Dispersion calculations

Dispersion calculations can relate to the dispersion of odour, pollutants and nitrogen emissions. The aim of such calculations is to assess the impact of certain emissions at ground level. This impact may be expressed in the following ways:

  • Immission: the concentration at ground level.
  • Deposition: the amount of a substance that enters the soil.


In most cases, the used dispersion models result in a map showing immission or deposition contours.

Relationship emission and immission or deposition

To translate emissions into immission or deposition, we have various dispersion models available. The choice of the model depends on the type of source and emission.

Some examples of dispersion models:

  • Odour emissions from conducted source:
    • The most obvious example is a chimney.
    • The odour dispersion and immission concentrations in the local environment are modelled using IMPACT (Flanders) or Geomilieu (the Netherlands)
  • NOx emissions from vehicles:
    • If the project is located in the built environment, the impact on the local air quality is modelled using CAR-Vlaanderen. The model provides an overview of the following elements:
      • the background concentration
      • the contribution of the traffic
      • the number of exceedances of the test value
    • In more open locations, CAR-Vlaanderen is not reliable enough and IMPACT-traffic is used.
  • NOx emissions from conducted source:
    • The dispersion and influence on the local air quality and deposition in the local environment are modelled using IMPACT.
    • Specifically for assessing the nitrogen deposition, the following tools are used:
      • Flanders: the online Pretest and the Impact score tool
      • The Netherlands: Aerius, the standard dispersion model for determining deposition in the local environment.

Within Witteveen+Bos, we have experience with all the above-mentioned dispersion models, which we can apply in your specific project.

Witteveen+Bos’ calculation tool models shipping emissions

Besides using existing models, Witteveen+Bos has also developed its own calculation tool to model the effect of shipping emissions. For its development, we work closely with internal data and GIS specialists.

More information?

Arnout Soumillion

Air quality engineer