2. ATMOSPHERIC PHOTOCHEMICAL MODELS To carry out the required assessments most efficiently and realistically, two photochemical models are used: a one-dimensional (1-D) model and a two- dimensional (2-D) model. The 1-D model, which contains over 50 species, is most useful for investigating globally or hemispherically averaged changes in composition; it is also capable of simulating a much larger system of species and reactions than the 2-D model, which is limited to about 25 chemical species at altitudes below 55 km (34 mi) and 9 species above 55 km. The 2-D model, on the other hand, can simulate meridional variations in composition changes and is thus able to treat possible seasonal and “corridor” effects; the latter refers to enhanced changes in a latitudinal zone containing the launch or reentry windows. In the present 2-D model, the bulk velocities were obtained by zonally averaging the wind fields predicted by three- dimensional dynamic models; the eddy diffusion tensor elements were obtained by an adjustment procedure based on matching computed tracer distributions with their observed distributions. Because both models have been described in detail in the literature (2-7), only a brief description is given of them here. Both models are based on the continuity equations for trace constituents, where P, and L, are production and loss rates, respectively, for the /th constituent, v is the meridional bulk velocity, </>, is the “eddy flux” representing large-scale eddy motions, and nt is the number density of the zth constituent. In the 1-D model the velocity v is, of course, absent, and the eddy flux is written as
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