DRY DEPOSITION PARAMETERIZATION IN A CHEMISTRY GENERAL-CIRCULATION MODEL AND ITS INFLUENCE ON THE DISTRIBUTION OF REACTIVE TRACE GASES

A dry deposition scheme has been developed for the chemistry general circulation model to improve the description of the removal of chemically reactive trace gases at the earth's surface. The chemistry scheme simulates background CH4-CO-NOx-HOx photochemistry and calculates concentrations of, for example, HNO3, NOx, and O-3. A resistance analog is used to parameterize the dry deposition velocity for these gases. The aerodynamic resistance is calculated from the model boundary layer stability, wind speed, and surface roughness, and a quasi-laminar boundary layer resistance is incorporated. The stomatal resistance is explicitly calculated and combined with representative cuticle and mesophyll resistances for each trace gas. The new scheme contributes to internal consistency in the model, in particular with respect to diurnal and seasonal cycles in both the chemistry and the planetary boundary layer processes and surface characteristics that control dry deposition. Evaluation of the model indicates satisfactory agreement between calculated and observed deposition velocities. Comparison of the results with model simulations in which the deposition velocity was kept constant indicates significant relative differences in deposition fluxes and surface layer trace gas concentrations up to about +/- 35%. Shortcomings are discussed, for example, violation of the constant flux approach for the surface layer, the lacking canopy description, and effects of surface water layers.