Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Figure 4-4 shows the global increases of total O 3 from aircraft emissions in 2015 and 2050 relative to those in 1992-that is, the difference of O 3 budgets for scenarios
listed in Table 4-4 (D with respect to B and F with respect to B). The same figure also shows the increases of O 3 in 2015 and 2050 from the effects of changes in
surface emissions (Section 4.2.2.1)-that is, the differences for scenarios C with respect to A and E with respect to A. Aircraft emissions account for approximately 15-
30% of the total O 3 increase in 2015 and 15-20% in 2050. It should be noted, however, that the projections of aircraft emissions and the IPCC IS92a scenario
underlying the increase of surface emissions are extremely uncertain. Changes in aircraft or surface emissions scenarios could change the relative contribution from
aircraft emissions to O 3 perturbations significantly. Using scenario G (high demand), an approximately 45% higher increase of O 3 from aircraft is calculated in 2050 by
the UiO model (see sensitivity studies).
4.2.3.4. Influence of Changing OH on CH 4 Lifetime
As discussed in Section 2.1.4, aircraft NO x emissions lead to higher OH concentrations. In
the troposphere, CH 4 is removed mainly by reaction with the OH radical. Therefore, a higher
OH concentration will lead to more rapid removal of CH 4 from the atmosphere. Table 4-5
presents the chemical lifetime of CH 4 and changes from aircraft emissions for scenarios A-
F. The lifetime in Table 4-5 is defined as the CH 4 amount up to 300 hPa divided by the
amount annually destroyed by chemical processes. There are large differences in CH4 lifetimes calculated by the models for base cases A, C, and E. It is beyond the scope of this
report to assess what causes these differences, but it can be generally said that global OH
is very sensitive to photolysis rates, parameterization of lightning NOx emissions, and the
amount and distribution of surface NO x and other emissions. Comparing simulations A, C,
and E, which show enhancements from changes in surface emissions, CH 4 lifetimes
increase by 0.5-3.2% from 1992 to 2015 and by 7-12% from 1992 to 2050. The decrease of
OH concentrations is a result of the strong effect of anthropogenic CO emissions and higher
background CH 4 concentrations, which dominate the effect of surface emissions of NO x .
The models are rather consistent in their estimates of changes of CH 4 lifetimes from aircraft
emissions. Comparing simulations with and without aircraft emissions, CH 4 lifetimes are
calculated to decrease globally by 1.2-1.5% in 1992, 1.6-2.9% in 2015, and 2.3-4.3% in
2050.
Changes in calculated CH 4 lifetime from aircraft emissions for the three time periods
Figure 4-4: Increases in global total tropospheric ozone abundances
(Tg O3) in 2015 and 2050 from aircraft and other anthropogenic
(industrial) emissions relative to 1992.
considered are surprisingly similar in the model studies. With the exception of the ECHAm 3 /CHEM model, which gives smaller perturbations than the other models
because it uses a fixed mixing ratio boundary condition for CO, the differences among models for aircraft impacts are within 20%. This perturbation of CH 4 residence
time from aircraft emissions is significantly larger than that obtained in previous studies (IPCC, 1995; Fuglestvedt et al., 1996) using 2-D models. CH 4 loss is
dominated by OH changes in the tropical and subtropical regions of the lower troposphere. These previous studies showed OH changes that were largely restricted to
the UT, where OH perturbations have little impact on CH 4 residence time. Figure 4-5a shows the perturbation in the zonally averaged OH field (July) for 2015 aircraft
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