The Mediterranean Intensive Oxidant Study, performed in the summer of 2001, uncovered air pollution layers from the surface to an altitude of 15 kilometers. In the boundary layer, air pollution standards are exceeded throughout the region, caused by West and East European pollution from the north. Aerosol particles also reduce solar radiation penetration to the surface, which can suppress precipitation. In the middle troposphere, Asian and to a lesser extent North American pollution is transported from the west. Additional Asian pollution from the east, transported from the monsoon in the upper troposphere, crosses the Mediterranean tropopause, which pollutes the lower stratosphere at middle latitudes.
[1] This paper provides a review of stratosphere-troposphere exchange (STE), with a focus on processes in the extratropics. It also addresses the relevance of STE for tropospheric chemistry, particularly its influence on the oxidative capacity of the troposphere. After summarizing the current state of knowledge, the objectives of the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), recently funded by the European Union, are outlined. Several papers in this Journal of Geophysical ResearchAtmospheres special section present the results of this project, of which this paper gives an overview. STACCATO developed a new concept of STE in the extratropics, explored the capacities of different types of methods and models to diagnose STE, and identified their various strengths and shortcomings. Extensive measurements were made in central Europe, including the first monitoring over an extended period of time of beryllium-10 ( 10 Be), to provide a suitable database for case studies of stratospheric intrusions and for model validation. Photochemical models were used to examine the impact of STE on tropospheric ozone and the oxidizing capacity of the troposphere. Studies of the present interannual variability of STE and projections into the future were made using reanalysis data and climate models.
Abstract. A dry deposition scheme, originally developed to calculate the deposition velocities for the trace gases 03, 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 p H, derived from a highresolution global soil p H 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. IntroductionAtmospheric sulfur has been studied extensively in relation to deleterious health effects and the decline of ecosystems due to anthropogenic emissions. The initial interest focused on the impact of sulfur oxides on plant nutrition [Chamberlain, 1980]
[1] Soils are an important though uncertain source of oxidized nitrogen (NO x ) to the atmosphere. One of the main uncertainties in the source estimates is the role of the canopy interactions between NO x emissions, dry deposition, turbulence, and chemistry. Previous studies, in which only dry deposition has been considered, indicate a reduction of about 50% of the globally emitted NO x by soils. We have implemented a multilayer trace gas exchange model in a chemistry general circulation model to explicitly calculate the role of canopy interactions in regulating the effective NO x emissions to the atmosphere. Our new NO x emission algorithm interactively calculates a global soil emission flux of about 12 Tg N yr À1. For a sensitivity analysis we have also included a fixed global soil NO x emissions inventory of about 21 Tg N yr À1. It appears that the enhancement of NO x and O 3 concentrations in response to the soil emission flux is suppressed by the compensating effect of dry deposition. For sites that are exposed to relatively large emission fluxes, our multilayer and the previously used big leaf approach, which does not consider canopy interactions, calculate similar surface NO x fluxes. This confirms the validity of the big leaf approach for most polluted regions at midlatitudes. However, for relatively pristine sites in the subtropics and tropics, where NO x is a limiting factor in ozone and hydroxyl chemistry, there are distinct differences between the multilayer and big leaf NO x surface fluxes. This justifies the use of more comprehensive atmosphere-biosphere exchange descriptions in global models.
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