Environmental permitting: air dispersion modelling reports
Find out what you must include in an air dispersion modelling report to get an environmental permit.
Applies to England
You must carry out detailed modelling assessment on any emissions that you did not or could not screen out through your air emissions risk assessment. Detailed modelling will identify if these emissions could harm the environment or human health.
You need to include all the information explained in this guide, in your report. If you do not include some information, you’ll need to explain why. There needs to be enough information in the report to make an exact copy of the model.
You need to demonstrate an understanding of the environmental implications of the emissions based on the best available science at the time of the submission, recognising any potential gaps in knowledge for new and developing technologies or processes. This includes an understanding of their environmental fate.
If your risk assessment identifies that your operation is likely to cause odour problems, you may need to model odour exposure. The guide H4 odour management has more information.
Explain your report
At the start of your report, you must:
- explain the purpose of the study
- describe the site
- explain the process by which your emissions occur, their duration and frequency
- explain the modelled emission scenarios and how they represent your operations
Include a location map
You must include a map that:
- shows the location of the proposed site, site boundary and surrounding land use
- shows the size of the modelled area
- uses National Grid referencing and indicates terrain contours, such as the Ordnance Survey Landranger series (1:50,000)
List emissions and environmental standards for air
You must provide a list of the chemical substances modelled.
You must identify the relevant air emission environmental standards for these substances.
For substances for which no air emission environmental standard is listed, you must provide a unique identifier, for example, the CAS registry number if available, or the chemical specification.
You must also provide a unique identifier where:
- you have derived a new environmental assessment level (EAL) for a substance
- you have proposed an alternative EAL for a substance
Work out ambient and background levels
You must:
- work out a representative value for the background concentration
- explain how your background concentrations are representative of the local environment and assessment timeframe
Find this information from:
- local authority ambient monitoring data
- data from Department for Environment, Food and Rural Affairs (Defra) national monitoring networks
- local air quality management background maps and modelled background pollution data produced by Defra
If you use different sources for this information, you need to explain why and explain why it is appropriate.
You should consider whether there are any other plans, projects or permissions that are not currently included in the background concentrations. You may need to model and provide the emissions parameters for other plans, projects or emissions, where relevant.
Explain the model
The model you use must be:
- fit for purpose
- based on established science
- validated
- independently reviewed
You must justify that the model you use is suitable to represent the transport, dispersion and transformation processes of the substances you emit.
You must list the:
- dispersion modelling software used
- software name, including version number
- type of model, for example Gaussian plume model
- supplier
Explain emission parameters
You must explain:
- how your modelled source terms represent your operational scenarios and emissions
- the source type and any assumptions or simplifications
You must list all the relevant parameters and their units in a table for point source emissions. You must explain any parameters not included or not relevant.
| Parameter | Units |
|---|---|
| Emission point location | (grid reference) |
| Release height | (metres) |
| Exit diameter | (metres) |
| Exit temperature | (kelvin, degrees Celsius) |
| Efflux velocity | (metres per second) |
| Volumetric flow rate (at actual conditions) | (cubic metres per second) |
| Volumetric flow rate (at reference conditions) | (normal cubic metres per second) |
| Substance emission rate | (grammes per second) |
You must explain how you have worked out the emission rates used in your model. You need to demonstrate that the emissions are appropriate to the assessment purpose.
You can calculate emission rates using:
- emission concentrations from the relevant BAT conclusion document
- measured concentrations from your own monitoring
- benchmark concentration values from similar plant
Where you use measured or benchmark concentrations, you need to include the raw data and the source of the information.
You must also include any other relevant data to demonstrate the emissions used in your model are a valid representation of the operational scenario being modelled, including:
- temperature and pressure conditions
- actual oxygen and moisture levels
- emission rates or other source parameters that vary with time
If you have modelled emissions from source types other than point source emissions (for example, emissions from area, line or volume sources) you need to explain the source type and how it represents the emissions from your operations.
Explain the modelled domain and receptors
You need to justify the resolution of the model receptor grid you use.
If relevant to your model, you must give the assumed height above ground level for receptors.
You must also give details of any discrete receptor and include grid references used to assess the impact at sensitive locations.
You must consult any local development plans to include potential future sensitive receptors in your assessment.
Explain meteorological data and surface characteristics
You must explain your choice of meteorological data and say why it’s representative. You also need to explain how the surface characteristics you have chosen represent the land use within your model. For example, surface roughness, albedo, Bowen ratio or Priestley-Taylor parameter.
Your report must include:
- the location of the meteorological data in relation to the model domain
- the number of years covered by the data – 5 is recommended, but it must be a minimum of 3
- the source of the data, for example, the UK Met Office
- whether the meteorological data is observed or from a numerical weather prediction (NWP) model with an appropriate resolution
- a description of the data quality and uncertainties relating to any alternative meteorological data
- any data processing or filling you have done to improve the data quality
- a wind rose showing the distribution of wind speed and direction
Explain terrain and building treatments
You must justify why you have included terrain and building treatments in your assessment. If you have not, you’ll need to explain why.
