Nitrogen dioxide diffusion tube surveys for planning

Nitrogen dioxide diffusion tubes are the lowest cost reliable way to establish what the air at a site is actually doing, and the most common reason a diffusion tube survey fails at a planning application is that it ran for six weeks. The annual mean objective for nitrogen dioxide is 40 micrograms per cubic metre, and comparing a six week average against an annual objective is not a comparison at all. Everything else in a diffusion tube survey for planning follows from that: duration, bias adjustment, annualisation and honest presentation of uncertainty.

Tubes are passive samplers. They cost little, need no power and can be sited almost anywhere, which is why they remain the workhorse of local air quality management.

When monitoring is needed rather than modelling

Most planning applications with an air quality dimension are assessed by dispersion modelling rather than by monitoring, because a model can predict concentrations at receptors that do not exist yet. Monitoring earns its place in three situations.

Where the model needs verifying, which is the main one. A model's road contribution is adjusted by comparing predicted concentrations against measured ones at the same locations, and that comparison needs local measured data.

Where the council holds no representative data near the site. Authorities run their own diffusion tube networks and publish results in annual status reports, and using existing data is faster and usually more defensible than starting a new survey. Ask the environmental health team first.

And where the question is about existing exposure at a specific facade, for example a proposed residential use beside a busy junction, and the council wants measurement rather than prediction before it accepts the scheme.

Start the survey early. A survey that needs a year and a bias adjustment cannot be compressed, so the decision to monitor belongs at pre-application, not after a holding objection arrives.

Siting tubes at relevant receptors

Siting decides whether the data means anything. Four rules cover most sites.

Place the tube where relevant public exposure occurs, which for the annual mean objective means a facade where people are likely to be present over the averaging period, such as a dwelling, a school or a hospital. A kerbside tube on a lamppost measures the kerb, not the bedroom.

Mount at roughly 2 to 2.5 metres above ground, clear of the ground level turbulence and out of casual reach. Keep the tube away from the corner of a building and out of any enclosed or sheltered pocket, because restricted air flow biases the result low.

Co-locate at least three tubes at any site where a local bias adjustment factor is being derived, alongside a reference analyser.

And include a background location away from the road, because the assessment needs to separate the road contribution from the regional background.

Photograph and grid reference every tube location, and record the mounting height. An air quality report without tube location photographs invites a request for them.

Survey duration and annualisation

Tubes are exposed for roughly monthly periods and changed on a published national calendar, which keeps results comparable between sites and between authorities.

A full calendar year of monitoring is the ideal, because the annual mean objective is an annual mean and nitrogen dioxide varies strongly by season. Where a shorter survey is unavoidable, Defra's local air quality management technical guidance sets out an annualisation method using long-running reference monitors in the region, scaling the short period result to an estimated annual mean. The method is accepted, it is less reliable the shorter the survey, and the report has to say so.

Two further points on data quality. Aim for at least 75 per cent data capture across the period, counting only valid exposures. And record every lost, damaged or contaminated tube with the reason, because an unexplained gap looks like selective reporting.

Bias adjustment and laboratory performance

Diffusion tubes are known to over-read or under-read compared with reference chemiluminescence analysers, depending on the laboratory and the preparation method. The raw result is therefore adjusted by a bias adjustment factor before it is compared with anything.

State in the report which factor was used, its value, its source and the period it relates to. Also state the laboratory, its accreditation and its performance in the relevant proficiency testing scheme. An assessment that reports adjusted concentrations without naming the factor cannot be checked, and a reviewing officer who cannot check a number tends to discount it.

Comparing the results to the objectives

The annual mean objective for nitrogen dioxide is 40 micrograms per cubic metre. There is also a one hour mean objective of 200 micrograms per cubic metre, not to be exceeded more than 18 times a year, which diffusion tubes cannot measure because they produce an average over weeks.

Where the annual mean at a facade exceeds roughly 60 micrograms per cubic metre, national guidance indicates that exceedences of the one hour objective become likely, so a result in that range prompts a different conversation. Below it, the annual mean is the operative test.

Report the adjusted annual mean at each location against the objective, state whether the site lies within an Air Quality Management Area, and compare against the authority's own published monitoring where it exists. Where results sit close to the objective, say so plainly and give the uncertainty rather than rounding towards the answer you want.

Using the data to verify a model

Model verification is the reason most planning surveys exist. The process is straightforward and the reporting is where people cut corners.

1. Model the monitored locations for the same year as the survey, using the same traffic data as the assessment.
2. Separate the modelled road contribution from the background at each location.
3. Compare the modelled road contribution with the measured road contribution, derived by subtracting background from the adjusted measured total.
4. Derive an adjustment factor from the comparison, using all suitable monitoring sites rather than the convenient ones.
5. Apply the factor to the modelled road contribution at every assessed receptor.
6. Report the verification in full, including the sites used, those excluded and why, and the residual difference after adjustment.

Traffic inputs matter as much as the tubes. A verification built on traffic counts taken in an unrepresentative month will be wrong in a way no amount of monitoring fixes, which is why the count data behind a Transport Assessment and the air quality work should come from the same agreed survey.

Presenting the results in the assessment

Put the monitoring into the Air Quality Assessment as a standalone section with a plan, a table and a short uncertainty statement. The plan shows tube locations, receptors and the road network. The table shows, for each tube, the grid reference, the mounting height, the number of valid periods, the raw mean, the bias adjustment factor, the adjusted mean and, where relevant, the annualised figure.

Then write two paragraphs that most reports omit. One stating the limitations honestly: tubes do not measure short term peaks, the adjustment carries uncertainty, and a partial year survey has been annualised. One stating what the results mean for the application, in terms the planning officer needs, being whether existing exposure at the site is acceptable and whether the development makes measured concentrations worse.

Where the scheme also raises deposition or agricultural emissions rather than traffic, the methods differ, as ammonia deposition from livestock near designated sites sets out.

Enter your postcode on PlanScope to see which environmental reports a site is likely to need, then commission the monitoring early enough for a full year of data.

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