A Review of NASA's 'Atmospheric Effects of Stratospheric Aircraft' Project (1999) / Chapter Skim
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2 Scientific Discussion
2 Scientific Discussion
Pages 8-34
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From page 8... ...
It has been suspected since the early 1970s that emissions, particularly of nitrogen oxides, from stratospheric aircraft could affect the level and distribution of stratospheric ozone. More recently, concern has been raised over the effect of particles and water vapor emissions on the ozone budget and on regional and global climate.
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From page 9... ...
co2 H2O NOT . SOx ~ · Infrared radiative forcing and associated climate impact · Infrared radiative forcing and associated climate impact · Formation of contrails and cirrus clouds and associated climate impact · Formation of polar stratospheric clouds and related impact on heterogeneous chemistry · Source of HOx and impact on atmospheric chemistry · Formation of ozone in the upper troposphere and lower stratosphere · Increase in the abundance of tropospheric OH · Enhanced catalytic ozone destruction in the middle stratosphere · Reduction in stratospheric ozone depletion by HOx, C10x, BrOx - Conversion to HNO3 and formation of type I polar stratospheric clouds with potential chlorine activation and ozone depletion Source of H2SO4 in young plume · Source of sulphate aerosols and associated climate impact · Change in cirrus cloud properties and related climate impact · Activation of soot as cloud condensation nuclei and ice nuclei · Increase in particle surface area with NOx reduction, chlorine activation, and ozone depletion Soot*
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From page 10... ...
NASA has projected emission indices for CO2 and water vapor of 3155 and 1237 g/kg fuel burned, respectively, for the year 2015, the same values used for current generation aircraft turbine engines. Carbon monoxide and hydrocarbons are emitted by aircraft engines even at the high combustion efficiency projected for the HSCT.
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From page 11... ...
It is also recommended that additional emphasis be placed on sulfur emissions. Improved understanding is needed of the chemistry and kinetics of fuel sulfur combustion in the engine, and sulfur particle formation in the near-field exhaust plume.
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From page 12... ...
This finding has led to a large number of modeling studies and subsequent measurements, but for subsonic aircraft only (SUCCESS campaign, SULFUR-5 and 6 campaigns4~. Modeling studies also have indicated that the conversion rates should decrease when the 4SUCCESS (Subsonic Aircraft: Contrail and Cloud Effects Special Study)
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From page 13... ...
Resolving this fundamental relationship between the production of particles and the fuel sulfur content, as well as the rate of conversion of fuel sulfur to sulfuric acid, is central to understanding particle production in jet engines. The magnitude and mechanisms of particle formation processes remain very unclear.
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From page 14... ...
In addition, even with the ions, the models still have difficulties reproducing the measured size distributions, because the particles are still smaller than the observed ones. Another hypothesis
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From page 15... ...
Rapid conversion of fuel sulfur to small particles in the plume leads to greater enhancement in lower stratospheric aerosol surface area than does dispersion of sulfur gases followed by oxidation and condensation on pre-existing particles (Fahey et al., 1995; Weisenstein et al., 19961. It is important to improve understanding of these different particle formation pathways, to assure that the scenarios used to simulate the impacts of aircraft emissions are as realistic as possible.
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From page 16... ...
This mean meridional transport, now known as the Brewer-Dobson circulation, was originally postulated based on stratospheric water vapor and ozone measurements. The extreme dryness of the middle and high latitude stratosphere can be explained by a "freeze drying" of the air by passage through the cold tropical tropopause and subsequent poleward transport (Brewer, 1949~.
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From page 17... ...
Results show that at 20 km, mean age varies from approximately 1 year in the tropics to approximately 5.5 years at high latitudes, while at 30 km it varies from 4 years in the tropics to 5-8 years at high latitudes. The residence time of HSCT exhaust should be related to mean age, as both mean age and HSCT residence time depend on the strength of the mean meridional circulation and the isentropic mixing in the stratosphere between the tropics and middle latitudes.
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From page 18... ...
The major transport uncertainty in regards to assessing HSCT effluent is determining how much mixing occurs between the tropics and middle latitudes in the lower stratosphere. The amount of isentropic mixing affects the age of stratospheric air and the stratospheric residence time of HSCT exhaust.
