Finding air quality monitoring data for a site

Finding air quality monitoring data for a site means collecting from two places that measure different things in different ways. The national network run for Defra uses continuous reference analysers at a relatively small number of locations and publishes hourly data. Local authorities run far denser networks of diffusion tubes, which are inexpensive, passive and good enough for annual means once they have been corrected. Neither gives you a number you can use raw. Tube results need bias adjustment and often annualisation, and national background maps come on a one kilometre grid that tells you nothing about the kerbside. Work out which you need before you commission anything.

The national network and what it covers

Defra's air information resource publishes data from the automatic urban and rural network and from associated networks, with continuous reference-grade analysers measuring nitrogen dioxide, particulate matter and other pollutants hour by hour.

Those sites are few and deliberately sited to represent particular exposure types: urban background, roadside, kerbside, rural. The classification matters more than the distance. A roadside monitor 3 kilometres away tells you more about a busy high street than an urban background monitor 500 metres away does.

The same service publishes modelled background concentration maps on a one kilometre grid, projected forward year by year. These are the starting point for background concentrations in almost every assessment in England, and they are explicitly background rather than roadside.

Download the raw data rather than reading a dashboard. You want data capture rates alongside the concentrations, because a monitor with poor capture in a given year produces an annual mean nobody should rely on.

Local authority diffusion tube data

Every English local authority has duties under the local air quality management regime to review and assess air quality in its area, and most publish an annual status report.

That report is the single most useful document for a site-specific assessment. It gives the location of every diffusion tube in the borough with a grid reference and a site classification, the annual mean for each tube over several years, the bias adjustment factor applied, the declared air quality management areas, and the authority's own commentary on trends.

Diffusion tubes are passive samplers exposed for a month at a time. They measure nitrogen dioxide only. They cost little enough to deploy in dozens, which is why the local network is far denser than the national one, and why the data is usually more relevant to a particular site.

Ask the environmental health team for the raw monthly results rather than taking annual means from the report. You will sometimes find a tube 40 metres from your site that never made it into the summary table.

Bias adjustment and annualisation

Raw tube results are not usable, and submitting them is the quickest way to have a report sent back.

Bias adjustment corrects for the systematic difference between tube results and co-located reference analysers, which varies by laboratory and preparation method. Authorities either derive a local factor from their own co-location study or use the national factor for the relevant laboratory and year. State which was used and give the number.

Annualisation corrects a short dataset to represent a full calendar year. Where a tube has fewer than the required number of valid monthly periods, the result has to be scaled using data from nearby long-running monitors. The method is set out in the national technical guidance, and it is not optional.

Both corrections move numbers meaningfully. A raw tube result sitting just under the annual mean objective can land above it once bias adjusted, and that changes the conclusion of the whole assessment.

Checking whether the site is in a management area

An air quality management area is declared where an authority finds that an objective is unlikely to be met. Declaration triggers an action plan, and development within or affecting one attracts closer scrutiny.

Check it in two places and expect them to disagree occasionally. The national service publishes an interactive map of declared areas with the declaration documents attached. The council's own website holds the order and the map, and the council's version governs. Boundaries are sometimes revised or revoked, so take the current order rather than a figure from an older report.

The objectives themselves sit in the Air Quality Standards Regulations 2010 and in the local air quality management regime. The annual mean objective for nitrogen dioxide is 40 micrograms per cubic metre, with a separate hourly objective, and the annual mean objective for PM10 is also 40 micrograms per cubic metre with a separate daily objective. Targets for fine particulate matter have been tightened under more recent legislation, so check the current target and date before citing one.

Background maps and their resolution

A one kilometre grid square covers an area roughly the size of a small neighbourhood, including the park, the dual carriageway and the cul-de-sac. The published background concentration for that square is an average across all of it.

Use it for what it is designed for: the background contribution at a site away from a dominant local source. Do not use it to describe concentrations at a facade three metres from a bus route, and do not interpolate between squares to invent precision that is not there.

Where a site sits close to a busy road, the roadside increment has to come from local monitoring or from dispersion modelling with traffic data. That is a different exercise with a different cost, and it is where most of an air quality assessment budget goes.

Projections are the other trap. Background maps are projected forward from a base year, and projected figures have historically tended to improve faster than measurements. Say which base year and which projection year you used.

Using the data in a screening assessment

Industry guidance on planning and air quality sets out screening criteria, and most schemes are resolved at that stage without modelling.

A screening assessment asks whether the development introduces exposure into an area where objectives are exceeded or close to being exceeded, and whether the traffic generated changes concentrations at existing receptors by enough to matter. The outputs that follow are usually construction dust mitigation, a commitment to low emission plant and, where relevant, electric vehicle charging provision.

Construction dust is assessed separately and is risk-based rather than numerical, classifying the site by scale and by the sensitivity of nearby receptors and producing a mitigation schedule. On most small commercial schemes this is the whole of the air quality work.

Where strategic noise mapping is also relevant to the layout, the two datasets are often gathered in the same early appraisal, as set out in checking strategic noise maps before layout.

What the environmental health officer asks for

Five things, consistently.

The baseline, with the monitoring sites named, classified and located relative to the site, and the corrections stated. The receptors, both existing ones around the site and the new ones the development creates. The construction phase dust risk assessment and the mitigation that follows from it. The operational assessment, proportionate to the scheme. And the mitigation written in terms a condition can be drafted around.

Scale the work honestly. A single hot food unit does not need a dispersion model. A scheme adding an entrance onto a street inside a management area does. The structure belongs in an air quality assessment, and where plant and extraction are involved the noise case runs alongside it in a noise impact assessment.

Enter your postcode on PlanScope to see whether your site needs an air quality assessment and what goes with it.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect an air quality assessment from a suitably qualified author, and professional review is strongly advised.

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