Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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4.1. Introduction
To assess the impact on the chemical composition of the atmosphere from a current and future fleet of aircraft, it is necessary to take the following steps:
Identify emissions associated with aircraft operation
Evaluate how each emission would change concentration of corresponding species in the atmosphere
Determine how those changes could alter concentrations of other species in the atmosphere.
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For aircraft with conventional engines that use hydrocarbon fuels, emissions include H 2 O, carbon dioxide CO 2 ), NO x , oxidation products from sulfur impurities,
hydrocarbons, carbon moNO x ide (CO), and soot. These products have the potential to perturb the atmosphere if their emissions are large enough to change
background concentrations substantially. Other emissions, such as metallic particulates from engine wear and paints, have been considered and are thought to have
minimal atmospheric effects (Stolarski and Wesoky, 1993). One environmental concern is how emissions could affect O 3 in the atmosphere-both in their potential to
deplete O 3 in the lower stratosphere, leading to increases in UV-B radiation at the ground, and in their potential to increase O 3 in the upper troposphere, leading to
greenhouse warming. In addition, increases in water vapor in the LS could have a direct effect on radiative balance as well as chemistry.
To evaluate how emission of a species could change its background concentration, one could estimate the expected change in concentration from emission rate and
residence time and compare that with the background concentration, or one could compare the emission rate directly with sources that sustain the background
concentration. Consideration of either criterion points to different impacts from material emitted either to the UT or the LS. The residence time for material emitted in
the UT is typically on the order of weeks, whereas residence time in the LS is on the order of months to a few years (Johnston, 1989). For NO x and H 2 O, background
sources also differ in the UT and LS. In the UT, NO x is affected by surface sources as well as lightning sources in the whole troposphere. In contrast, NO x in the LS is
sustained by downward transport of NO x and nitric acid (HNO 3 ) from the middle stratosphere and transport of NO x produced by lightning in the tropical UT. In the case
of H 2 O, the background concentration in the UT is orders of magnitude larger than that in the LS.
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