Air quality dispersion modelling and who should run it
Air quality modelling is not a licensed activity, so the test is whether the author can show verification against local monitoring data and defend every input. Here is what to check.
Air quality modelling competence is not licensed in the way ecology licences are, so the test an authority applies is practical: can the author show the model was verified against local monitoring data, and can they defend the inputs. A dispersion assessment that reports concentrations to one decimal place without a verification section is a number generator, and environmental health officers have been rejecting them for years.
What the assessment has to predict
A planning air quality assessment usually has to answer three separate questions, and the methods differ.
The first is construction dust and its effect on nearby receptors, which is a risk-based exercise rather than a modelled one. The second is whether traffic generated by the development worsens concentrations at existing homes and schools. The third is whether future occupants of the development will themselves be exposed to concentrations above the objectives.
Only the second and third normally need dispersion modelling. Where a scheme has a combustion plant, a biomass boiler, a standby generator or an energy centre, add a fourth question about stack emissions, which is a different modelling exercise with different inputs.
The objectives being tested are set out in the Air Quality Standards Regulations 2010. Nitrogen dioxide is assessed against an annual mean of 40 micrograms per cubic metre and a one-hour mean of 200 that must not be exceeded more than 18 times a year. PM10 has an annual mean of 40 and a 24-hour mean of 50 not to be exceeded more than 35 times. The fine particulate target is tightening under the Environment Act 2021 framework, so check the current figure rather than reusing a number from an older report.
Model selection and competence
The choice of model follows the source. Roadside assessments use a street-scale dispersion model fed with link-by-link traffic flows, speeds and fleet composition. Point sources need a model that handles plume rise, building downwash and terrain. Street canyon geometry needs a module that represents it, because an open-road model will understate concentrations in a narrow urban street.
There is no statutory competence requirement, which puts the burden on you to check. Membership of the Institute of Air Quality Management at a qualified grade, a track record of assessments accepted by the authority you are dealing with, and professional indemnity cover appropriate to the scheme size are the practical filters. An Air Quality Assessment sits in the category where councils usually expect a suitably qualified author, and professional review is strongly advised before submission.
Ask who will actually run the model. On larger consultancies the modelling is often done by a graduate and signed by a chartered or qualified member, which is normal and fine, provided the supervision is real. That arrangement is worth understanding before you appoint, and supervision chains and graduate work signed by a chartered author sets out what to look for.
Monitoring data and model verification
Verification is the step that separates a defensible assessment from a spreadsheet. Dispersion models systematically under-predict the road contribution to nitrogen oxides in UK conditions, so the modeller runs the model at locations where measured data exists, compares the modelled road contribution with the measured one, and derives an adjustment factor applied to all receptors.
Three things make a verification credible. The monitoring sites used have to be representative of the modelled road type, not a background site three streets away. There should be more than one, where the authority's network allows it. And the resulting factor should be reported, with the scatter between sites visible rather than averaged into a single reassuring number.
A factor of around 1.5 to 3 is common on urban roads. Where the factor is very large, that is a signal that the traffic data, the emission factors or the model setup needs revisiting rather than correcting away. The approach is set out in Defra's local air quality management technical guidance, which is the document the environmental health officer will have open.
Background concentrations come from the national background maps, and the year matters. Using a projected background for a future assessment year is right, but the base year of the maps should be stated and the maps should be the current release.
Meteorological data choices
Pick the met station on the basis of representativeness, not proximity. A coastal station five miles away may be less representative of an inland valley site than one further off with similar exposure, and the report should say why the station was chosen.
Use several years of data where the assessment is sensitive to wind direction, and report the variation between years rather than modelling the most favourable one. Surface roughness and the minimum Monin-Obukhov length both need setting to values appropriate for the surrounding land use, and both should appear in the input appendix. If they are not stated, you cannot tell whether an urban site was modelled as open countryside.
