differences from IMAGES. First, the model extends up to the pressure level of 2 mb and therefore includes a large part of the stratosphere. Second, the horizontal resolution corresponds to a spectral model with 42 waves in the meridional and zonal directions, respectively, with triangular truncation (T42). Versions of the model at lower resolution are also available. Third, the winds are provided by CCM2 at relatively short intervals (e.g., six hours). The model can be run either in an online mode (synchronously with the dynamical model) or an offline mode (using the winds previously calculated and stored). Tests are currently performed to validate the offline approach and determine the most appropriate way by which dynamical variables (including the information on vertical convection) should be transferred from the GCM to the transport model. The model will also be run with analyzed winds (e.g., from the European Centre for Medium-Range Weather Forecasts), especially when model results will be compared to observations at specific sites.

One of the major challenges is incorporating heterogeneous reactions into models. As discussed in Chapter 5, we now know that during Antarctic springtime reactions on the surface of polar stratospheric cloud particles are instrumental in the destruction of polar ozone. Similar heterogeneous reactions on sulfate aerosol particles at middle latitudes are also possible. There is also concern about enhancements of the stratospheric aerosol burden by large volcanic injection events (e.g., El Chichon) and the release of aerosols through industrial activity and their diffusion into the stratosphere. Although a substantial database on aerosols exists (both satellite and ground-based data), global atmospheric chemistry models typically do not include aerosol effects.

In addition to chemistry issues, a number of shortcomings in current models are related to dynamical processes and thereby affect the ability of the models to predict the distribution of chemically reactive species. For example, global models typically do not simulate those equatorial wave modes (Kelvin and Rossby gravity waves) that are thought to force the semiannual and quasi-biennial oscillations in the stratosphere. This inadequacy of the models is either a result of insufficient resolution or failure to include tropospheric convective processes believed to be the source of these waves. Some atmospheric chemistry models (notably two-dimensional models) have attempted to include these effects by ad hoc methods.

Perhaps an even more important deficiency in models used to study atmospheric chemistry is the failure to include, or to treat adequately, cloud processes and the hydrological cycle. This fault results from both inadequacy of computational resources and incomplete understanding of the hydrological cycle. The consequence of this deficiency is typically a poor simulation of the observed water distribution, and this implies an inadequate treatment of gas sources and sinks, particularly in terrestrial systems.

In sum, there is an emerging consensus that both two- and three-dimensional



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