IMAGES - A 3-DIMENSIONAL CHEMICAL-TRANSPORT MODEL OF THE GLOBAL TROPOSPHERE

A new three-dimensional chemical transport model of the troposphere is presented. This model, named intermediate model of global evolution o f species, has been developed to study the global distributions, budge ts, and trends of 41 chemical compounds, including the most important species that determine the oxidation capacity of the atmosphere. The c hemical mechanism is made of approximately 125 chemical reactions and 26 photodissociations. The model accounts for surface emissions, chemi cal transformations, dry and wet deposition, and aerosol reactions of trace constituents. The model is formulated in sigma coordinates and i ncludes 25 layers in the vertical. Its horizontal resolution is 5 degr ees in longitude and 5 degrees in latitude. To keep the requirements i n computer time limited, a simplified representation of the transport is adopted: the advection, solved by a semi-Lagrangian scheme, is driv en by monthly mean climatological winds provided by an European Center for Medium-Range Weather Forecasts analysis, The effect of wind varia bility at timescales smaller than a month is taken into account by an eddy diffusion parameterization. Convection in cumulonimbus clouds is also represented. All input field, such as the distribution of winds, clouds, eddy diffusion coefficients, and the boundary conditions, ate monthly means constrained by observational data. The modeled global di stributions of species such as methane, carbon monoxide, nitrogen oxid es, and ozone are generally in good agreement with observations. The l ifetime of methane, which can be regarded as a measure of the oxidizin g capacity of the atmosphere, is found to be equal to 11 years, in agr eement with recent estimates. The model also shows that the deposition of ozone at the Earth's surface (1100 Tg/yr) balances the sum of the net photochemical production (550 Tg/yr) and the flux from the stratos phere (550 Tg/yr). In the case of carbon monoxide, surface emissions ( 1400 Tg/yr) are approximately 50% larger than in situ production by hy drocarbon oxidation (900 Tg/yr).