Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
D models by eddy diffusion terms. Measurements of the NO Y -to-ozone ratio in the LS have provided evidence for distinctly different airmass characteristics that are not
well represented in 2-D models (Murphy et al., 1993; Minschwaner et al., 1996; Volk et al., 1996; Schoeberl et al., 1997). One method for improving the 2-D
representation of tropical/extratropical air mass difference has been to reduce the horizontal eddy coefficient in the subtropical region. Efforts such as these have
underscored the fact that an accurate model representation of tropical/mid-latitude air mass distinctions, including the extent of transport of tropical air into midlatitudes,
remains an important assessment uncertainty.
Model representation of bulk, global-scale vertical exchange between the stratosphere and the troposphere by diabatic circulation is likely adequate (Holton et al.,
1995). However, most models do not adequately resolve tropopause-folding events or stratosphere-troposphere exchange along isentropic surfaces. To the extent that
these processes are important, calculated aviation impacts will be sensitive to model horizontal and vertical resolution.
Model representation of gas-phase photochemical links between ozone and atmospheric trace species such as HOx and NOx may be the most mature area of model
construction, although rate parameter uncertainties increase with decreasing temperature. This representation is facilitated by the existence of evaluated compilations
of photochemical rate parameters (IUPAC, 1997a,b; JPL, 1997). Because of the sensitivity of reaction rates to temperature and photolysis rates to solar zenith angle,
model treatments must account for temperature and solar flux changes as air parcels move around the globe and encounter day and nighttime conditions. Diurnal
variations in calculated radical concentrations can be reproduced either by invoking an explicit time marching kinetic scheme or by applying a correction factor to
concentrations calculated from averaged solar zenith angles.
The dependence of reaction rate coefficients on temperature, especially for PSC processes, can present a particular problem for 2-D models, which are constrained to
zonal-mean temperature fields. One strategy to address zonal variations has been to describe the zonal mean temperature by a probability distribution (Considine et
al., 1994). The applicability of this approach to PSC processes is an area of active investigation. Type II PSC particles, consisting of water-ice and uniformally formed
at temperatures below 188 K, can be adequately captured in 2-D formulations. However, the temperature thresholds for PSC type I particle formation are highly
variable because of the multitude of possible particle compositions, and they depend more heavily on the temperature histories of air parcels. Some of the type I PSCs
considered in stratospheric models include solid nitric acid hydrates (e.g., trihydrate and dihydrate), mixed hydrates, and supercooled sulfate, nitrate, and water ternary
solutions (Worsnop et al., 1993; Carslaw et al., 1994; Tabazadeh et al., 1994; Fox et al., 1995). Compositional details of modeled PSC type Is are important because
they determine what the model will calculate for the size, density, and removal rates (by sedimentation) of the particles as well as the partitioning of NOy between
gaseous and condensed phases.
Finally, stratospheric models must describe background sulfate and carbonaceous aerosol formation and evolution adequately to gauge perturbations from aircraft
SO x O and soot emissions. In past studies, models have merely prescribed aerosol suface area distributions based on satellite observations. Recognition that aircraft
exhaust may contain a large number of small-diameter sulfate particles has motivated development of aerosol microphysical schemes (Weisenstein et al., 1996).
2.3.1.4. Stratospheric Model Evaluation
The growing body of satellite, balloon, and aircraft chemical and meteorological data for the middle atmosphere has made it possible to devise tests of photochemistry
and transport within stratospheric models. A number of 2-D and 3-D models have participated in two major intercomparison efforts, Models and Measurements (M&M)
I and II (Prather and Remsberg, 1993; Park, 1999). These comparisons have focused on testing the ability of these models to estimate the atmospheric effects of a
proposed fleet of supersonic aircraft that would operate near 20 km. As a direct result of the first M&M effort, a number of errors in the models were identified and
corrected. Both M&M efforts have served to highlight important tests of model representations. Because of the supersonic aircraft focus, however, less analysis has
been directed at model performance in the lowermost stratosphere, where subsonic aviation effects are expected. With the exception of ozone representation, rigorous
tests of model representation of the dynamics and chemistry of the lowermost stratosphere and UT have not been performed to date. Poor agreement between model
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