Aviation and the Global Atmosphere

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2.1.4. Indirect Effects Involving Atmospheric CO and CH4

The tropospheric modeling tools detailed in Section 2.2.1 produce an increased global inventory of OH radicals in response to subsonic aircraft injection of NOx. This characteristic model response to increased NOx levels in air traffic corridors arises through the reaction sequence:

HO2 + NO NO2 + OH (3)

so that increased ozone production is accompanied by an increase in the OH radical concentration and the concentration ratio of OH to HO2. Increased OH levels in air traffic corridors then lead to decreased CO concentrations through the following reaction series:

OH + CO H + CO2 (1)
H + O2 + M HO2 + M (2)

CO lifetimes may approach 2 months and are therefore much longer than NOx lifetimes. As a result, a region of decreased CO concentrations spreads out from air traffic corridors toward lower altitudes and toward the Equator. Ultimately, as a result of subsonic aircraft NOx emissions, decreased CO levels are found in tropical and sub-tropical regions where much of the CH4 is oxidized. Because CO levels are lower, OH levels are higher; hence CH4 concentrations decrease slightly, so the total flux through the OH + CH4 reaction (35) remains in balance with CH4 emissions:
OH + CH4 CH3 + H2O (35)

As a result, the global CH4 distribution adjusts slowly to higher OH levels, and a new steady-state is established in which CH4 concentrations are reduced slightly. This adjustment in CH4 itself drives a further increase in OH levels, with the result that CH4 concentrations build up more slowly over 10-15 years. These indirect or feedback processes have been described in some detail in previous IPCC (1995, 1996) reports.

As quantified in the following section, aircraft NOx emissions produce an increase in the total global inventory of OH of about 2%, which should be reflected in a corresponding change in CH4 loss rate (IPCC, 1995, 1996). The CH4 chemical feedback will then amplify this change in loss rate, producing a decrease in CH4 concentrations that is about 1.4 times the change in loss rate. CH4 concentrations should then decrease by about 3%. These reduced CH4 burdens will then decrease tropospheric ozone production, although the effect may be considered negligible (Fuglestvedt et al., 1999). Because these readjustments take place over 10-15 years, they are exceedingly difficult to represent fully in global 3-D models, though they have been fully explored in 2-D models (Fuglestvedt et al., 1996; Johnson and Derwent, 1996).

The stratospheric modeling tools described in Chapter 4 indicate that supersonic aircraft flying in the LS may lead to stratospheric ozone depletion. Under conditions of stratospheric ozone depletion, there is greater penetration of solar ultraviolet radiation through the stratosphere. N2O transported up from the troposphere therefore has higher photolysis rates lower in the stratosphere, leading to a shorter photolysis lifetime. The global N2O distribution will then slowly adjust over decades to the increase in stratospheric destruction, leading to decreased N2O concentrations, assuming constant emissions. Because N2O is an important greenhouse gas, there is an indirect radiative forcing impact of supersonic aircraft over 50-200 years. This indirect impact is relatively straightforward to take into account using modeling tools described in Chapter 4.

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