Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
4.2.3.5. Sensitivity Studies
In this section, we focus on a limited set of sensitivity studies that help define the uncertainty
range of the model calculations. Ideally, a large number of simulations should have been
performed by all the participating models. However, only a limited number of model
simulations was possible because of time constraints and the demand of computer
resources for 3-D CTM studies. Therefore, only one or two models have performed each of
the sensitivity studies. Also, some uncertainties cannot be addressed properly. For example,
modeled upper tropospheric and lower stratospheric NO x and NOy concentrations are
extremely uncertain and are difficult to compare to measurements because of large
temporal and spatial variations and a limited number of observations (see Chapter 2). There
may be additional uncertainties from unknown processes that feed back on increases in
NO x and O 3 in a future modified atmosphere.
The sensitivity studies focus on the impact on O 3 . The following studies were performed:
Sensitivity of aircraft-induced O 3 perturbations to background NO x levels from
lightning. This finding is obtained by increasing global average NO x production from
5 Tg N yr -1 , which is used in the reference case, to 12 Tg N yr -1 . The same spatial
distribution is used in both cases.
Sensitivity of O 3 perturbation to different regional growth in emissions. In the base
case (IS92a), a uniform growth rate was used for the surface emission of pollutants.
The sensitivity run was performed with the same global growth rate as in the base
case but with the different regional growth rates given in Table 4-3.
Sensitivity to different projections of aircraft emissions. Instead of NASA 2015
emissions, the ANCAT-2015 aircraft emission data set was used. Total ANCAT
emissions for the year 2015 are about 15% larger than the NASA emissions (see
Section 9.3.4). Further differences relate to the location and seasonal variations of
emissions.
Sensitivity to inclusion of NMHC chemistry. This study was conducted by making
runs in which NMHC chemistry was excluded and comparing the results with those
in which NMHC chemistry was included.
Sensitivity to neglecting heterogeneous chemistry on background sulfate aerosols.
This sensitivity was estimated by comparing results with and without the
heterogeneous nitrogen pentoxide (N 2 O 5 ) + H 2 O reaction on aerosol. The aerosol
Figure 4-5: Zonally and monthly averaged change in concentration of
OH (106 molecules/cm3) and CO (%) in July as a result of emissions
from aircraft in 2015, calculated by the UiO model.
surface area was derived from model calculations of the sulfur cycle. Hydrolysis of N 2 O 5 on wet aerosol converts active NO x into the reservoir species HNO 3 ,
which is effectively removed by rain out. Hence, in the base case, less NOy is present in the free troposphere and LS, and emissions by aircraft are more
effective in producing O 3 .
Sensitivity to interannual variability in meteorology.
http://www.ipcc.ch/ipccreports/sres/aviation/048.htm (8 von 10)08.05.2008 02:42:21