Aviation is an integral part of the global transportation network, and the number of flights worldwide is expected to grow rapidly in the coming decades. Yet, the effects that subsonic aircraft emissions may be having upon atmospheric composition and climate are not fully understood. To study such issues, NASA sponsors the Atmospheric Effects of Aviation Program (AEAP). The NRC Panel on Atmospheric Effects of Aviation is charged to evaluate AEAP, and in this report, the panel is focusing on the subsonic assessment (SASS) component of the program. This evaluation of SASS/AEAP was based on the report Atmospheric Effects of Subsonic Aircraft: Interim Assessment Report of the Advanced Sub-sonic Technology Program (Friedl, 1997), on a strategic plan developed by SASS managers, and on other relevant documents.

While SASS has made significant progress over the last couple of years, significant uncertainties remain, particularly with regard to the chemical and radiative impacts of particles, including sulfur and carbon aerosols, contrails, and modification of natural cirrus clouds. In this report, the panel reviews recent scientific developments that it sees as the most relevant to AEAP and notes many specific areas where more research attention would be particularly useful. Overall the panel identified the following as its highest priority concerns and recommendations:

• Impacts of heterogeneous chemistry on ozone. Heterogeneous reactions couple strongly with gas-phase processes and may significantly affect ozone concentrations; thus, assessments of the impacts of aviation on ozone levels must

• include the uncertainties associated with the climatology of aerosols and the heterogeneous processes occurring on these aerosols.
• Emissions characterization. More aircraft engine emissions characterization studies, particularly for particles, sulfur oxidation products, and hydro-carbons would be worthwhile. To accurately interpret field measurements of these exhaust species, methods need to be developed to distinguish in situ between emissions from aircraft and those from other sources.
• Aerosol climatology. Aircraft particulate emissions must be assessed in the context of the ambient particle field. It is recommended that SASS assemble currently available baseline data on the ambient particle fields in the upper troposphere and lower stratosphere, so that the relative effects of aircraft-emitted particles can be properly gauged.
• Troposphere-stratosphere exchange. Cross-tropopause transport processes are not well enough known to permit appropriate representation in models, and thus need to be an important focus of AEAP analyses. It is important that SASS and AESA sort out this composite assessment with whatever type of collaboration will be most effective.
• The Global Modeling Initiative. Carefully coordinated sensitivity studies are needed to test the dependence of the Global Modeling Initiative (GMI) results upon the parameterizations of sub-grid processes, stratosphere-troposphere exchanges, and the chemical schemes selected. Likewise, the tropospheric components of the GMI should participate in international model-intercomparison studies. SASS's large commitment to the GMI can be justified only if heterogeneous processes (including the formation and chemistry of aerosols, contrails, and cirrus) can be incorporated into the model framework in the near future; otherwise, it may be wise to invest in alternative modeling tools. Finally, the GMI efforts should be more closely coordinated with the SASS-sponsored climate modeling studies.
• Planning of field campaigns. The results of the recent field campaigns SUCCESS and SONEX should be used to guide the planning and conduct of SASS's next campaign, helping to pinpoint the most important scientific questions and the most effective measurement methodologies.
• Cooperation with other research programs. SASS experimental research should focus as much as possible directly on the effects of aircraft emissions; the importance of these perturbations, however, can only be evaluated in the context of the natural distribution and variability of the relevant chemical species, as well as the magnitude of other anthropogenic perturbations. SASS

• should give much higher priority then, to exploring how data accumulated by other programs can be used to provide some context in which aircraft effects can be evaluated. We recommend that SASS devote more of its time and money to establishing cooperative efforts with other research programs, both within and beyond NASA, and to evaluating the possible applicability of datasets and information gathered by others. Likewise, SASS should be preparing to use the existing and anticipated data from satellite platforms, identifying the specific data products that will be relevant to evaluating the impacts of aircraft emissions.