Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
predictions and observations of ozone in this region of the atmosphere (typical errors greater than 50%) suggests that significant improvement will be required before
stratospheric assessment models can be used to examine the impact of aviation (or, for that matter, any perturbation) on the lowermost stratosphere and UT. In the
following paragraphs, we summarize comparison efforts for the following key issues (for altitudes above 15 km):
● Photochemistry
● Dynamics
● Comparison of model data and observations of stratospheric ozone.
2.3.1.4.1. Photochemistry
The photochemical mechanisms employed by most of the models compare well with each other. Tests of the photochemical mechanisms were performed by
comparing predicted concentrations of short-lived reactive chemicals from these models against a benchmark photo-stationary state model constrained by the
distribution of precursors from each 2-D model. These comparisons provide a means of accounting for differences in the transport of long-lived species, such as NOy,
and O 3 , within the models. The distribution of NOy versus altitude and the mixing ratio of N 2 O was markedly different among the various 2-D models. Most of the
differences for calculated concentrations of hydrogen, nitrogen, and chlorine free radicals among the various 2-D models were shown to be caused by differences in
NOy and to a lesser degree ozone. The benchmark model has been tested extensively against atmospheric observations and has been shown to generally reproduce
observed concentrations of OH, HO 2 , NO, NO 2 , and ClO in the stratosphere to within ±30%, provided precursor fields and aerosol surface areas are accurately known.
However, no significant tests of the model photochemistry of the lowermost stratosphere
were performed during the recent M&M workshop. The chemistry of this region is
considerably different. For example, the relatively high ratio of CO to ozone implies that
ozone production from the oxidation of CO is much more important in this region than at
higher altitudes. Furthermore, at the tropopause and below, saturated conditions often exist;
therefore, chemical processes occurring on ice particles may be important. In addition,
because this air is influenced by mixing of reactive trace gases from the lower troposphere,
these models must consider transport of a larger number of reactive species than they
typically do.
2.3.1.4.2. Dynamics
Tests of the dynamics within the 2-D and 3-D models during both M&M I and II revealed a
number of problems. In general, the mean age of air within the stratosphere is much older
than predicted by these models. Measurements of CO 2 (Boering et al., 1996) and sulfur
hexafluoride (SF 6 ) (Elkins et al., 1996), both of which are increasing rapidly, provide a
means of dating stratospheric air. The models had a high dispersion in predicted conversion
rate of N 2 O to NOy. It is unclear whether this dispersion reflects errors in dynamics or
chemistry related to the high-altitude sink of NOy. This is a key point: If the assessment
models are unable to accurately simulate observed concentrations of total NOy, their ability
to predict the influence of additional NOy from aircraft on ozone will remain relatively
http://www.ipcc.ch/ipccreports/sres/aviation/028.htm (8 von 11)08.05.2008 02:41:47
Figure 2-8: Estimates of northern mid-latitude total ozone column
changes (%) from NOx emission in the troposphere and stratosphere