Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Differences are most pronounced in the free troposphere, especially close to the tropopause (see Figure 2-7).
As part of the International Global Atmospheric Chemistry Project/Global Integration and Modeling Activity (IGAC/GIM) study, an intercomparison exercise is currently
being attempted of ozone concentrations calculated by 12 global 3-D CTMs (Kanikidou et al., 1998). Many of these CTMs have already performed assessments of the
impacts of subsonic aircraft NO x emissions on tropospheric ozone; their results have been included in Table 2-1. Furthermore, all of the tropospheric assessment
models employed in Chapter 4 have submitted results to the IGAC/GIM intercomparison.
The GIM intercomparison extends the intercomparisons described above in that it employs some of the available observational database to evaluate intermodel
differences. Figure 2-7 presents some of the model intercomparison results for seasonal cycles of ozone at 300 mb at three widely separated sites.
The GIM model intercomparison with monthly mean values of observations demonstrates that the models capture some of the considerable variability within the
observations. The range in observations may approach 20 ppb at 500 mb and up to 40 ppb at 300 mb, with the ranges in the models significantly greater. These
ranges are significantly greater than the tropospheric ozone impacts of about 8 ppb anticipated from subsonic aircraft NO x emissions (Table 2-1).
2.3.1.3. Key Issues and Processes for Stratospheric Models
CFC and HSCT assessment activities have engaged 2-D (height and latitude) and, to some extent, 3-D (height, latitude, and longitude) models focused on the
stratosphere over the past 10 years. These efforts have served to highlight a number of critical stratospheric model issues:
● Aircraft plume processes
● Stratospheric transport
● Stratospheric gas-phase and heterogeneous chemistry
● Sulfate aerosol
● PSCs
● Soot.
The issue of soot has been raised only to a small extent by the HSCT studies, although it has assumed a more prominent role in the subsonic aviation case (see
Section 2.1.3). In the following paragraphs, we discuss these issues in the context of current model treatments of subsonic aviation impacts.
Aircraft emissions, whether supersonic or subsonic, are deposited primarily at northern mid-latitudes and over a limited vertical range. A key issue for models is how
fast these emissions are dispersed to other regions of the atmosphere, such as the tropical stratosphere or the mid-latitude troposphere, where the response of ozone
to the emissions will be substantially different. In addition, it is important to consider the chemistry occurring in the aircraft plume and wake before it has been
expanded to the model grid scale. Initial attempts to combine near field, far field, and global models in series (Danilin et al., 1997) are the first global impact studies to
be based directly on detailed microphysics and chemical kinetics occurring in the aircraft plume and wake. An increasingly robust plume and wake observational
database is being collected to validate this approach (Kärcher, 1998; Kärcher et al., 1998b).
To date, most models used to assess the impact of aviation on the atmosphere have been 2-D, in which the time-consuming complexity of the real 3-D atmosphere is
reduced to a manageable calculation by averaging around latitude circles. Because of this simplification, 2-D models do not adequately simulate all dynamic features
of the atmosphere. Horizontal transport between mid-latitudes and the tropics (or polar vortex) is an inherently episodic, wave-driven process that is parameterized in 2-
http://www.ipcc.ch/ipccreports/sres/aviation/028.htm (6 von 11)08.05.2008 02:41:47