Search

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

Cross-tropopause transport of O3 is a significant factor in the tropospheric budget and distribution of O3. Nevertheless, the distribution in the troposphere of O3 that originates from the stratosphere is uncertain. We study this with a chemistry - general circulation model with relatively high spatial and temporal resolution. The model simulates background tropospheric CH4-CO-NOx-HOx photochemistry, and includes a tracer for stratospheric O3. Since this tracer is not photochemically produced in the troposphere but only destroyed, comparing its budget and distribution with that of total tropospheric O3 yields an estimate of the contribution of the stratospheric O3 source in the troposphere. Model results suggest that transport from the stratosphere and net photochemical formation in the troposphere, considering present-day emissions, are of comparable magnitude. The model predicts efficient transport of upper tropospheric O3-rich air to the surface by large-scale subsidence in the subtropics and by synoptic disturbances in the NH middle and high latitudes. O3 from the stratosphere contributes significantly to surface O3 in winter and spring when the photochemical lifetime of O3 is relatively long. In summer and in the tropics, little O3 from the stratosphere reaches the surface due to strong photochemical destruction, so that surface O3 is largely determined by photochemical production. Photochemically produced O3 maximizes in the free troposphere where the O3 surface warming efficiency is higher compared to the boundary layer.