Executive Summary

Exhaust products emitted by the current subsonic-transport fleet may influence tropospheric ozone and clouds, and thus Earth's climate. Such emissions in the upper troposphere may affect climate at the surface of the Earth by means of many chemical and meteorological processes. Some of these processes are poorly understood, and thus cannot be quantified with much certainty. NASA's Atmospheric Effects of Aviation Project (AEAP) has as its objective the development of a scientific basis for assessment of the atmospheric impacts of the exhaust components of subsonic and supersonic fleets of civil aircraft. The NRC's Panel on Atmospheric Effects of Aviation (PAEAN) has been requested to evaluate the appropriateness of AEAP's research plan, appraise the project's results, and suggest how best to reduce the remaining uncertainties.

It is essential that AEAP apply its research funds where they are most likely to reduce the major uncertainties. This report evaluates the scientific questions being addressed in AEAP's Subsonic Assessment (SASS) project that are related to the perturbation of the ozone and aerosol concentrations in the upper troposphere, and discusses setting priorities for this research support. It provides preliminary recommendations related to work in each of the four main atmospheric-science foci of SASS: laboratory studies; two groups of observations designed to increase our knowledge of relevant processes in the troposphere (chemical and radiative); and the Global Modeling Initiative. The main thrust of these recommendations is the need to (1) study the sensitivity of key species in the upper troposphere and lower stratosphere to various chemical and dynamic processes, and (2) endeavor to characterize the same region as regards aerosols.



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--> Executive Summary Exhaust products emitted by the current subsonic-transport fleet may influence tropospheric ozone and clouds, and thus Earth's climate. Such emissions in the upper troposphere may affect climate at the surface of the Earth by means of many chemical and meteorological processes. Some of these processes are poorly understood, and thus cannot be quantified with much certainty. NASA's Atmospheric Effects of Aviation Project (AEAP) has as its objective the development of a scientific basis for assessment of the atmospheric impacts of the exhaust components of subsonic and supersonic fleets of civil aircraft. The NRC's Panel on Atmospheric Effects of Aviation (PAEAN) has been requested to evaluate the appropriateness of AEAP's research plan, appraise the project's results, and suggest how best to reduce the remaining uncertainties. It is essential that AEAP apply its research funds where they are most likely to reduce the major uncertainties. This report evaluates the scientific questions being addressed in AEAP's Subsonic Assessment (SASS) project that are related to the perturbation of the ozone and aerosol concentrations in the upper troposphere, and discusses setting priorities for this research support. It provides preliminary recommendations related to work in each of the four main atmospheric-science foci of SASS: laboratory studies; two groups of observations designed to increase our knowledge of relevant processes in the troposphere (chemical and radiative); and the Global Modeling Initiative. The main thrust of these recommendations is the need to (1) study the sensitivity of key species in the upper troposphere and lower stratosphere to various chemical and dynamic processes, and (2) endeavor to characterize the same region as regards aerosols.

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--> These steps will make it possible to quantify uncertainties (or at least bound them), focus fieldwork most profitably, and optimize model design. In addition to making these scientific recommendations, the panel recommends two important program-management steps. To achieve the desired results from the SASS project, and to employ the available funds most effectively, it is essential that AEAP first draw up and execute an adequately detailed, prioritized, unambiguous research working strategy and plan. Some of the elements needed exist in various documents, but nowhere is there a single strategy that sets out the important topics, evaluates them in terms of costs and benefits, and assigns them priorities that also reflect the associated uncertainties and sensitivities. Second, AEAP needs to give SASS strong scientific leadership. To ensure that the many tasks involved contribute measurably to the goals of this single research strategy, the science must be managed by a person with recognized experience and authority, who is in a position to manage the implementation of the strategy through the end of the project.