The Atmospheric Effects of Stratospheric Aircraft Project An Interim Review of Science and Progress (1998) / Chapter Skim
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4 Outstanding Issues
4 Outstanding Issues
Pages 36-45
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From page 36... ...
As explained in Chapter 2, after global atmospheric forces have pushed air up into the tropical stratosphere, much of that air fans out in the mid-latitude middle stratosphere (where the ozone distribution is dominated by ultraviolet radiation 36
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From page 37... ...
Further measurements, and continuing analysis of existing data, will be necessary to reduce the existing uncertainties, particularly above 20 km and in tropical regions. POLAR PROCESSING OF HSCT EXHAUST Since late 1978, instruments aboard circumpolar NASA satellites have measured global ozone and its vertical distribution every day over the sunlit portion of Earth.
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From page 38... ...
This effect raises the possibility of substantial additional ozone depletion in the polar regions in response to HSCTs via heterogeneous processes involving PSC particles. Furthermore, ozone in the polar regions appears to be sensitive to interannual fluctuations of temperature and aerosol to a degree that may indicate particular sensitivity to HSCT emissions.
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From page 39... ...
Interactions among the NOX, HOX, and ClOX chemical cycles in the gas phase mean that the NOX in the exhaust may decrease rather than increase polar ozone depletion under some circumstances (Considine et al., 1994, 1995~. While HSCT emissions at non-polar latitudes may influence polar chemistry, as noted earlier, polar processes may also alter the effect of HSCT emissions occurring at non-polar latitudes.
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From page 40... ...
The sensitivity of the ozone balance to horizontal transport in the region of transition between chemical and dynamical control places strong constraints on assessment models. It is not clear that parameterized transport in 2-D models can realistically compute the evolution of the ozone mixing ratio in regions where the balance between chemistry and dynamics will be highly dependent on whether transport takes place by rapid, large-scale wave breaking (in which air parcels move through large latitudinal ranges in a few days)
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From page 41... ...
Uncertainties related to the instruments measuring particle sizes, however, made it necessary to use crude size estimates in converting the number of particles measured into inferred particle mass (Fahey et al., 1995~. These percentages thus should be viewed with caution, and evaluations of potential HSCT impact need to include the related uncertainties about surface area available for heterogeneous reactions and other details of the relevant microphysical processes.
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From page 42... ...
They may also be found to have a significant global impact on radiative forcing or other atmospheric properties. The consequences of an aviation-related increase in sulfate aerosols, in the context of these existing processes, must be considered.
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From page 43... ...
They suggested that AESA should "conduct an immediate first-cut assessment" of possible effects of HSCT effluents on climate, including the modification of the thermal balance of the atmosphere by changes in the vertical ozone distribution, the alteration of the number or size distribution of cloud- or ice-nucleating aerosols, and the influence of added water vapor on cirrus clouds and contrails. From the research summarized above, it does not seem that significant impacts on climate from HSCT exhaust can be expected, although many uncertainties remain.
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From page 44... ...
Similarly, in Figure 7 five AESA assessment models exhibit good agreement for calculated change of ozone vertical column, with a maximum spread of 1 percent. Figure 8, however, shows the uncertainty in the vertical distribution of the calculated changes in ozone concentration at 45°N.
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From page 45... ...
OUTSTANDING ISSUES 45 microphysics, and heterogeneous chemistry will be challenging to resolve. AESA needs to identify the most critical uncertainties, and develop a plan establishing research priorities that will reduce those uncertainties as much as possible during AESA's lifetime.
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