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--> Executive Summary Scientists and policy-makers alike are concerned that operation of a fleet of high-speed civil transport (HSCT) aircraft could significantly affect the global atmosphere. HSCT emissions may have a direct effect on the chemistry of the atmosphere, leading to changes in the distribution of ozone; they may also have indirect effects on ozone and on global climate through coupling with radiative and dynamical processes in the atmosphere. An assessment of the atmospheric impact of a fleet of HSCTs thus requires not only an understanding of the chemistry of the natural stratosphere and its possible perturbations by HSCT emissions, but also an understanding of the pathways for transport of HSCT emissions within the atmosphere, and the resulting temporal and spatial distribution of HSCT emissions. The results of NASA's Atmospheric Effects of Stratospheric Aircraft (AESA) project were summarized in a 1995 NASA assessment. The present report looks at that summary and at more recent work to evaluate the state of the science. AESA has made good progress in the past few years. Satellite and aircraft observations have elucidated important aspects of large-scale transport processes. Field campaigns have provided a much better picture of the relative importance, below 20 km altitude, of the major catalytic cycles for ozone destruction. Careful intercomparisons of assessment models have led to reduction of some of the differences among the models. However, a number of uncertainties and inconsistencies still remain.
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--> Transport Processes Subsonic aircraft fly in the lowermost part of the stratosphere, where material is exchanged across the tropopause on time scales of a few months. HSCTs, however, would fly in the region of the stratosphere above about 16 km, from which exchange with the troposphere occurs only through a slow, global-scale vertical circulation. This slow movement determines the rate at which HSCT emissions are transported into the regions of the stratosphere in which exhaust components could be important catalysts for ozone destruction. Within the region above 16 km, horizontal transport by eddy motions is important for determining the distribution of chemical constituents. Accurate model treatment of these transport processes is necessary for estimating the spatial and temporal distributions of HSCT emissions, and is thus essential to prediction of the emissions' potential effects on ozone distribution and on climate. Chemical Processes Uncertainties related to gas-phase chemistry are less than those associated with heterogeneous chemistry on aerosol particles. Stratospheric aerosols arising from natural sources play a key role in the chemistry of the ozone layer as a result of the reactions that occur on their surfaces. Because emissions from a fleet of HSCTs might be a significant additional source of aerosols, and because reactions on aerosols appear to be important in converting nitric oxides to nonreactive species, it is important that reactions on both natural and aircraft-related aerosols be better understood. Predicted ozone reduction is greatly diminished if the heterogeneous reactions that occur on aerosols in the mid-latitudes are included in assessment models. However, substantial additional ozone depletion may occur in polar regions in response to HSCT emissions, because of heterogeneous chemistry involving polar stratospheric clouds (PSCs). Assessment Models The models used by AESA for assessment of HSCT-induced changes in the ozone layer have been two-dimensional (2-D) models with highly parameterized representations of transport and mixing. Model intercomparisons indicate that the 2-D models all predict similarly small impacts of HSCTs on total ozone. Within that total column amount, however, their predictions vary noticeably at some altitudes. There are also a number of differences between models and measurements that need to be resolved. How well 2-D models treat the sensitive role of transport can be tested only by comparison with 3-D chemical-transport models, such as the 3-D model being developed under AESA's Global Modeling Initiative.
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--> Climate Impacts Recent general-circulation model studies have examined the impact on surface climate of HSCT-induced changes in the global ozone and water-vapor distributions. These studies suggest that such emissions should have no significant direct impact on climate. HSCT-emitted sulfur aerosols are also expected to play a small role in climate compared to other aerosol sources, but more analysis is needed. Contrail formation is not expected to occur at the altitudes at which HSCTs will fly, although indirect radiative impacts may result if HSCT emissions enhance the formation of PSCs. Recommendations On the basis of its evaluation of the science and AESA's progress, the Panel on Atmospheric Effects of Aviation makes the following recommendations to NASA: Transport Processes: There is still considerable uncertainty about atmospheric transport above 20 Kilometers. PAEAN recommends that AESA emphasize analysis 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, since the evolution of polar stratospheric cloud particles is not yet understood. 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: Three-dimensional transport effects are of crucial importance for understanding the potential impact of a fleet of HCSTs, and the use of the NASA's GMI model will be required for future assessments. PAEAN recommends that AEAP continue to support the development and testing of the Global Modeling Initiative model. Microphysics in the Plume-Wake Region: 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, further climate studies by AESA may not yield significant progress in the near future. Research to date suggests that the globally averaged direct radiative impact of
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--> 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 discussed in this volume have not yet been sufficiently reduced to permit an informed assessment of the possible impact of a fleet of HSCTs to be made. Research in the years to come must focus on those critical uncertainties first. 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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