A dry deposition parameterization for sulfur oxides in a chemistry and general circulation model

Abstract

A dry deposition scheme, originally developed to calculate the deposition velocities for the trace gases O3, NO2, NO, and HNO3 in the chemistry and general circulation European Centre Hamburg Model (ECHAM), is extended to sulfur dioxide (SO2) and sulfate (SO42−). In order to reduce some of the shortcomings of the previous model version a local surface roughness and a more realistic leaf area index (LAI), derived from a high‐resolution ecosystem database are introduced. The current model calculates the deposition velocities from the aerodynamic resistance, a quasi‐laminary boundary layer resistance and a surface resistance of the surface cover, e.g., snow/ice, bare soil, vegetation, wetted surfaces, and ocean. The SO2 deposition velocity over vegetated surfaces is calculated as a function of the vegetation activity, the canopy wetness, turbulent transport through the canopy to the soil, and uptake by the soil. The soil resistance is explicitly calculated from the relative humidity and the soil pH, derived from a high‐resolution global soil pH database. The snow/ice resistance of SO2 is a function of temperature. The SO2 deposition velocity over the oceans is controlled by turbulence. The sulfate deposition velocity is calculated considering diffusion, impaction, and sedimentation. Over sea surfaces the effect of bubble bursting, causing the breakdown of the quasi‐laminary boundary layer, scavenging of the sulfate aerosol by sea spray, and aerosol growth due to high local relative humidities are considered. An integrated sulfate deposition velocity is calculated, applying a unimodal mass size distribution over land and a bimodal mass size distribution over sea. The calculated sulfate deposition velocity is about an order of magnitude larger than that based on a monodisperse aerosol, which is often applied in chemistry‐transport models. Incorporation of the new dry deposition scheme in the ECHAM model yields significant relative differences (up to ∼50%) in mass flux densities and surface layer concentrations compared to those calculated with a simple, constant dry deposition scheme.