scope:

Scope and objective

This guideline describes a numerical model for simulating the dispersion and calculating the concentrations of trace species in the atmosphere.¹⁾Da ta required for the model include the mean wind field, turbulence parameters, emission data and, depending on the specific case, further application-specific input data. These input data normally have to be obtained from pre-processors which are not the subject of this guideline. In the same way, post-processor models are required for calculating effects such as odour perception, for instance, or radioactive radiation doses. Die Spurenstoffe k¨onnen als Gas oder als Aerosol vorliegen. Simuliert werden die Trajektorien einer großen Anzahl von Gas- bzw. Aerosolpartikel, die unabh ¨angig voneinander mit der turbulenten Str¨omung verlagert werden. Die Ausbreitung kann in freiem Gel¨ande, im Bereich von topographischen Strukturen oder von Hindernissen, wie Geb¨auden, Industrieanlagen, W¨allen, Br¨ucken, B¨aumen und W¨aldern erfolgen, wobei die Einfl¨usse der Hindernisse auf Wind- und Turbulenzfeld durch das Vorschaltmodell richtig beschrieben werden m¨ussen. Chemische Umwandlungen, Sedimentation, Ablagerung am Boden und an der Vegetation, Aufwirbelung vom Boden, Auswaschen durch den Niederschlag, Filterung durch por¨ose Hindernisse und Auftriebseffekte k¨onnen ber¨ucksichtigt werden.

The trace species may be present in gaseous or aerosol form. The model simulates the trajectories of a large number of gaseous and/or aerosol particles which are transported independently of each other by the turbulent flow. Terrain regimes like open terrain, topographical structures or obstacles such as buildings, industrial plants, embankments, bridges, trees and forests can be accounted for in the dispersion calculation, provided that the impacts of such obstacles on the wind and turbulence fields are correctly described by the pre-processor. Chemical transformation processes, sedimentation, deposition on the ground and the vegetation, resuspension into the atmosphere, washout by precipitation, filtering through porous obstacles and buoyancy effects can be accommodated.

The spatial structure and the temporal behaviour of the source(s)can be selected at discretion. The model region investigated may reach from several tens of metres (building complexes)to several hundred kilometres (regions)with resolutions between one metre and several kilometres. The time scale for typical applications ranges from about 10 minutes to several days, the treatment of climatological problems over longer time spans also being possible.

The model outputs are the three-dimensional concentration field and the two-dimensional distribution of the deposition, if applicable together with their respective temporal variation. The model lends itself to the solution of current, diagnostic or planning problems. Typical applications include:

• Dispersion calculations within the scope of permitting applications for industrial plants in complex terrain, in the area of obstacles and during meteorological conditions with low wind speed
• Prediction of regions where the limit values will be exceeded within the scope of safety analyses for chemical and nuclear plants
• Prediction of affected areas in the case of accidents involving the release of hazardous gases or aerosols for fire services and emergency response
• Basis for the estimation of odour nuisance
• Determination of pollutant dispersion from fugitive sources such as waste water treatment plants, landfills, contaminated sites, composting plants, factory farming and chemical plants
• Environmental compatibility analyses

¹⁾ A computer program for this model, which has also been used to compute the examples given in Annex D, is available free of charge on the Internet at http://www.vdi.de/partikelmodell.