Ammonia Emission Dispersion Modelling

Guidance on Ammonia Emission Dispersion Modelling and SCAIL calculation.

Initial model

Initial model used for Screening: SCAIL (Simple Calculation of Atmospheric Impact Limits)

SCAIL is a simple online screening tool that can be used to estimate the effect of an agricultural emission (e.g. poultry shed) on a habitat (e.g. ASSI). This estimate may then be used to determine the exceedance or non-exceedance of the habitat's impact limit and will help users in deciding whether more detailed modelling or site specific investigation is required.

Information required

Information Detail
Source Location                    Easting and Northing
Source Type                          Housing, litter/manure storage, land spreading
Ventilation Type                    Naturally ventilated or force ventilated
Building Height                     Metres
Livestock Number                Number of animals
Housing Floor Area             Square metres
Type Broilers, layers etc
Fan Location                        Roof or side of building
Number Of Fans                  Number
Fan Diameter                      Metres
Fan Flowrate                        M3/s
Designated Site Details       Detail
Search Radius                     Kilometres
User Specified Site              Detail
Site Name                            Name of site
Site Location                         Easting and Northing
Habitat Within Site                Acid grassland, bog etc

Use the APIS website (Air Pollution Information System) to identify the Critical Level of each sensitive species/habitat: It is important to distinguish between a critical load and a critical level. The critical load relates to the quantity of pollutant deposited from air to the ground, whereas the critical level is the gaseous concentration of a pollutant in the air. (For more detail on Critical Loads/Levels visit the Air Pollution Information System website.

More detailed dispersion modelling

More detailed dispersion modelling could be provided by the following models:


ADMS is a dispersion model used to model the air quality impact of existing and proposed industrial installations. It has been developed by Cambridge Environmental Research Consultants (CERC) in collaboration with government bodies.

The model predicts wet and dry deposition using meteorological data and typography.

It covers dispersion from point, area, volume and line sources with a straight-line plume trajectory from source to receptor or grid point.

Concentrations are modelled on an hour by hour basis using an appropriate regional meteorological data set.

This modelling software is the industry standard in the United Kingdom.


AERMOD is another dispersion modelling software package which was produced by American Meteorological Society/United States Environmental Protection Agency.

It is a steady-state plume model that incorporates air dispersion based on  planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.

Further guidance

NIEA (2015) 'Guidance for Operators on producing an Air Dispersion Modelling Report for a PPC Farming Application' will provide more detail on the type of information required when published.

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