Environmental Engineering Reference
In-Depth Information
~0.04Wm
-2
for 1992. There are several major sources of uncertainty in the radiative
forcing estimates for line-shaped persistent contrails and enhanced cirrus cloudiness:
contrail coverage calculations, optical depths of the clouds and ice crystal size distri-
butions are all critical factors (Lee et al, 2000).
E
FFECTS
OF
SUPERSONIC
AVIATION
It has been suggested over the past 30 years or so that a significant fleet of supersonic
transports (SSTs) would become economically and technically feasible. Most recently,
the NASA high-speed civil transport (HSCT) research programme investigated
technological and environmental possibilities and constraints. Ambitious technolog-
ical goals were set in the HSCT programme, including an overall emissions index
(EI) for NO
x
of 5, under the supposition that the principal environmental effect
would potentially be stratospheric O
3
depletion via NO
x
chemistry.
Hypothetical fleet and emissions scenarios were designed that included different
fleet sizes (500 and 1000) and SO
2
to particle conversion efficiencies. The aircraft
were assumed to cruise at altitudes of approximately 17-20km. Although sulphur is
only present in fuel at very low concentrations, much of it is thought to be con-
verted to SO
2
, which subsequently oxidizes to H
2
SO
4
, forming particles. Polar O
3
depletion is dominated by heterogeneous chemistry on polar stratospheric clouds
(PSCs). This process is therefore highly dependent upon the surface area density of
aerosols. As such, future SO
2
emissions may increase the surface area density and there-
fore possibly lead to further O
3
loss.
The calculation of the above effects is usually done in chemical/transport mod-
els of the stratosphere, treating the troposphere as a lower boundary condition. Mod-
els are often 2D (latitude versus height), although some more advanced 3D models
are being developed and used (eg Rogers et al, 2000).
In the IPCC (1999) report, a major stratospheric modelling effort was made to
assess supersonic potential aviation impacts (see IPCC, 1999, Chapter 4). Two sig-
nificant results emerged from these studies: first, for the particular scenarios studied,
the maximum ozone column change under scenario Fa1H (2050, 1000 SSTs, EI of
NO
x
of 5) was -0.4 per cent (-1.3 and +0.9 per cent from supersonic and subsonic
aircraft, respectively); second, the most significant effect of supersonic aircraft on
the atmosphere was from emissions of water vapour. The change in O
3
column was
smaller than previously calculated and had only a small effect on increased UV-B
penetration to the Earth's surface. The substantial increase in radiative forcing from
water vapour was something of a surprise. Increases in H
2
O of 0.4-0.7 parts per
million (ppm) were found in the Northern Hemisphere, compared to background
values of 3-4 ppm. However, these calculations were assessed to have large uncer-
tainties.
R
ECENT
ACTIVITIES
AND
FUTURE
DIRECTIONS
That the IPCC (1999) report
Aviation and the Global Atmosphere
was a landmark,
there can be no doubt. Since, and during, its writing and publication, there have
