Find out what you must include in an air dispersion modelling report to get an environmental permit.
You must carry out detailed modelling assessment on any emissions that you didn’t 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 don’t include some information, you’ll need to explain why.
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 (or summarise) the modelled scenarios you’ve used
Include a location map
You must include a map that:
- shows the location of the proposed site 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 air quality standards
You must provide a list of the emissions modelled, including their chemical specification. For example, oxides of nitrogen and halogenated compounds.
You must also identify the relevant environmental standard for air emissions for these substances.
Work out ambient and background levels
You must work out a representative value for the background concentration. Find this information from:
- local authority ambient monitoring data
- background maps produced by Defra
If you use a different source for this information, you need to explain why.
Explain the model
You must list the:
- dispersion modelling software used
- software name, including version number
- type of model, for example Gaussian or new-generation
The model you use must be fit for purpose, based on established science, and be validated and independently reviewed.
Explain emission parameters
You must provide the following information, including relevant units, in a table.
|Stack location||(grid reference)|
|Pollutant emission rate||(grammes per second)|
|Exit temperature||(kelvin, degrees celcius)|
If relevant to your model, you must also provide information about one or more of the following parameters.
|Efflux velocity||(metres per second)|
|Volumetric flow rate (actual)||(cubic metres per second)|
|Volumetric flow rate (at reference conditions)||(normal cubic metres per second)|
You must explain how you’ve worked out the emission rates used in your model.
You can calculate these rates using chimney exit concentration values from the appropriate BREF note or by using a measured concentration value.
You must also include other relevant data, including:
- actual oxygen and moisture levels used to adjust volumetric flows
- emissions that vary with time, if appropriate
Explain the modelled domain and receptors
You need to justify the resolution of the model receptor grid you use.
If it’s relevant to your model, you must give the assumed height above ground level for the receptors.
You must also give details of any discrete receptors and include grid references used to assess impact at sensitive locations.
Explain weather data and surface characteristics
You must explain your choice of weather data and say why it’s representative. You also need to explain how the surface characteristics you’ve chosen represent land use. For example, surface roughness, albedo, Bowen ratio or Priestley-Taylor parameter.
Your report must include:
- the location of the chosen Met Office weather station in relation to the modelled 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, UK Met Office
- a description of the data quality and uncertainties relating to any alternative weather data
- the format of the weather data - hourly sequential or long-term statistical
- a wind rose showing the distribution of wind speed and direction
Explain terrain and building treatments
You must justify why you’ve included terrain and building treatments in your assessment. If you haven’t, you’ll need to explain why.
You report should:
- justify why you’ve included building treatment in your assessment - if you haven’t , explain why
- explain the source, format and processing of digital terrain data used in the model
- show the location and dimensions of all buildings included in the model, including National Grid reference, height, width and rotation
- show the location and relative orientation of buildings and their dimensions on a site plan
Estimate model uncertainty
You must show that you have accurately estimated the level of uncertainty in your predictions.
Look at validation documents for examples of the differences between measured values and those estimated by models.
Where the validation documents indicate levels of uncertainty that might affect your conclusions, run your data through another model to check the differences between models and the level of uncertainty in your predictions.
You will need to carry out sensitivity analysis to deal with variability in your input data.
Carry out sensitivity analysis
You must show how the model is affected by:
- weather data, such as different Met Office weather stations, data sources, inter-annual variation and surface characteristics
- emission parameters, such as stack parameters, pollutant release rates and different plant operating scenarios
- the receptor grid resolution
- treatment of terrain and buildings
Describe the input parameters and any assumptions you’ve made on specialised model treatments, such as:
- coastal models
- wet/dry deposition
- flare releases
- short-term (puff) releases
- post-processing - to measure if short-term emissions could coincide with peak weather conditions
Carry out impact assessment
Your assessment must present the post-processing of relevant percentile values with the addition of background concentrations. You must explain how your background concentrations are representative of the local environment.
You must also justify any assumptions you’ve made relating to pollutant conversion processes (such as nitric oxide and nitrogen dioxide photochemistry) for different averaging times.
Breaches of air quality standards
You must comment on any potential breaches of air quality standards or objectives. You must take into account model uncertainty and provide an assessment of different:
- stack heights
- emission characteristics
- process operation scenarios
You must provide contour plots for each air quality objective you assess. These must:
- include the pollutant name and modelling scenario
- provide averaging time and appropriate percentile plotted, clearly indicating areas of exceedance
- use the same colour scale for all contour plots relating to a particular air quality objective
Present your results
You must present your results in a table showing:
- total (process plus background) concentration values
- locations of maximum air quality impacts and the process contribution to this
- the percentage impact upon the relevant air quality standard or objective
Include input files
You must include the input files for the air dispersion model you’ve used as an appendix to your report.
You must provide enough data so that we can audit the model configuration you’ve used.
We’ll also check parameter values you’ve used to define all source and meteorological inputs to the model.
Please contact the Environment Agency for more information about producing an air dispersion modelling report.
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