Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Typically, these models have horizontal resolutions of 3-6°, with the exception of the UiO model, which has a horizontal resolution of 8°x10°. In the vertical dimension,
the IMAGES/ BISA model has 25 levels; the Tm 3 /KNMI and ECHAm 3 / CHEM models have 19 levels; and the HARVARD, UKMO, and UiO models have nine levels.
Four of the models (ECHAm 3 /CHEM, HARVARD, Tm 3 /KNMI, UiO) have a top layer located at 10 mb; the IMAGES/BISA and UKMO models have top layers located at
50 mb and 100 mb, respectively. Because vertical model levels are defined on sigma coordinates and not on pressure coordinates, the number of model levels
between fixed pressure surfaces can vary in time. Between the surface and 850 mb, the HARVARD, UiO, and UKMO models have about two vertical levels; the
ECHAm 3 /CHEM and Tm 3 /KNMI models have about five vertical levels, and the IMAGES/BISA model has eight vertical levels. In the UT/LS region between 100 and
300 mb, the HARVARD model has one vertical level, the UKMO model has about two levels, and the other four models have about four vertical levels.
4.2.1.3. Coupling to the Stratosphere
With the exception of the ECHAm3 /CHEM model, the models have little or no representation of explicit stratospheric chemistry. Instead, either the cross-tropopause
fluxes of O3 and NOy are specified or the mixing ratios of these species are specified in the LS based on observations. In the Tm3 /KNMI, UiO, and IMAGES/BISA
models, however, the upper boundaries are higher in an attempt to minimize their influence on regions of maximum perturbations by aircraft. It should be noted that
this condition may not be satisfied for the IMAGES/BISA model because the model top is at 50 mb.
4.2.1.4. Tropospheric NO x Sources
All of the models include anthropogenic and biogenic tropospheric NO x sources. For present-day conditions, the magnitudes of surface NO x sources in the various
models are ~21 Tg nitrogen (N) yr -1 from surface-based fossil-fuel combustion, 5-12 Tg N yr -1 from biomass burning, and 4-6 Tg N yr -1 from soils. The present-day
magnitude of the lightning source is 5 Tg N yr -1 in the IMAGES/BISA, Tm 3 /KNMI, UKMO, and UiO models; 4 Tg N yr -1 in the ECHAm 3 /CHEM model (increased to 5 Tg
N yr -1 in the 2015 and 2050 simulations); and 3 Tg N yr -1 in the HARVARD model. It should be noted, however, that the simulated impact of lightning on NOy species
in the troposphere can differ from model to model even if the magnitude of the lightning source of NO x is the same, as a result of differences in factors such as duration
and intensity of convective events, land/ocean differences in convection, height of NO x emissions, and so forth. Sensitivity tests were run to evaluate the effect of this
lightning assumption on calculated aircraft perturbation.
4.2.1.5. Tropospheric Chemistry
Most of the models include a comprehensive description of the CH 4 -CO-NO x -HO x -O 3 chemical system. With the exception of ECHAm 3 /CHEM and Tm 3 /KNMI, the
models include representations of NMHC chemistry. However, the details of NMHC chemistry differ significantly from model to model. The ECHAm 3 /CHEM model
includes a stratosphere with a chemistry scheme more suited to the stratosphere, however, it does not include some of the species that are important for tropospheric
chemistry.
Table 4-2: Increase from 1992 to 2015 and 2050 for emissions of CO, NO x , and VOCs (based on IPCC scenario
IS92a).
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