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听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听information about the听dataset versions, see the听听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听听to calculate ammonia (NH3) emissions.
You can also enter your own听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鈥檚听
- ammonia (NH3) from Air Quality Consultants鈥櫶�
The fleet composition data and projections used in CREAM were taken from the听.
You can also enter your own听emission rates.
Dispersion modelling inputs
The service uses Cambridge Environmental Research Consultants鈥櫶齮o 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听observational meteorological data or听numerical weather prediction (NWP)听computer-modelled data.听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听albedo听and Priestley-Taylor parameter, but听by听default are set at:
- 30 metres (m) for minimum Monin-Obukhov length
- 0.23 for surface albedo
- 1 for Priestley-Taylor parameter
In 鈥榓dvanced options鈥� mode, you can input custom values for surface albedo and Priestley-Taylor parameter.
鈥楥alm conditions鈥� 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鈥檚 wind direction. If that information is not available, the service will ignore the calm hour of meteorological data in its dispersion calculations.
Read the听听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 脳 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 脳 64).
Street canyons
The service uses ADMS-Urban鈥檚 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听fractions (fNO2) to convert concentrations of NOx听to NO2听at each assessment point. This is the same approach as Defra鈥檚听.
You can also enter your own听primary NO2听蹿谤补肠迟颈辞苍蝉.
Calculating deposition and plume depletion
The service calculates deposition results using predicted NO2听and NH3听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听and NH3, and on predicted nitrogen deposition.
Background concentrations and deposition fluxes
鈥楤ackground鈥� 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听and NH3听are from the听. The background deposition fluxes of nitrogen are听calculated听from the background concentrations of NOx听and NH3, and听a听surface roughness map derived from the UKCEH land听cover maps.
The service adds these background values to the predicted concentrations and deposition fluxes to calculate the total predicted impact.
Nature sites听
The locations听of听Sites of Special Scientific Interest (SSSIs),听Areas of Special Scientific Interest (ASSIs), Special Areas of Conservation (SACs)听and Special Protection Areas (SPAs), as well as the听protected feature and sensitivity information,听are听from the听.
Where in APIS听a feature鈥檚听critical level for NH3听is listed as 1 or听3听micrograms per cubic metre (碌驳/尘鲁),听in this service a critical level of 1 碌驳/尘鲁听is used,听except for听a selection of SPAs for which 3 碌驳/尘鲁听is used. These sites are:
- Belfast Lough Special Protection Area (SPA)听
- Belfast Lough Open Water SPA听
- Carlingford Lough SPA听
- Larne Lough SPA听
- Lough Foyle SPA听
- Sheep Island SPA听
- Killough Bay SPA听
- Strangford Lough SPA
Features of interest听for SACs that are not part of the official designation of the site (also called Category D features) have not听been included in this service.
Calculation听resolution and the听fixed assessment grid
When using the Formal assessment or Quick run calculation methods,听the results will automatically include听all听designated nature sites within the zone of influence听of your project.听The听receptor points and hexagons generated听for these calculations听are referred to as the fixed assessment grid.听It has a minimum resolution of 4 hectares (the hexagons are听approximately听215 metres from听edge to听edge and 250 metres from point to point), with higher resolutions used听for听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听by听adding custom assessment points.
Get help with this service
If you need help using this service, contact the air pollution assessment听helpdesk.
Email: get.an.air.pollution.assessment@defra.gov.uk听
We aim to reply within 2 working days.
Telephone: 03301 242 162听听
Monday to Friday, 8:30am to 4pm (except statutory holidays)听
Find out about call charges听