Atmospheric Effects of Aviation A Review of NASA's Subsonic Assessment Project (1999) / Chapter Skim
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2 Discussion of the Science Issues
2 Discussion of the Science Issues
Pages 9-26
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From page 9... ...
Some highly uncertain issues remain, such as the lightning NOx source, and the need for more upper troposphere/lower stratosphere trace gas measurements to constrain model estimates. In recent years, though, some significant progress has been made.
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From page 10... ...
notes on p.36, "Aircraft exhaust is known to contain a large number of C2-C~7 species, although the relative amounts are not well established." This means that assessments must consider the effects on ozone chemistry of unburned hydrocarbons and hydrocarbon oxidation products in the exhaust plume. Our knowledge of the hydrocarbons present in current jet fuels is reasonably good, but some important minor components remain ill-defined.
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From page 11... ...
The atmospheric lifetime of hydrocarbons in this mass range is determined largely by reactions with OH radicals; it is relatively short for the conditions typical of the upper troposphere and lower stratosphere, but some of the initial products of oxidation of the large alkanes (e.g., the ketones) have longer lifetimes than the original alkanes.
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From page 12... ...
Local emissions of NO titrate existing ozone, forming NO2, but as transport and dilution of the air mass occurs, NO2 photolysis will eventually form additional ozone. The SASS project needs to take into account the possible influence of tropospheric ozone increases in the years ahead, which would affect model results and uncertainties.
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From page 13... ...
These data, coupled with the extensive and growing database of j-values measured for various amounts and types of clouds, should be used to develop algorithms that can estimate realistic j-values for use in the predictive modeling of the effects of aircraft emissions on the ozone column. PARTICLE EMISSIONS, CHARACTERIZATION, AND CHEMISTRY The SASS strategic plan reflects an increased recognition of the importance of aerosol-related issues, and more of its resources are now being directed toward this area.
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From page 14... ...
Heated at 192°C, a large fraction of these submicron particles was volatilized, and their composition was consistent with that of sulfuric acid. While the Concorde engine design is quite different than that of today's subsonic fleet, these studies are still relevant to SASS, as they confirmed that a large number of aerosols can be generated in the aircraft wake by the simultaneous condensation of sulfuric acid and water vapors (heteromolecular nucleation)
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From page 15... ...
is formed should be explicable in terms of the complex kinetics of the many reactions involved, but the rate-coefficient data needed for the calculations are often not available for the temperature-pressure regimes occurring in the aircraft engines. Although this "kinetic solution" is the ultimate goal to aid our understanding of sulfur conversion, the thermodynamic properties of the sulfur gases also yield useful information.
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From page 16... ...
Such regions have been reported to extend over hundreds of kilometers, for outflow from deep convection near the Intertropical Convergence Zone (Clarke, 1993~. More recently, outflow from lower clouds in mid-latitudes has been shown to produce similarly high particle concentrations in the daytime, by photochemistry linked to high sulfuric acid production near these clouds (Clarke et al., 1998a,b)
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From page 17... ...
In a 1995 paper, Blake and Kato drew several conclusions that are relevant to aircraft soot emissions: " (i) During volcanically quiescent periods, the calculated total surface area of black carbon soot aerosol is of the same order of magnitude as that of the background sulfuric acid aerosol .
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From page 18... ...
Reducing this uncertainty, and putting aircraft measurements of soot in the context of this uncertainty, should remain a priority of the SASS program. Aerosol Size Distribution "Measured concentrations of volatile particles in commercial aircraft wakes are large and show significant unexplained variability." This statement, or something like it, has appeared repeatedly in various papers and documents, with particular reference to the high particle-number concentrations observed behind the Concorde by Fahey et al.
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From page 19... ...
it is argued that "in the worst case, aircraft are only responsible for <0.5% of the total global soot emissions." Even if this figure is correct on a mass basis, most emission indices for soot are obtained for combustors that generally emit larger carbon-soot particles than aircraft engines and, thus, fewer particles for a given amount of soot mass. Hence, the number of soot particles emitted by aircraft engines could be much larger than the 0.5% mentioned.
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From page 20... ...
Contrail Formation Contrails may be formed entirely from direct emission of precursor substances, or may result from the emissions of smaller particles that modify or enhance natural cirrus. Compared to natural cirrus clouds, crystals in aged contrails have a greater number density and smaller size (Gayet et al., 1996~.
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From page 21... ...
Several pathways of contrail formation have been considered, including heterogeneous nucleation of water vapor on frozen H2SO4/H2O aerosols, on soot, and on soot coated with H2SO4. This last case can occur either by adsorption of H2SO4 from the gas phase or by coagulation of soot particles and supercooled H2SO4-H2O aerosols.
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From page 22... ...
All experimental measurements are in relatively good agreement on the magnitude of the uptake coefficient for aqueous sulfuric acid aerosols, and the dependence of this value on temperature and relative humidity (Fried et al., 1994~. Experimental estimates of the uptake coefficient for aqueous ammonium sulfate and ammonium bisulfate aerosols, common to the lower troposphere, show some scatter, but all lie within the range 0.02-0.1 (flu et al., 1997~.
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From page 23... ...
As mentioned earlier, it is not yet clear whether the abundance of carbon aerosols in the upper troposphere and lower stratosphere is significantly increased by aircraft exhaust emissions. If this is indeed the case, it is important to determine experimentally the possible extent of alteration of the ozone column due to heterogeneous reactions involving those aerosols (whether coated with H2SO4 or uncoated)
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From page 24... ...
Clearly, if there were to be variability or trends in the frequency of occurrence of cirrus clouds or in their distribution, this could add substantially to their impact on the ozone layer. As our knowledge of the influence of aircraft emissions on contrail formation and the possible enhancement of cirrus cloud formation improves, the resulting effects on the ozone column should be tested in the modeling studies.
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From page 25... ...
Understanding the ozone changes in this altitude range will be critical to assessing climate forcing and response; however, modeling or observing such perturbations in the region around the tropopause is very difficult because large variations occur in both transport patterns and concentrations of key species. Effects of Particles and Contrails The direct radiative forcing caused by jet aircraft soot and sulfur emissions is thought to be relatively insignificant.
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From page 26... ...
efforts should be made to gauge aircraft perturbations against other major natural and anthropogenic inputs (biomass burning, urban pollution) that can impact tropospheric aerosol size, concentration, composition, and cloud nucleating capability.
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