Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Examination of the 3-D CTM results in Chapter 4 shows a very wide variation in the range of models for estimated ozone
differences in the upper troposphere. Arguments can be advanced for adopting either absolute or percentage changes
to apply to measured ozone columns; until the reasons for the variations in the range of models are clear, however,
neither approach is clearly superior to the other.
The current limitations of multidimensional CTMs generate additional complications that must be addressed in assessing
the impacts of aviation on the composition of the atmosphere. The impact of subsonic fleets on ozone, for the present
and for 2015 and 2050, has been discussed in Chapter 4, based on a range of 3-D CTMs. In their present state, these
predominantly tropospheric models are unable to take into account adequately changes in the chemistry of the
stratosphere. Between 1992 and 2050, for example, these changes would be induced predominantly by changes in the
concentrations of inorganic chlorine and bromine compounds in the stratosphere. If the principal concern were to
evaluate the impact of aviation on ozone for a given year, these limitations would not be too severe. However, if in
addition one wishes to compare the effects of aircraft emissions with those that can be attributed to changes in
stratospheric processes over the period 1970 to 2050, then one requires more information than current 3-D CTMs can
supply. Furthermore, it is expected that future fleets may contain a supersonic component. These supersonic aircraft will
fly in the stratosphere; to calculate the effects of this hybrid subsonic/supersonic fleet, models capable of assessing
stratospheric changes must be used.
Calculations of the effects of the hybrid subsonic/supersonic fleets were carried out in Chapter 4 in the following way.
For a given year (2015 for example), a 3-D CTM calculation using the Oslo model was performed to determine the
change in ozone concentration for the subsonic fleet relative to the background atmosphere for that year. A 2-D
calculation was then carried out with the AER 2-D model to determine the change in mixing ratio for ozone between the
hybrid fleet and subsonic-only fleet scenarios. For the latitudes and seasons shown in Table 5-1, these mixing ratio
differences were added to the mixing ratio differences, calculated by the Oslo 3-D CTM, between the subsonic fleet and
the background atmosphere. The scenarios used in Chapter 4 to provide the ozone differences for the subsonic impact
were (A,B, 1992), (C,D, 2015), and (E,F, 2050), as defined in Table 4-4. The hybrid fleet impacts were obtained by
comparing scenario S1k, defined in Table 4-11, with scenario D of Table 4-10 for 2015 and scenario S9h, defined in
Table 4-11, with scenario D9 from Table 4-10 for 2050. Results from these sets of scenarios correspond to the
calculated impacts of 500 HSCTs in 2015 and 1,000 HSCTs in 2050.
Calculations designed to show the effects of aviation relative to changes in
ozone or UV resulting from changes in the composition of the stratosphere
require a slightly more convoluted approach. This approach is illustrated in
Figure 5-4. The top panel shows the differences in background
atmospheres calculated by the Oslo 3-D CTM for the years 2050 and
1970 (2050bg -1 970bg), 2015 and 1970 (2015bg -1 970bg), and 1992 and
1970 (1992bg -1 970bg). The middle panel shows the results obtained
when these calculations are performed with the AER 2-D CTM. The
background surface concentrations used in these calculations for 1992,
2015, and 2050 are those shown in Table 4-8. The background surface
concentrations used for the 1970 calculation are those given in Table 6.3
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Figure 5-4: Method used to combine 3-D
CTM and 2-D CTM results for changes in
background atmospheres.