The Atmospheric Effects of Stratospheric Aircraft Project An Interim Review of Science and Progress (1998) / Chapter Skim
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2 Scientific Background for HSCT Concerns
2 Scientific Background for HSCT Concerns
Pages 8-22
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From page 8... ...
who pointed out that the observed aridity of the stratosphere (water-vapor mixing ratios of a few ppmv) could be understood only if water vapor were transported into the stratosphere by an upwelling circulation confined to the tropics, where air parcels would be "freeze-dried" as they went through the cold tropical tropopause.
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From page 9... ...
The temperature inversion throughout the stratosphere confers stability on the air against vertical mixing and vertical motion, but it does not inhibit horizontal motions. In contrast, tropospheric air undergoes strong vertical mixing, mainly because of moist convection.
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From page 10... ...
This wave-induced pumping pulls air upward and poleward from the tropical troposphere into the stratosphere and pushes it poleward and downward into the extratropical lowermost stratosphere (Horton et al., 1995~. Note that vertical transport from the overworld into the lowermost stratosphere, not quasi-horizontal exchange across the tropopause,
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From page 11... ...
The longer residence times of i4C in the overworld, by comparison with i4C and other radioactive debris in the lowermost stratosphere, demonstrate that for transfer of HSCT exhaust from the stratosphere to the troposphere, the rate-limiting step is the relatively easily evaluated vertical transport from the overworld into the lowermost stratosphere, not the intermittent, very complicated, quasi-horizontal exchange across the tropopause. A simple mental picture of stratosphere-troposphere transport for both subsonic-aircraft and HSCT exhaust is to visualize the lowermost stratosphere as a large reservoir for chemical constituents that is replenished from above by diabetic descent across the boundary of the overworld, and undergoes horizontal leakage on its equator-facing side, especially in the springtime, as well as some downdirected leakage.
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From page 12... ...
Ozone production is quenched sooner than ozone destruction in the air entering polar night, so the ozone mixing ratio decreases. The changing photochemical steady state in that region also affects ozone concentration.
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From page 13... ...
Lower panel: Mixing ratios in parts per million by volume. The superimposed heavy lines indicate where the local ozone photochemical replacement time ([O3]
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From page 14... ...
. The simultaneous observations of longlived gases, in conjunction with recently developed understanding of atmospheric dynamics, provided investigators with quantitative new information about transport.
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From page 15... ...
Since 18-21 km is the altitude range at which HSCT emissions will occur, it is important that accurate quantitative estimates (including temporal and spatial variability) be developed for the rates at which mass and trace constituents are transported between the mid-latitude northern hemisphere air corridors and the polar vortex as well as those corridors and the tropical-upwelling region.
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From page 16... ...
Throughout the stratosphere, ozone is continually destroyed by several homogeneous gas-phase chemical reactions, including direct ozone destruction by the reaction O + O3 ~ 2O2, and by reactions with active atoms and radicals, illustrated here for the case of nitrogen oxides (NOX) , nitric oxide (NO)
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From page 17... ...
According to model calculations (see Figure 5 in Chapter 3) , ozone destruction by NOX is about 60 percent of the total in the tropics, 70 percent at mid-latitudes, and 80 percent in sunlit polar regions in the ozoneformation region between 28 and 38 km.
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From page 18... ...
Discovery of the Antarctic ozone hole in 1985 sparked coordinated campaigns of field measurement, laboratory studies, and modeling, including reconsideration of the potential role of heterogeneous processes in the chemistry of ozone at mid-latitudes. The special perturbed chemistry of the polar stratosphere requires the presence of at least one of the two types of polar stratospheric cloud (PSC)
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From page 19... ...
However, there is a pulse of OH and HOO radicals shortly after sunrise; some substance had been accumulating all night and was dissociated by the early morning sunlight, and the observed OH and HOO were higher than the modeled values throughout the sunlit day. Also, the observed ratio ClO/Cl was higher than the model values, which is consistent with high OH, since the reaction OH + HCl = H2O + Cl increases active chlorine, including ClO.
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From page 20... ...
on sulfate aerosol into reaction schemes for describing midlatitude ozone chemistry (Rodriguez et al., 1991~. Considerable improvement in agreement between predicted and observed vertical patterns of ozone depletion caused by chlorofluorocarbons and high C1O concentrations was obtained as a result, and the significance of the region below 20 km became apparent.
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From page 21... ...
It should be noted, however, that the physical and chemical changes in PSCs caused by HSCT emissions, and the magnitude of the response of polar ozone chemistry to those PSC changes, will depend on both microphysical properties such as the aerosol surface-area-to-volume ratio and the specific composition of the aerosol (e.g., the proportions of NAT, STS, and sulfuric acid tetrahydrate)
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From page 22... ...
Observations of the spatial and temporal variability of such trace constituents, together with high-resolution Doppler imager (HRDI) wind data and meteorological analyses from the U.K.
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