What is air dispersion modelling and why is it needed?

Air dispersion modelling is the use of mathematical formulations to simulate atmospheric processes and predict the dispersion of pollutants. It is a tool used as part of an air quality risk assessment, usually to support either Environmental Permit or Planning Applications. Any facility or development that emits pollutants to air could need a dispersion modelling assessment for example:

• Industrial sites, chemical plants;
• Food and drink factories;
• Boilers and Combined Heat and Power plants (CHP) on any of the above sites;
• Medium Combustion Plants and Specified Generators;
• Power stations, Waste to Energy, Carbon Capture plants;
• Data centers – emergency back-up generators; and
• Animal rearing / intensive agriculture / farms e.g. for odour emissions, ammonia emissions.

How is air dispersion modelling undertaken?

Air dispersion modelling is typically undertaken using computer software programs such as AERMOD and ADMS-6. They are complex programs and require expert knowledge to be used effectively. The information input to a dispersion model includes:

Emission parameters: stack height, diameter, velocity, temperature, mass pollutant emission rates, including details on variation through time;

Meteorological data: multiple years of hourly meteorological data are required that cover the more obvious parameters like wind speed, direction and temperature, but also include complex information on atmospheric boundary layers and require processing for local conditions, surface roughness, albedo, bowen ratio, and urban heat island effects;

Building data to assess building downwash: building downwash occurs when turbulence, caused by nearby structures (building wake effects), results in the emissions from an elevated source being dispersed rapidly towards the ground, resulting in elevated ground level concentrations;

Topography: The presence of elevated terrain can significantly affect the dispersion of pollutants and the resulting ground level concentration in a number of ways, for example elevated terrain reduces the distance between the plume centre line and the ground level, thereby increasing ground level concentrations;

Mapping data and identification of sensitive receptors: most models now include some kind of satellite imagery or mapping enabling specific receptor locations of interest to be identified within a model such as potential sensitive human exposure locations (schools, hospitals, care homes etc) and designated ecological receptors (e.g. Sites of Special Scientific Interest - SSSIs).

What are the regulator requirements for dispersion modelling assessments?

In the UK, the Environment Agency (EA) have specific requirements for dispersion modelling assessments and reports. The EA guidance ‘Air emissions risk assessment for your environmental permit’ [1] details when a dispersion modelling assessment will be required – most of the regulatory agencies across the UK (Natural Resource Wales (NRW), Scottish Environmental Protection Agency (SEPA) and Northern Ireland Environment Agency (NIEA)) either use this same guidance or have similar guidance. A staged approach is adopted, i.e. a screening assessment (known as Stage 1 and 2 or H1) to identify those emissions that can be considered insignificant and the requirement (if any) for detailed dispersion modelling (Stage 3).

In addition, the Environment Agency has guidance on the content of air dispersion modelling reports [2]. This detailed guidance requires that sufficient information is included to allow the assessment to be replicated and detailed justification for the various modelling inputs.

What are the benefits of dispersion modelling?

Whilst dispersion modelling is more time and cost expensive than basic assessments, it can also result in capital expenditure savings especially where it is employed early in the design process and feeds into cost-benefit-analysis, for example: a dispersion model can:

• identify potential constraints early in the design stage and assist in findings solutions;
• more accurately define appropriate stack heights than basic methods (like D1 calculations), and usually shows lower stack heights are sufficient; and
• more accurately define abatement requirements e.g. a combination of increased stack height and less expensive lower maintenance abatement system may achieve the same results as multi-stage abatement (identify cross-environmental benefits).

Who to contact for a Dispersion Modelling Assessment

The SLR Air Quality Team can support with all aspects of air quality assessments and dispersion modelling needed to support your project, including:

• Detailed dispersion modelling assessments
• Stack height assessment
• Assessments for Medium Combustion plant and Specified Generators
• Odour Impact Assessments
• Ammonia Emissions Risk Assessments
• Plume Visibility Assessments
• Dispersion modelling to support Habitats Risk Assessments
• Stack Emissions Testing

If you would like further information please send us an enquiry and we’ll get back to you.

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References

[1] https://www.gov.uk/guidance/air-emissions-risk-assessment-for-your-environmental-permit

[2] https://www.gov.uk/guidance/environmental-permitting-air-dispersion-modelling-reports