How the Get an air pollution assessment service calculates emissions
Information for air quality consultants and regulators in Northern Ireland about the data the air pollution assessment service uses to calculate emissions.
Applies to Northern Ireland
The service currently only calculates assessments in Northern Ireland. It will be evaluated before a decision is made to roll it out further across the UK.
The Get an air pollution assessment service uses dispersion modelling to calculate the air quality impacts of developments at sensitive ecological sites.
The calculations use data on:
- emissions
- meteorological conditions
- background pollutant concentrations
- nature site location and sensitivity
The service also uses some standard input values and assumptions.
For information about the dataset versions, see the Data Licences for this service.
There may be delays between the release of a new dataset and its integration in the service.
Emissions data the service uses
Agriculture sources
The service uses emission factors from the Simple Calculation of Atmospheric Impact Limits (SCAIL) to calculate ammonia (NH3) emissions.
You can also enter your own emission rates.
Energy, industry and other sources
You will need to enter emission rates for energy, industry and other source types (for example, emissions from ships).
Road sources
To calculate emissions from road sources, the service will use emission rates for:
- nitrogen oxides (NOx) from Defra’s Emission Factor Toolkit (EFT)
- ammonia (NH3) from Air Quality Consultants’ Calculator for Road Emissions of Ammonia (CREAM)
The fleet composition data and projections used in CREAM were taken from the National Atmospheric Emissions Inventory data.
You can also enter your own emission rates.
Dispersion modelling inputs
The service uses Cambridge Environmental Research Consultants’ Atmospheric Dispersion Modelling System (ADMS-Urban) to carry out dispersion modelling. ADMS-Urban predicts concentrations of NOx and NH3 at the sensitive ecological sites.
Meteorological data
You must choose one of the meteorological data sets within the service. You cannot use your own meteorological dataset.
You can choose observational meteorological data or numerical weather prediction (NWP) computer-modelled data. The observational meteorological data is derived from UK Met Office datasets and the NWP meteorological data is from European Centre for Medium-range Weather Forecasts.
You can input a custom value for the minimum Monin-Obukhov length, surface albedo and Priestley-Taylor parameter, but by default are set at:
- 30 metres (m) for minimum Monin-Obukhov length
- 0.23 for surface albedo
- 1 for Priestley-Taylor parameter
In ‘advanced options’ mode, you can input custom values for surface albedo and Priestley-Taylor parameter.
‘Calm conditions’ are when the wind speed is less than 0.75 metres per second at a height of 10m.
For hours with calm conditions, the service resets the wind speed to 0.75 metres per second and uses the previous hour’s wind direction. If that information is not available, the service will ignore the calm hour of meteorological data in its dispersion calculations.
Read the ADMS-Urban user guide for further information on how the service handles calm conditions and flow calculations.
Surface roughness and terrain
The surface roughness values are based on land cover maps published by the UK Centre for Ecology and Hydrology (UKCEH). You cannot define these yourself.
The service averages the UKCEH 10m × 10m grid to match its output grid.
The service can model the effects of complex terrain on pollutant dispersion using terrain maps published by the Ordnance Survey (OS) at a 50m resolution. It processes the terrain files to ensure they cover the correct size for the modelling area. It uses the default ADMS FLOWSTAR internal grid resolution (64 × 64).
Street canyons
The service uses ADMS-Urban’s advanced street canyon module to model the effects of barriers on the dispersion of pollutants from road sources. The standard porosity values are based on previous project work and dispersion modelling experience.
Nitrogen oxides (NOx) to nitrogen dioxide (NO2)
The service uses primary NO2 fractions (fNO2) to convert concentrations of NOx to NO2 at each assessment point. This is the same approach as Defra’s NOx to NO2 calculator.
You can also enter your own primary NO2 fractions.
Calculating deposition and plume depletion
The service calculates deposition results using predicted NO2 and NH3 concentrations. These are based on surface roughness and dry deposition velocity data derived from the UKCEH land cover maps.
These deposition velocities are accepted and recommended by the Air Quality Technical Advisory Group (AQTAG).
The service can also include the effects of plume depletion on predicted concentrations of NO2 and NH3, and on predicted nitrogen deposition.
Background concentrations and deposition fluxes
‘Background’ describes the existing air quality conditions, without accounting for new or recent emission sources. This includes background concentrations of NOx and NH3, and background deposition fluxes of nitrogen.
The background concentrations of NOx and NH3 are from the UK Air Pollution Information System (APIS). The background deposition fluxes of nitrogen are calculated from the background concentrations of NOx and NH3, and a surface roughness map derived from the UKCEH land cover maps.
The service adds these background values to the predicted concentrations and deposition fluxes to calculate the total predicted impact.
Nature sites
The locations of Sites of Special Scientific Interest (SSSIs), Areas of Special Scientific Interest (ASSIs), Special Areas of Conservation (SACs) and Special Protection Areas (SPAs), as well as the protected feature and sensitivity information, are from the UK Air Pollution Information System (APIS).
Where in APIS a feature’s critical level for NH3 is listed as 1 or 3 micrograms per cubic metre (µg/m³), in this service a critical level of 1 µg/m³ is used, except for a selection of SPAs for which 3 µg/m³ is used. These sites are:
- Belfast Lough Special Protection Area (SPA)
- Belfast Lough Open Water SPA
- Carlingford Lough SPA
- Larne Lough SPA
- Lough Foyle SPA
- Sheep Island SPA
- Killough Bay SPA
- Strangford Lough SPA
Features of interest for SACs that are not part of the official designation of the site (also called Category D features) have not been included in this service.
Calculation resolution and the fixed assessment grid
When using the Formal assessment or Quick run calculation methods, the results will automatically include all designated nature sites within the zone of influence of your project. The receptor points and hexagons generated for these calculations are referred to as the fixed assessment grid. It has a minimum resolution of 4 hectares (the hexagons are approximately 215 metres from edge to edge and 250 metres from point to point), with higher resolutions used for concentration depending on how close the nature site is to the emission source.
You can calculate results for any locations not covered by the fixed assessment grid by adding custom assessment points.
Get help with this service
If you need help using this service, contact the air pollution assessment helpdesk.
Email: get.an.air.pollution.assessment@defra.gov.uk
We aim to reply within 2 working days.
Telephone: 03301 242 162
Monday to Friday, 8:30am to 4pm (except statutory holidays)
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