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From page 20... ...
It is likely that revision of the relevant rate constant will resolve the NOx/NOy issue (Gao et al., 1999~. As a consequence of the developments described above, there is now improved confidence in the representation of chemistry of the nonpolar regions in the assessment models, i.e., earlier concerns about "missing chemistry" have abated, particularly for the region below ~22 km.
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From page 21... ...
Heterogeneous Chemistry Heterogeneous chemistry and gas phase chemistry in the stratosphere are strongly coupled. For example, a fundamental element of the AESA program was developing an understanding of the significance of formation of nitric acid on background sulfuric acid aerosol at mid-latitude (Fahey et al., 1993~.
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From page 22... ...
Recent laboratory studies of these reaction rates have identified kinetics for chlorine formation for some postulated types of polar stratospheric cloud particles. At equilibrium, the cloud conditions in the stratosphere are consistent with the formation of two-phase PSC particles in which a solid core is coated with a supercooled H2O/HNO3 liquid layer.
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From page 23... ...
PAEAN believes that the importance placed on PSC uncertainties in the AESA assessment report is warranted, but the report failed to highlight the continued need for laboratory studies to unravel the fundamental kinetics of the complicated particle compositions that can exist under PSC-forming conditions. Although significant progress has been made in identifying the composition- and temperature-dependence of these processes, detailed laboratory studies in these areas need to continue in order to provide the required process information for modeling studies.
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From page 24... ...
The panel supports further assessment of the impact of volcanic eruptions and other sources of stratospheric aerosol on the global aerosol climatology, through aircraft and satellite measurements and field studies to help characterize the concentration and phase of consensible species entering the stratosphere in the tropical tropopause region. In general, the details of how PSC particles form and grow in even the unperturbed atmosphere are not well understood.
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From page 25... ...
have made some initial attempts to simulate future climatic changes due to aviation using a general circulation model (Figure 3~. The radiative forcing due to HSCTs has been estimated to be up to several tenths of a W/m2, due mainly to water vapor; and as the size of the HSCT fleet approaches 1,000 aircraft (in the year 2050)
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From page 26... ...
For instance, the relative importance of radiative forcing from HSCT emissions will depend upon feedbacks associated with stratospheric temperatures, water vapor concentration, and ozone levels (which, in turn, can be affected by anthropogenic halogen emissions, subsonic aviation emissions, and greenhouse gas buildup) (Forster and Shine, 1997; Ponater et al., 1996~.
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From page 27... ...
Thus the AESA box models, combined with the powerful observational data set obtained by the measurement program and up-to-date photolytic and homogeneous gas phase kinetic data, have helped pinpoint many problem areas and, in turn, have pointed the way towards possible resolutions to these problems. They remain important diagnostic tools for many applications.
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From page 28... ...
and at model properties through studies of the mean age of stratospheric air and the correlation of species. On the chemistry front, there have been strenuous efforts to standardize and update the gas phase photochemistry in the 2D models, so that there is now little difference between the chemical schemes in the different models.
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From page 29... ...
Although certain characteristics were common, such as general shape and location of the maxima, the size of the maxima were very different. Stratospheric mean age-of-air and the NOy lower stratospheric distribution have both been used for model-model and model-measurement comparisons.
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From page 30... ...
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From page 31... ...
Also, water vapor plays an important role in stratospheric chemistry. A changing atmosphere may change tropospheric conditions, and in particular, may modify the temperature of the tropical tropopause, thus affecting the lower stratospheric water vapor distribution.
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From page 32... ...
; Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) ; Stratospheric Tracers of Atmospheric Transport (STRAT)
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From page 33... ...
Specifically, flights were designed to examine the relative roles of vortex air transported to mid-latitudes and in situ loss induced by heterogeneous chemistry on sulfuric acid. Flights were also designed to study the exchange of air between the tropics and middle latitudes of both hemispheres, in order to aid in the assessment of HSCTs.
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From page 34... ...
Additional field measurements (along with laboratory studies) would likely help determine whether there is missing chemistry in the assessment models.
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