Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
4.4.1. Uncertainties in Calculated Effects of Subsonic Aircraft Emissions
4.4.1.1. Transport and the Fuel Study
One means of assessing model uncertainties from transport is by comparing concentrations of inert or passive tracers with specified input and removal rates. For such
species, it may be assumed that differences in distribution are solely a result of transport and its representation because chemistry plays no role. The results of such
an experiment are described in Section 3.3.4.1 and Danilin et al. (1998). Because the emissions in this experiment are based on fuel use by subsonic aircraft, the
experiment tests the transport characteristics of models in the UT and LS. Mixing ratios of fuel tracer are roughly 10-100 ng/g between 8-16 km north of 30°N.
Southern Hemisphere stratospheric concentrations are about a factor of 4 less than in the Northern Hemisphere. Although the general pattern of the test tracer is
qualitatively similar among various models, the maximum tracer concentrations deviate by a factor of 10 among the models. Some of this difference may be an artifact
of low vertical resolution, but the results clearly point to uncertainties that need to be resolved.
One of the major problems with model tranport is that stratospheric-tropospheric exchange (STE) is poorly represented in current models of any dimension. Current
theory has air entering the stratosphere through the tropics and returning at mid-latitudes. However, recent measurements of chemical tracers such as H 2 O and CO,
diagnostic of tropospheric air, suggest that the lowest few kilometers of the lowermost stratosphere can be impacted by air being convected from the lower troposphere
(Dessler et al., 1995; Lelieveld et al., 1997). These areas require more evaluation.
One of the problems that has not been addressed in these studies is sub-grid-scale parameterizations. This issue includes dispersion of material in the plume to the
regional scale, which is typical of the grid resolution of most models. In addition, exchange of air between the troposphere and the stratosphere takes place on a
variety of scales, and only the larger scales are modeled with any reliability.
Thus, it is very difficult to assess inherent uncertainties in model results that are products of limitations in modeling transport. Nevertheless, as explained in Section 4.5,
we attempt such an assessment by comparing the different models in a semi-quantitative manner.
4.4.1.2. Chemistry
The chemistry of VOCs (NMHC, acetone) and the transport of peroxides to the upper troposphere have recently been found to play important roles in the HOx radical
balance (Singh et al., 1995; Folkins et al., 1997; Jaeglé et al., 1997; McKeen et al., 1997; Prather and Jacob, 1997; Wennberg et al., 1998). An additional source of
HOx in the UT could significantly influence O3 production efficiencies. This uncertainty would propagate through the 3-D tropospheric models because there are large
differences in chemistry modules. To some degree, this effect has been explored and the results tabulated in Table 4-6. Further model studies clearly are needed.
NO x plays a key role in the O 3 generation process in the UT and LS, and there are significant differences in predicted NO x levels among the models. However,
because of the limited number of observations and the large temporal and spatial variations of these observations, it is difficult to validate modeled upper tropospheric
and lower stratospheric concentrations (see Chapter 2).
Although the process studies referred to in Chapter 2 point to a strong dependence of O 3 production on NO x levels, the standard simulations and sensitivity studies
presented in Chapter 4 show a limited impact of nonlinear chemistry on O 3 perturbations. For example, all of the 3-D CTM standard simulations for 1992, 2015, and
http://www.ipcc.ch/ipccreports/sres/aviation/051.htm (2 von 10)08.05.2008 02:42:31