Environmental Engineering Reference
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Figure 5.9 (a) Modelled nitrogen dioxide concentrations in parts per trillion (ppt) and
(b) ozone in parts per billion (ppb) from aircraft at cruise altitudes for 1992 and a 2050
scenario (Fe1)
the perturbation induced by aircraft. Such a model typically divides the global
atmosphere into grid boxes in three-dimensional space; wind fields are used to
'drive' the model with a typical time interval of six hours to transport the source
gases around, over which period chemical reactions are calculated over shorter time
periods, typically 5 to 30 minutes.
Perturbations attributable to aviation NO x emissions and the resultant O 3 formed
are shown from our own modelling studies (Lee and Kingdon, 2002) in Figures 5.9a
and 5.9b. These figures show the contribution ('delta') of NO 2 (in parts per trillion
(10 -12 ), ppt) and O 3 (in parts per billion (10 -9 ), ppb) for 1992 emissions and a sam-
ple 2050 scenario at cruise altitudes for July conditions. The NO 2 concentrations
show a marked resemblance to the pattern of NO x emissions, with some smoothing,
the NO 2 having a residence time of a few days at these altitudes. By contrast, the O 3
perturbation shows a much more 'smeared-out' pattern, varying across the Northern
Hemisphere, where much of the emissions occur, with tongues of ozone being trans-
ported across the hemispheres because of the longer residence time of approximately
one month at these altitudes.
From equivalent simulations undertaken for the IPCC report (see IPCC, 1999,
Chapter 4), it was found that the aircraft NO x emissions resulted in a chain of chem-
ical reactions that reduced ambient concentrations of methane (CH 4 ) emitted from
other sources; CH 4 is itself a powerful greenhouse gas. The cause of this effect is
rather complex and the magnitude of it was something of a surprise, as previous sim-
ulations with two-dimensional (2D) chemical/transport models of the atmosphere
(latitude by height) had demonstrated such an effect but at a much smaller magni-
tude (eg Fuglestvedt et al, 1996). Essentially, aircraft NO x emissions result in an
enhancement of OH concentrations, which then reduce CH 4 concentrations via
several chemical reactions and transport processes. Thus, aircraft NO x emissions ulti-
mately result in an enhancement of O 3 in the source regions - primarily the North-
ern Hemisphere - and an almost uniform reduction in CH 4 concentrations across
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