IFDM (Immission Frequency Distribution Model) is a bi-Gaussian dispersion model developed by VITO to calculate the local dispersion of pollutants in the atmosphere based on meteorological data such as wind speed, wind direction and temperature. The figure below schematically illustrates the elements in a typical Gaussian dispersion model. 

IFDM was originally developed to assess the local impact of point sources as part of Environmental Impact Assessments. Later is was extended to line sources to include the dispersion of road traffic emissions.  Over the past decade IFDM is mainly being used,to produce high resolution maps of the air quality over a region or city.  IFDM is able to capture the strong gradients observed along a road network, which is particularly important for assessing the varying concentrations of NO2 across an urban area. IFDM can also be coupled to the OSPM street-canyon module to account for street canyon increments in urban environment.

In order to produce regional maps, IFDM can be coupled to any regional model or other source of background concentrations. In such a coupled model chain IFDM is taking care of the local dispersion of traffic and industrial sources whereas other sources of air pollution such as agriculture and residential heating are included in background. Unlike point or line sources, the emissions from agriculture and residential heating are much more distributed over larger areas and are best taken into account in the background concentrations or in a chemistry transport model (CTM) coupled to the IFDM-model.  To avoid double counting of traffic and industrial sources in both IFDM and the regional model, a "double count correction" can be applied.

The IFDM model has been well validated for use in urban environments and is published in several Atmospheric Environment papers ( W. Lefebvre, Van Poppel, Maiheu, Janssen, & Dons, 2013; Wouter Lefebvre & Vranckx, 2013).


Coordinate system and elements of a bi-Gaussian dispersion model
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