The Images Model

One of the exciting challenges for the scientific community in the coming decade will be the development of coupled earth system models that account for the interactions between the biogeochem-ical cycles and the physical climate system. At present, most simulations of the atmospheric composition are performed using chemical transport models in which the atmospheric dynamics are prescribed (based on meteorological analyses) or calculated by an atmospheric general circulation model. Like most chemical transport models, IMAGES, which is used in the present study, incorporates several basic elements: surface emissions, atmospheric transport, chemical transformations, and surface deposition. The horizontal resolution is 5° in longitude and latitude with finely resolved vertical layering (25 layers) and an atmosphere that extends 22.5 km high. The transport time step for IMAGES is typically 6 h. A global climatology of wind from the European Center for Medium-Range Weather Forecasts was used to drive transport. The model represents advection (Smolarkiewicz and Rasch, 1991) and accounts for subgrid transport through diffusive mixing in the boundary layer (Müller, 1993), deep convection (Costen, 1988) in the free tropopshere, and eddy diffusive mixing to account for unresolved wind variability. IMAGES derives the concentration of 41 species, including seven different hydrocarbons, and several oxygenated organics, including PAN and MPAN (Müller and Brasseur, 1995). It includes a relatively detailed chemical scheme with 125 reactions (including 26 photolytic reactions and a few heterogeneous reactions),

The spatial distributions of the deposited species, NO, N02, UNO,, and 0?, depend on interactions of the transport and chemical schemes with both wet and dry deposition. IMAGES parameterizes wet deposition or wash out as a first-order loss rate calculated as a function of the precipitation rate (Müller and

Brasseur, 1995). The precipitation rate was taken from the climatology of Shea (1986). IMAGES expresses dry deposition as a function of a prescribed deposition velocity (which is specific to vegetation type). Wet deposition depends on the rate of precipitation, but the formulation accounts for the different types of precipitations and the species-dependent solubility coefficients. Dry deposition velocities vary considerably from species to species. HNO? has by far the largest deposition velocity.

Emission estimates for a number of relevant chemical species considered in modeling the contemporary atmosphere are provided in Table 1 (Muller, 1992; Muller and Brasseur, 1995). The relative contributions of the anthropogenic and biogenic sources vary a great deal depending on the chemical compounds considered. Sulfur dioxide emissions are almost entirely produced by fossil fuel burning, while technological sources account for only 13% of volatile organic carbon emissions. For carbon monoxide and methane, technological sources provide only 25% of the total budget. For the remainder of the CO budget, biomass burning constitutes the bulk of the respective budgets. For NOx emissions, fossil fuel combustion is the largest term in the budget but biogenic production, biomass burning, and lightning all contribute a substantial portion of the total budget. For modeling the future atmosphere, we use the IS92a scenario developed for IPCC 1995.

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