Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
As discussed in Section 2.1.2.4, the response of ozone to increasing NO x depends on the strength of
the HO x source. Recent evidence (Brune et al., 1998; Wennberg et al., 1998) supports the presence
of additional upper tropospheric HO x sources from organic precursors that are not included in many
current models. Improved model treatments of HO x production from precursors such as acetone,
peroxides, and aldehydes will require additional data on the mechanisms and kinetics of a number of
NMHC reactions. The role of heterogeneous chemistry in influencing HO x and NO x levels in the UT
has not been investigated fully yet and is expected to become an increasingly important issue for
tropospheric models.
2.3.1.2. Tropospheric Model Evaluation
Previous IPCC reports (IPCC, 1996) identified tropospheric ozone modeling as one of the more
difficult tasks in atmospheric chemistry. Difficulties arise, in part, from the large number of processes
that control tropospheric ozone and its precursors and, in part, from the large range of spatial and
temporal scales that must be resolved. Global 3-D CTMs attempt to simulate the life cycles of many
trace gases and the impacts of subsonic aircraft NO x emissions on them. We need to understand the
level of confidence that is to be ascribed to these model studies.
There is a significant amount of scatter in current model assessments of the impacts of subsonic
aircraft NO x emissions on all aspects of tropospheric composition. With respect to reducing the range
of uncertainty, it would be helpful if we could point to particular aspects of model performance and
gauge models against specified benchmarks. Some model evaluation studies have begun the difficult
task of identifying the current level of model performance and defining the level of confidence that
should be placed in them. To this end, a number of model intercomparison exercises have been
completed; some are in hand, and some are only at the planning stage. These exercises have
involved the following elements:
● Transport of 222Rn
● Fast photochemistry
● Transport of NO x
● Comparison of model data and observations of tropospheric ozone.
2.3.1.2.1. Transport of 222Rn
Figure 2-7: Comparison of modeled and observed ozone
concentrations at 300 mb pressure-height for three
locations: (a) Hohenpeissenberg, Germany (48°N, 11°E);
(b) Hilo, Hawaii, USA (20°N, 155°W); and (c) Wallops
Island, Virginia, USA (38°N, 76°W). Descriptions of the
models are given in Chapter 4. Measurements are from
ozonesondes.
Twenty atmospheric models participated in the 222Rn intercomparison for global CTMs (IPCC, 1996;
Jacob et al., 1997). Differences between model-calculated distributions of this short-lived (e-folding
lifetime of 5.5 days) radioactive decay product emitted at the surface from soils were large, which enabled the drawing of conclusions about the general adequacy of
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