Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The character of the O 3 budget is also very different in the UT and LS. In the UT, the transport and chemical time scales are on the order of weeks. The chemical
transformation is dominated by reactions among oxides of hydrogen (HO x ) and NO x radicals, which affect local production and removal rates of O 3 . Local
concentrations of HO x species in the UT are controlled by concentrations of water, hydrocarbons, NO x , and CO, each of which is affected by how contributions from
surface sources are redistributed in the UT by convection. In the LS, transport and chemical time scales are on the order of months. The O 3 budget in the LS is
maintained by a balance between transport and chemistry (chemical production balanced by transport out of the region in the tropical LS, and chemical removal
balanced by transport into the region in the mid-latitude LS). Addition of NO x and H 2 O to the LS would modify the chemical production and destruction rates of O 3 .
However, the efficiency of the added radicals in removing O 3 is dependent on the amount of chlorine radicals in the background atmosphere and the extent of surface
reactions that occur on sulfate particles and polar stratospheric clouds (PSCs). Previous modeling studies (Danilin et al., 1997; Weisenstein et al., 1998) have shown
that sulfur emissions from supersonic aircraft can increase the surface area of the sulfate layer by about 50% in the Northern Hemisphere LS. Effects from the current
subsonic fleet are less clear. Subsonic aircraft cruise in the troposphere or the very lowest part of the stratosphere (just above the tropopause); thus, the stratospheric
impact from subsonic aircraft sulfur emissions would probably be less than that computed for the projected supersonic fleet. Whether any observed trend in the sulfate
layer in the past decade can be ascribed to subsonic aircraft is currently under debate (see Hofmann, 1991, and Section 3.3.4.1). The amount of PSCs also would be
increased as a result of H 2 O and NO x emissions from aircraft (see Section 3.3.6).
Numerical models of the atmosphere are used to calculate these changes. By solving a system of equations, these models simulate the transport and chemical
interactions of trace gases to obtain their spatial and seasonal distributions. A typical model keeps track of the distributions of 50 species that interact via more than
100 reactions. Transport in the models is driven by winds and parameterization of mixing, which change with seasons. There are several ways to classify the models
into different classes. One is by dimensionality: Two-dimensional (2-D) versus three-dimensional (3-D). 3-D models simulate the distributions of trace gases as
functions of altitude, latitude, longitude, as well as season. 2-D models of the stratosphere simulate the zonal mean (averaged over longitudes) concentrations of
species, taking advantage of the fact that many of the trace gases have uniform concentrations along latitude circles in the stratosphere. Another way to distinguish
different types of models is to note whether the transport circulation is fixed or calculated in a consistent way with the model-generated trace gas concentrations.
General circulation models (GCMs) calculate temperature and transport circulation along with chemical composition. Alternatively, chemistry-transport models (CTMs)
simulate the distribution of trace gases using temperature and transport circulation either from pre-calculated GCM results or derived from observations.
Because of intrinsic differences in chemistry and dynamics that control O3 and precursor species in the UT and LS, different models have been developed to examine
the different regions. Models for the troposphere require better resolution immediately above the planetary boundary layer and a proper description of convection that
carries material from the boundary layer to the free troposphere. The chemical scheme in these models places more emphasis on the role of non-methane
hydrocarbons (NMHCs), acetone, and peroxyacetalnitrate (PAN). Models with emphasis on the stratosphere concentrate on large-scale transport from the equatorial
LS to the mid-latitudes and the exchange of material between the stratosphere and the UT. The chemical scheme pays more attention to the coupling between the
nitrogen, hydrogen, and halogen species and their sources in the stratosphere. The computation requirements are such that it has not been possible to develop a
model that will treat both the UT and the LS in a satisfactory manner. Historically, two sets of models have been used to evaluate aircraft impact in the UT and LS. This
approach is clearly unsatisfactory because a portion of the subsonic fleet operates in the LS. In this report, we essentially continue to use this approach.
A large number of model studies of the impact of NOx emissions from subsonic aircraft have been performed over the past 20 years (see Chapter 2 for an overview).
During the past few years, these studies have been based on 3-D CTMs. Recent assessments of the atmospheric effects of aircraft emissions were completed by the
National Aeronautics and Space Administration (NASA) (Friedl, 1997) and the European Community (Brasseur et al., 1998). For these reports, 3-D CTM studies of the
ozone perturbation from the present-day aircraft fleet were performed with several models. The model studies used the NASA database in Friedl (1997) and the
Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) (DLR)-2 (Schmitt and Brunner, 1997) database in Brasseur et al. (1998). Although there are
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