resulting from aerosol scattering and increased cloudiness. The following chapter looks at each of the atmospheric-science topics of the SASS project as they appear in Table 1–2 of the first SASS report (Thompson et al., 1996), notes some questions and comments, and makes specific recommendations. Last, two overarching recommendations are presented for the project as a whole, which PAEAN considers to be of the utmost importance: the development of a complete, coherent, detailed strategic research plan, and the designation of a strong, experienced scientist to be charged with its implementation. This PAEAN report discusses only tropospheric issues; emittant deposition in, or exchanges and/or interactions with, the lower stratosphere will be reviewed in a later report. Also, the project elements that deal with emissions characterization and interactions in the near field (the region in which the emissions are still influenced by aircraft-related effects) are reviewed in a separate report now in press, An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements (NRC, 1997). A report on the AESA project is in preparation.
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