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 c irculation model to improve the description of the removal of chemical ly reactive trace gases at the earth's surface. The chemistry scheme s imulates background CH4-CO-NOx-HOx photochemistry and calculates conce ntrations 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 sta bility, wind speed, and surface roughness, and a quasi-laminar boundar y layer resistance is incorporated. The stomatal resistance is explici tly 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 s easonal cycles in both the chemistry and the planetary boundary layer processes and surface characteristics that control dry deposition. Eva luation of the model indicates satisfactory agreement between calculat ed and observed deposition velocities. Comparison of the results with model simulations in which the deposition velocity was kept constant i ndicates significant relative differences in deposition fluxes and sur face layer trace gas concentrations up to about +/- 35%. Shortcomings are discussed, for example, violation of the constant flux approach fo r the surface layer, the lacking canopy description, and effects of su rface water layers.