Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
One study of the effect of increasing CO 2 amounts (Pitari and Visconti, 1994) showed that
stratospheric cooling mitigated the increase in NO x . However, this study did not take into
consideration the increase in H 2 O associated with aircraft; therefore, more complete studies
need to be done with a wide range of models.
Another major uncertainty regarding knowledge of the future stratosphere is that associated
with source gases, in particular H 2 O. This uncertainty is underscored by our lack of
understanding of current H 2 O trends. Global trends in stratospheric humidity over the period
1992 to 1996 inclusive have been presented recently by Evans et al. (1998) using Upper
Atmosphere Research Satellite (UARS) measurements of H 2 O and CH 4 from the Halogen
Occultation Experiment (HALOE) instrument. The authors find that the combined budget of
2CH 4 + H 2 O is approximately constant with altitude and is increasing with a global mean
value of 61 ppbv yr-1 . A more local view of water vapor increase in the lower stratosphere
was presented by Oltmans and Hofmann (1995) over the period 1981 to 1994. They found a
maximum rate of increase in the 18-20 km layer of 32 ppbv yr-1 . The rate of increase of H2O observed in the stratosphere is greater than the increase in tropospheric CH4 . One
suggestion is that the increase could be accounted for by a change in tropopause
temperature of a few tenths of a degree, and the most recent changes might be a result of
changes in the dynamics of the tropics induced by heating of Mt. Pinatubo aerosols
(Schauffer and Daniel, 1994). This uncertainty highlights the delicate balance between
chemistry and climate.
4.4.3. Soot Emissions by Subsonic and Supersonic Aircraft
The chemistry of
soot has been
discussed in
Chapter 2; the
conclusion is that
it does not react
with O 3 in a
catalytic manner.
That is, O 3 can
be destroyed by
soot, but the soot
is consumed in
the process (see
Section 2.1.3.1).
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Figure 4-11: Ozone column (left panels) and ozone profile (right
panels) percentage difference as a function of latitude and month for
three models and three supersonic aircraft scenarios with respect to
scenario D: UNIVAQ-2D model for S1c; AER-2D model for S1k; and
GSFC-2D model for S1h. This figure mainly illustrates model
differences rather than scenario differences.