Sensitivity and uncertainty reporting
Good assessments are explicit about what they do not know. The useful form is a short section listing the main uncertainties and what happens to the conclusion if each is wrong.
Typical candidates are the traffic growth assumption, the fleet composition projection, the verification factor, the receptor heights chosen for upper-floor flats, and whether a nearby consented scheme has been included in the baseline. Running the model again with a pessimistic assumption on one or two of those takes an afternoon and removes the obvious line of attack.
Where an objection is already in, the sensitivity work is also your negotiating material. It shows which of the officer's concerns actually change the answer, and which do not, which is a better conversation than disputing method. Handling that disagreement is covered in when the council's expert disagrees with your consultant.
Mitigation and damage cost calculations
Where residual effects are not negligible, authorities commonly expect emissions mitigation, either in kind or as a financial contribution calculated using Defra's damage cost approach. The calculation takes the annual emissions of nitrogen oxides and particulates attributable to the development, applies published damage costs per tonne, and produces a figure for the section 106 schedule.
Two points cause arguments. The first is whether the emissions used are the gross development emissions or the net change against the existing use, which on a redevelopment can be a large difference. The second is whether measures already built into the scheme, such as electric vehicle charging provision or a car-capped layout, are credited before the calculation or offered as the mitigation itself.
Settle both in pre-application correspondence. A damage cost figure that arrives in a draft section 106 at week 11 is very hard to argue down.
Reviewing a report critically
Read the appendices before the summary. The inputs tell you whether to believe the conclusion.
- Check the modelled year, the background map release and whether the assessment year matches the expected opening year of the development.
- Find the verification section, the monitoring sites used and the adjustment factor, and check the factor was applied to the road contribution rather than the total.
- Look at receptor locations and heights, including whether upper-floor windows on the development frontage were assessed.
- Compare the traffic data against the transport assessment, because the two documents should use the same flows.
- Confirm the construction dust assessment assigns a risk category and ties mitigation to that category rather than listing generic measures.
- Check that any plant emissions, including standby generators tested monthly, have been included.
Where you need to commission or review this work, the scope of the report type is set out on the Air Quality Assessment page, and reviewers who can check and sign off a draft can be found through the professional directory. Every version of the report should carry a named author and a date, for the reasons set out in why every planning report needs a named author and date.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for specialist technical reports, qualified professional input may be required.
Frequently asked questions
Does an air quality assessment have to be done by an accredited person?
There is no statutory licence or accreditation requirement. Councils usually expect a suitably qualified author and professional review is strongly advised, so the practical filters are membership of the Institute of Air Quality Management at a qualified grade, a record of assessments accepted locally, and professional indemnity cover suited to the scheme size.
What is model verification and why does it matter?
UK dispersion models tend to under-predict the road contribution to nitrogen oxides, so the modeller runs the model where measured data exists, compares modelled and measured road contributions, and derives an adjustment factor applied to every receptor. Without that step the reported concentrations are unverified, and environmental health officers routinely reject assessments that omit it.
Which air quality objectives are tested?
The Air Quality Standards Regulations 2010 set the figures used in planning. Nitrogen dioxide is assessed against an annual mean of 40 micrograms per cubic metre and a one-hour mean of 200 not to be exceeded more than 18 times a year, and PM10 against an annual mean of 40 and a 24-hour mean of 50 not to be exceeded more than 35 times.
How is an air quality contribution calculated?
Authorities commonly use Defra's damage cost approach, applying published costs per tonne to the annual nitrogen oxide and particulate emissions attributable to the development. The arguments are usually about whether gross or net emissions are used, and whether scheme measures such as charging provision are credited before the calculation or offered as the mitigation.
Is construction dust modelled?
No. Construction dust is assessed through a risk-based method that scores the scale of demolition, earthworks, construction and trackout against the sensitivity of nearby receptors, then assigns mitigation by risk category. A good report ties each measure to the category it came from rather than listing generic site practices.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.