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3 Recommendations Because NASA's Atmospheric Effects of Aviation program at this point has few resources left to carry out new work, the panel recognizes that recommendations for future research will be of very limited use in the near term. However, it is also recognized that research on aviation's atmospheric impacts is by no means complete, and this issue will become all the more important in the coming decades as demand for air travel continues to increase. Likewise, although recent decisions by aircraft manufacturers make it unlikely that a fleet of HSCTs will be flying anytime soon, there is still substantial interest in supersonic aviation, and it seems inevitable that it will be considered again at some point in the future. Thus, it is worthwhile to highlight remaining uncertainties and make recommendations for research that would help reduce those uncertainties. These suggestions are applicable to any future research program on the atmospheric effects of stratospheric aircraft, whether it is carried out by NASA or by others. The fundamental reaction processes that occur in engines are not understood well enough to determine the potential magnitude, phase, and composition of emissions. Without this information, plume and wake processes cannot be quantified. In particular, the production of sulfur particles needs to be better understood, as well as the role played by chemi-ions and by trace species such as metals and hydrocarbons. PAEAN recommends more investigation of fundamental engine chemistry and particle formation processes, including laboratory, modeling, and field studies. Because future high-speed aircraft have not yet been fully built and tested,
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the emission indices of many critical species are not known with certainty. It important to keep in mind that any future changes in fuel composition (such as new lubricant compounds) and engine design (such as the new LPP design) can greatly affect emissions. PAEAN recommends that assessment studies continue to include a realistic range of potential emission indices, particularly for NOx and S compounds. Quantifying the aviation-related impacts on stratospheric aerosol requires some knowledge about the ambient climatology of aerosols and aerosol precursors in the lower stratosphere and upper troposphere. Very little information exists about the flux of aerosol particles and precursors into the stratosphere from the troposphere at low latitudes. PAEAN recommends continued emphasis on characterizing the global distribution and sources of lower stratospheric and upper tropospheric aerosol, in order to properly gauge the relative impact of aircraft particle emissions. Accurate modeling of transport processes is needed to fully assess the effects of aircraft emissions deposited in the upper troposphere and lower stratosphere, yet atmospheric dynamics remains a major source of uncertainty in assessment models. PAEAN recommends that theoretical and observational studies of transport processes such as troposphere-stratosphere exchange and midlatitudel/low-latitude mixing processes continue. There is still potential for surprises in regard to the impacts of stratospheric aircraft on ozone chemistry. Some particularly vulnerable areas include the region of the stratosphere above 22 km (where there are very limited in situ comprehensive measurement sets) and the polar regions. Although model predictions and atmospheric observations seem to be steadily converging, it is still not clear what processes may be missing from the models. PAEAN recommends continued support for in situ field measurement campaigns that advance understanding of stratospheric chemistry. PAEAN also recommends continued emphasis on laboratory and field studies of the composition of PSCs and the fundamental kinetics and temperature dependences of the chemical processes associated with PSCS. Assessing climate impacts has not been a major focus of AESA thus far, which is understandable given the complexity of the problem and the limits of current models. However, results coming from IPCC and elsewhere indicate that this issue is worthy of further investigation. Because the composition and climate of the atmosphere will continue to change, assessment models need the capability to look at an evolving atmosphere and to include progressively more realistic chemistry/climate feedback processes. PAEAN recommends that emphasis be placed on quantifying the radiative impacts of aircraft emissions, particu-
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larly water vapor, in the stratosphere, and the consequent feedbacks that may exist within the climate system. Good progress has been made in developing the GMI as a 3-D, diagnostic assessment modeling tool; however, a more diverse range of modeling tools that can include feedbacks and address processes on a variety of scales maybe needed to make real progress in forecasting future aviation impacts. PAEAN recommends that the next generation of stratospheric assessment models include chemical-dynamical feedbacks, higher vertical and horizontal resolution, and accurate representation of relevant tropospheric processes.
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