Your report should:
- explain the source, format and processing of digital terrain data used in the model
- justify why you have included building treatment in your assessment – if you have not, explain why
- show the location and dimensions of all buildings included in the model, including grid reference, height, width and rotation
- show the location and relative orientation of buildings and their dimensions on a site plan
- differentiate between on-site and off-site, and existing and planned structures.
Special treatments
Conversion of nitrogen oxides to nitrogen dioxide
You must justify any assumptions you have made relating to the conversion of nitric oxide and nitrogen dioxide photochemistry for different averaging times.
For combustion processes where no more than 10% of nitrogen oxides are emitted as nitrogen dioxide, you can assume worst case conversion ratios for nitrogen dioxide of:
- 35% for short-term average concentrations
- 70% for long-term average concentrations
Other special treatments
Explain and provide the input parameters and any assumptions you have made on specialised model treatments, such as:
- photochemistry
- chemical reaction models
- wet or dry deposition
- fluctuations
- short-term (puff) releases
- dense gas releases
- flare releases
- effects of wind turbines
- coastal effects
- calm wind conditions
- urban heat island effects
- urban canopy flow
- post-processing or short-term statistical analysis – to calculate the likelihood of intermittent emissions coinciding with meteorological conditions that could increase pollution
Estimate model uncertainty
You must show that you have estimated the level of uncertainty in your predictions.
Look at evaluation or validation documents for examples of the differences between measured values and those estimated by models.
Where these documents indicate levels of uncertainty that might affect your conclusions, you need to consider running another model to check the differences between models.
You must justify your input data and assumptions. You will need to carry out sensitivity analysis to deal with uncertainty and variability in your input data.
Your conclusions must show that you have taken the uncertainty of predictions into account.
Carry out sensitivity analysis
You must show how the model is affected by:
- meteorological data, such as different observed data, NWP data, data sources, inter-annual variation and surface characteristics
- emission parameters, such as stack parameters, substance release rates and different plant operating scenarios
- the receptor grid resolution
- treatment of terrain and buildings
- special model treatments
- alternative modelling software, if appropriate
Carry out impact assessment
Your assessment must present the post-processing of relevant percentile values with the addition of background concentrations.
Present your results
You must include results:
- for the locations of maximum air quality impacts
- at discrete sensitive receptor locations
Present them in a table showing:
- the process contribution (PC)
- predicted environmental concentration (PEC) – which is the PC plus the background concentration, and any other contributions from existing or planned sites, where relevant
- PCs and PECs as a percentage of the relevant environmental standard
You must include a discussion of results (what they mean and their significance) before you make your final conclusions.
For a detailed modelling assessment PCs are insignificant where they are less than:
- 10% of a short-term environmental standard
- 1% of a long-term environmental standard
At the detailed modelling stage there are no criteria to decide whether PCs are significant.
You must explain how you judged significance and the impact and effect of your PCs in the context of the PEC, the environmental standard and the site-specific circumstances.
Breaches of environmental standards for air
You must comment on any potential breaches of environmental standards from your emissions. You must consider model uncertainty and where relevant provide an assessment of different:
- stack heights
- emission characteristics
- process operation scenarios
If you judge that your model and assumptions lead to predictions that are overestimated and show exceedances of environmental standards, you need to either:
- present more realistic predictions
- explain why exceedances are not likely, or the impacts from your site emissions are not significant
Contour plots
You must provide contour plots for any PCs above the insignificant criteria. These must:
- include the pollutant name and modelling scenario
- provide the relevant averaging time and percentile plotted
- clearly indicate the areas exceeding the insignificant criteria and environmental standards
Include input files and input parameters
You must include the input files for the air dispersion model you have used, in addition to your report.
You must provide a separate annex with a table of all the input parameters you have used within your model that supports any:
- air quality impact assessment
- human health risk assessment
- odour impact assessment
You must provide enough data so that we can audit the model configuration you have used.
We may audit the model configuration and check the parameter values you have used to define sources of emissions and meteorological inputs to the model.
In doing so we will evaluate whether your conclusions are sound and can be used for regulatory decision making. We may recommend consultation with other specialists.
Contact
If you have a query about producing air dispersion modelling reports, email aqmau@environment-agency.gov.uk
If you have queries about a site-specific air dispersion modelling, you can request enhanced pre-application advice.
General enquiries
General enquiries
National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY
Email enquiries@environment-agency.gov.uk
Telephone 03708 506 506
Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312
Monday to Friday, 8am to 6pm.
Last updated 26 March 2024 + show all updates
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Added more information on what the Environment Agency expects from applicants conducting dispersion modelling assessments with emissions from emerging technologies, such as the carbon capture and storage industry. Made other updates to bring the guidance in line with current air dispersion modelling requirements.
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We have added the requirement that you must provide a separate annex with a table of all the input paramaters you have used.
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We have: added that we need enough information in the report to make an exact copy of the model; changed air quality standards to environmental standards for air; linked to Defra national monitoring networks; added more detail on emission rates calculations; added clarity on post-processing under special treatments; added information on impact assessment for combustion processes and added clarity to ‘present your results’.
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First published.