Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
researchers appreciated that 2-D models needed parameterizations to represent the main features of the distributions of NO x and other short-lifetime species. Some
research teams investigated 2-D channel models (altitude and longitude) models, but the development of 3-D chemistry transport models (CTMs) has been the main
thrust of tropospheric modeling efforts.
Global 3-D CTMs are now the main tools for the assessment of subsonic aircraft impacts in the troposphere. Typically, these models have horizontal resolutions of 2-6°
by 2-6°, limited by the resolution of the general circulation model (GCM) from which they have been derived. Emissions databases are now available with higher spatial
resolution, so model performance is limited by the meteorological databases used in CTMs and the necessary computing time. Vertical resolution is a major limitation
with current CTMs, in terms of the height taken as the top of the model and the number of layers into which the model domain is divided. Few models have enough
vertical resolution to fully resolve the atmospheric boundary layer and tropopause domain and to describe the exchange of trace gases between the UT and the LS.
There is a major concern with 3-D CTMs regarding the adequacy of time resolution required in emission inventories and in meteorological data used to transport trace
gases from their sources to their sinks. Initially, some CTMs used monthly averaged fields of horizontal and vertical winds, temperatures, clouds, and humidities. To
resolve major storm systems and convective events, the meteorological data have been updated in the CTMs on a steadily increasing frequency; fields are now usually
updated every 6 hours. On this basis, it is possible to resolve the changing stability of the atmospheric boundary layer, the developing behavior of major weather
systems, and large-scale convective events.
Time and spatial resolution are crucial issues in evaluating the impacts of subsonic aircraft. To evaluate whether the chosen time and spatial resolutions are adequate
in each of the tropospheric assessment tools, a number of sensitivity studies should be carried out in the near future.
Concentration changes occurring in the aircraft plume and wake take place on a spatial scale (i.e., < 20 km) that is less than the smallest global atmospheric model
scale (i.e., > 100 km). Consequently, global models do not typically treat aircraft near-field processes in detail. In fact, most current global model studies have input
aircraft emissions inventories (i.e., emissions indexes, or EI) by simple dilution of the aircraft plume at the altitude of injection, with no chemical changes taking place in
the near field. As a possible means of connecting near-field processes to the global model grid scale, Petry et al. (1998) and Karol et al. (1998) have proposed the
concept of effective emissions index (EEI) to account for changes in species concentrations in the plume dispersion region resulting from photochemical reactions. As
an example, EEI(NO x ) will be less than the corresponding EIs because of plume processes that convert NO x to NOy. However, first estimates show that EEIs are very
sensitive to temperature and light intensity, which results in a large variation of EEIs in latitude, altitude, and season (Karol et al., 1997; Meijer et al., 1997).
Recent ozone model studies have also pointed out the importance of background NOx sources in understanding ozone tendencies with respect to increasing, or
additional, NOx sources. In a sense, the aircraft case is one example of a broader issue regarding nonlinearity between ozone impacts and NOx levels. Model studies
have shown that the magnitude of ozone changes from aircraft NOx emissions may depend significantly on the amount of NOx from non-aircraft sources. One difficulty
with modeling background NOx sources, however, is the short lifetime of NOx (typically 1-5 days). In general, aircraft NOx impacts on upper tropospheric and lower
stratospheric ozone will be overstated if background NOx concentrations are underestimated, and vice versa.
Over the past 2 decades, a significant amount of research has been committed to improving our understanding of background NOx sources through model studies and
observations of aircraft NOx emissions. Much of the NOx in the troposphere comes from surface NOx sources, either via fast vertical transport as NOx (Ehhalt et al.,
1992) or by conversion to temporary reservoir NOy carriers such as PAN or HNO3 , followed by subsequent conversion back to NOx . An accurate representation of the
contribution made by surface NOx sources to the UT and LS requires full treatment of boundary layer chemistry, exchanges between the boundary layer and the free
troposphere, deposition and wet scavenging, free tropospheric chemistry and transport to the upper troposphere by convection, atmospheric circulation, and synoptic-
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