Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
of WMO (1994). It is clear from the top panel that the 3-D CTM does not
provide any information about changes occurring in the atmosphere above
about 18 km for the period from 1970 to 2050. The calculated increases
for ozone shown below 18 km and
reaching peak values in the lower troposphere result from increased
surface emissions, predominantly NO x , that are expected to occur over
this time period. The calculations reported in the middle panel show quite
different behavior. In this set of 2-D calculations, surface emissions of NOx were not increased between 1970 and 2050. The changes in ozone
concentrations shown in this panel are directly attributable to expected
changes in concentrations of inorganic chlorine and bromine compounds,
as well as those for nitrous oxide.
The bottom panel in Figure 5-4 shows the result of the linear combination
of the concentration changes calculated in the top and middle panels.
Results such as those in the bottom panel are then used to derive the
background atmospheres for 1970, 2015, and 2050 from the background
atmosphere used for 1992. The procedure of combining the results of 3-D
and 2-D CTM calculations in this manner is clearly questionable, but at the
present stage of model development this compromise is necessary.
5.4.1.2. Method for Referring Calculated Changes to 1970
As discussed elsewhere in this chapter, the calculated impact of aviation
on UV is to be compared with impacts on UV resulting from changes in
atmospheric composition from other sources. For this purpose, 1970 has
been taken as the reference year because it represents a period before
the expected onset of substantial changes in the concentration of
stratospheric ozone resulting from increases in the concentrations of
stratospheric chlorine and bromine. The required ozone columns for 1970
have been obtained in the following manner: It is assumed that the
amount of ozone in the lower troposphere of the background atmosphere
has not changed between 1970 and 1992. Just as for 2015 and 2050, a 2-
Figure 5-5: Ozone column changes for
July and October.
D calculation is performed to derive the 1970 background columns from corresponding values for 1992. As explained in Section 5.4.1.1, the background surface
concentrations used for the 1970 calculation are those given in Table 6.3 of WMO (1994). It is further assumed that changes in tropospheric ozone due to aviation
between 1970 and 1992 are proportional to the amount of NO x emitted. The amount of NO x emitted by aviation in 1970 is deduced from a linear extrapolation of the
values given in Table 9-4; this amount is estimated to be 0.72 Tg.
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