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--> 5 Recommendations AESA has made praiseworthy progress toward its goal of providing a scientific basis for assessing the potential impact of a fleet of HSCTs on the atmosphere, particularly on stratospheric ozone. Its models project the ozone reduction by a fleet of 500 Mach 2.4 HSCTs to be almost an order of magnitude less than was projected in 1990, under operating conditions that include a conceptual HSCT engine with a low NOx emission index, and a choice of flight altitude and Mach number that would minimize the effect of added NOx. Through laboratory measurements of the rates of newly recognized heterogeneous chemical reactions, and simultaneous in situ measurements of almost all active chemical species at altitudes up to 20 kilometers, AESA has shown that heterogeneous reactions involving sulfuric-acid aerosols deactivate NOx in the lower stratosphere. Looking at patterns of correlation between reactive species and tracers such as N2O and CH4 has yielded new understanding of atmospheric chemistry and atmospheric motions. Although major uncertainties remain in how numerical models treat atmospheric motions (especially above 20 km), AESA's modeling work has been effective in translating both laboratory and in situ measurements into projected impacts. AESA has also identified new and important questions, such as how HSCT exhaust gases would interact with the polar stratospheric clouds that contribute to ozone depletion over the Antarctic. PAEAN commends AESA for its work; the recommendations that follow indicate areas that the panel feels are currently in need of greatest emphasis.
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--> Transport Processes Observations from aircraft and satellite programs have greatly increased our understanding of transport processes in the stratosphere. Major uncertainties in understanding air transport remain, however, especially above the tropical region and at altitudes above 20 kilometers, where fewer data are available. PAEAN recommends that AESA emphasize the analysis and interpretation of data from aircraft missions and satellites, in order to quantify better the meridional and vertical transport in the stratosphere, particularly at altitudes between 20 and 30 kilometers. Polar Processing The processing of HSCT emission products in the polar regions is not adequately represented in current 2-D models. Only when the evolution of liquid and solid PSC particles in the winter polar stratosphere is known can the effects of the additional input of HSCT-related gases and aerosols be properly assessed. PAEAN recommends that AESA support field measurements as well as model developments that are specifically designed to unravel the complex formation process of polar stratospheric cloud particles. Assessment Models Current assessments of the impact of a fleet of HSCT still rely on 2-D models. However, 3-D transport effects are of crucial importance for understanding this impact, and the use of the GMI model will be required for future assessments. PAEAN recommends that AESA continue to support the development and testing of the Global Modeling Initiative model. Microphysics in the Plume-Wake Region As the earlier AESA panel noted in 1994, the processes (both gas-phase and heterogeneous) within the plume-wake region need to be properly described so that they can be incorporated into large-scale atmospheric models. PAEAN recommends that the development of microphysical models of the plume-wake regime continue, in order to assess the role of the chemical and physical transformations that may occur before engine effluents mix into the background atmosphere. Climate Studies Because of the complexity of the climate system, it is not clear that any expansion of the limited climate studies AESA has sponsored will yield signifi-
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--> cant further progress on the issue of climate impacts within the time frame of the 1998 assessment. Research to date suggests that the globally averaged direct radiative impact of stratospheric aircraft emissions can be expected to be small, although the effects of aerosols must be better quantified. PAEAN recommends that AESA continue its present policy of not undertaking model studies of the impact of aviation on climate, beyond calculating the degree of radiative forcing by components of HSCT exhaust. Beyond the 1998 Assessment Many of the uncertainties noted earlier, such as those related to transport processes, microphysics, and heterogeneous chemistry, have not yet been sufficiently reduced to permit an informed assessment of the possible atmospheric effects of a fleet of HSCTs to be made. AESA management needs to identify the most critical uncertainties, and develop a strategy that will enable them to be reduced as far as possible during AESA's remaining lifetime and even beyond. PAEAN recommends that AESA draw up and execute an adequately detailed plan that sets priorities for research to reduce the remaining critical uncertainties.
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