Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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1.3. Emissions and the Environment
Other reports in this collection
The global environmental issues addressed in this report are climate changes and depletion of stratospheric ozone (see Boxes 1-1 and 1-2). Major international
assessments of these issues are made periodically, and no attempt is made here to revisit such reports (for more information, see IPCC, 1996a,b; WMO, 1999, and
references therein). These reports are used for contextual information and as a background to the possible impact of aviation on the atmosphere. There have also
been several recent assessments of the impacts of aviation in Europe and the United States of America (Stolarski et al., 1995; Friedl et al., 1997; Schumann et al.,
1997; Brasseur et al., 1998).
Aviation represents only one of many perturbations associated with future scenarios. There are many sources of climate-active substances. For example, greenhouse
gases are emitted from a wide range of industrial, domestic, and agricultural activities, and there are numerous sources of aerosols (e.g., sulfate from fossil fuel
combustion and volcanoes and carbonaceous aerosols from fossil fuel and biomass burning). Increases in tropospheric ozone are expected to result from increasing
methane, nitrogen oxides, carbon moNO x ide, and hydrocarbon emissions. Similarly, perturbations to stratospheric ozone result from chlorofluorocarbons (CFCs) and
CFC-substitutes and from halon and methyl bromide emissions in addition to any potential perturbations resulting from aircraft emissions.
Some of the processes related to aviation emissions and their location in the atmosphere are shown in Figure 1-2. Subsonic aircraft fly in the troposphere and lower
stratosphere, whereas supersonic aircraft fly in the stratosphere 80-85% of the time, with cruise altitudes several kilometers above those of subsonic aircraft. The
differing chemical and physical processes in the two regions must be taken into account.
The troposphere is principally heated near the Earth's surface, and the temperature decreases with altitude. Warm, moist air tends to underlie cool, dry air, leading to
frequent vertical turbulent motions that exchange air throughout this region. In contrast, the temperature in the stratosphere generally increases with altitude; the result,
as reflected in its name, is that this region is stratified. Vertical motions are much slower here than in the troposphere. The stratosphere is also much drier than the
troposphere, and clouds rarely form at this level.
Table 1-1 lists aircraft emissions that are important from an atmospheric perspective, with summaries of the roles that they play. These emissions can be usefully
divided into two categories, depending on how they affect climate: Direct, as with CO 2 (where the emitted compound is the species that can modify climate), and
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