| Dispersion models
Using meteorology and emissions as input data to various models, it is possible to simulate the dispersion of different air pollutants and estimate the extent to which people and the environment are exposed. Ultimately, this allows for estimates on health and environmental impacts to be calculated. Each of these stages is vital in order to describe the impacts of a particular measure or development. SMHI Airviro Dispersion is used to calculate concentrations of air pollutants from emissions. Four different types of model are available: Wind model, Gauss model, Grid model and Street canyon. For street canyon calculations the SMHI Simair OSPM-model is also used. |
| Wind model Air pollutants are influenced by various meteorological processes. Winds carry pollutants, and turbulence mixes and dilutes them. Airviro uses a simplified wind model based on Danard (1977) to calculate the wind fields that are used in the dispersion model. The wind field calculation is based on the principal that small-scale winds can be seen as a local adaptation of large-scale winds (free winds) due to local fluxes of heat andmomentum from the sea or ground surface. All nonlinear interaction between scales is disregarded. It is also assumed that the adaptation process is very quick and that horizontal processes can be described by nonlinear equations, while vertical processes can be described as linear functions. Large-scale winds and vertical fluxes of momentum and temperatureare estimated from profile measurements in meteorological masts in Högdalen, south Stockholm, and in Marsta, outside Uppsala. The resolution of topography and land-use data in the wind model is 500 metres. |
| Gauss model Pollutants emitted into the atmosphere are dispersed due to transport and dilution processes. In the atmosphere, pollutants disperse through dilution and transport. The dispersion depends, in part, on the height at which pollutants are emitted, such as, at ground level or from a high chimney. The temperature of exhaust gases is also important; a hot gas will rise upwards and give a more effective emissions level. The Gauss model is used to calculate concentrations of pollutants above ground (open landscape) or rooftop (buildings). The default height for calculations is set at 2 m, but the user can specify other values. Dispersion from each individual source is described in the model with help of a Gaussian plume model. |
| Grid model The Grid model is used, like the Guass model, to calculate air pollutant concentrations 2 m (or at another specified height) above ground or rooftop. While the Guass model is suitable on the local/urban-scale, the Grid model is more suitable on a regional scale. The Grid model is primarily used for deposition calculations of, for example, acid deposition. |
| Street Canyon model Concentrations of pollutants in narrow street environments can vary widely from one side of the street to the other. This is dependent upon the formation of vortices (and recirculation of air flow) from the surrounding buildings. The street canyon’s design is crucial to ventilation and the dilution of air pollutants. Narrow streets are worse than wide streets, and streets with tall buildings are worse than streets with low or no buildings. Both the Gauss and Grid models calculate air pollutant concentrations at rooftop. SMHI Airviro Street Canyon is used for individual street calculations of air pollutant concentrations and is advantageous in streets with buildings on both sides. SMHI Simair OSPM-model is used for calculations both in streets with buildings on one side and for those with buildings on both sides. |