destruction of tropospheric ozone, which controls the chemical reactivity of the lower atmosphere and is itself a significant greenhouse gas. Tropospheric sulfate aerosols, on the other hand, are believed to significantly affect the Earth's radiation budget by scattering solar radiation.

Models that incorporate atmospheric chemical processes provide the basis for much of our current understanding in such critical problem areas as acid rain, photochemical smog production in the troposphere, and depletion of the ozone layer in the stratosphere.94 These formidable problems require that models include chemical, dynamical, and radiative processes, which through their mutual interactions determine the circulation, thermal structure, and distribution of constituents in the atmosphere. That is, the problems require a coupling of the physics and chemistry of the atmosphere. Furthermore, the models must be applicable on a variety of spatial (regional to global) and temporal (days to decades) scales.95 Fortunately, there have been advances in three-dimensional modeling of the chemistry of both the stratosphere and the troposphere, including modeling the tropospheric distribution of aerosols.96

Until relatively recently, atmospheric chemistry studies have often relied on two-dimensional (latitude and altitude) models.97 These models solve the zonally averaged momentum, thermodynamic, and mass continuity equations and include a detailed treatment of chemistry and radiative processes. Because of the demanding computational requirements, many two-dimensional models group related constituents into “families” to avoid explicit integration of a mass continuity equation for each individual chemical species (not unlike the grouping that occurs in ecosystem models). A major problem with two-dimensional models has been the necessity to include the effects of horizontal transport by zonally asymmetric motions (waves or eddies) by means of eddy diffusion terms analogous to the approach adopted for vertical transport in the one-dimensional models. As a consequence, these models do not correctly represent the interactive behavior of the chemical, radiative, and dynamical processes. Despite their shortcomings, the models have provided significant insight into atmospheric chemical processes through incorporation of horizontal motions. They will also continue to provide the basis for ozone assessment studies well into the next decade, until significant progress is made in developing three-dimensional models and acquiring and making available the essential, more powerful computing resources, since it is not just the computational cost of the fluid dynamic equations but the chemistry equations as well (which are often the most computationally expensive step).

Most effort in three-dimensional atmospheric chemistry models over the past decade has been in the use of transport models in the analysis of certain chemically active species (e.g., long-lived gases such as N2O or the CFCs). In part, the purpose of these studies was not to improve our understanding of the chemistry of the atmosphere but rather to improve the transport formulation associated with GCMs and, in association with this improvement, for understanding sources and sinks of carbon dioxide.98 More recently, attempts have been made to develop

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