Diffusion tubes against continuous analysers

Diffusion tube air quality monitoring gives you a monthly mean concentration of nitrogen dioxide at a point, adjusted for a known bias, and nothing else. A continuous analyser gives you hourly data against the reference method, which means short-term metrics, diurnal patterns and a defensible basis for verifying a dispersion model. Choosing between them is rarely a choice at all: the question is whether tubes are adequate for the assessment you have to write, and on most small and medium schemes they are.

What each method actually measures

A passive tube is a small acrylic cylinder with a coated grid at the closed end. Nitrogen dioxide diffuses up the tube and is absorbed, and the mass collected over the exposure period is converted to a mean concentration. There is no pump, no power supply and no housing, so tubes can be deployed in tens across a site and along a receptor facade for the cost of a single instrument. Exposure is normally over a calendar month, changed on a published schedule.

A chemiluminescent analyser draws air continuously, reacts nitric oxide with ozone and measures the light emitted. It is the reference method for nitrogen dioxide in the European standard, it reports at least hourly, and it needs mains power, a secure enclosure, regular calibration with certified gas and an operator who knows what a drifting zero looks like.

Bias adjustment and why the factor's provenance matters

Tubes read differently from the reference method, and the direction and size of the difference depends on the preparation laboratory, the coating method and the exposure conditions. A bias adjustment factor corrects for it. Defra publishes a national factor derived from pooled co-location studies, and a local factor can be derived where the authority runs tubes in triplicate alongside its own automatic monitor.

Use the local factor where one exists for the same laboratory and the same period. Say which you used, name the laboratory, give the factor and its year, and state whether it came from a national spreadsheet or a local co-location. An assessment that reports an adjusted annual mean without naming the factor is asking to be sent back.

Two further corrections come up. Where a tube has not run for a full calendar year, or data capture falls below the accepted threshold, the result is annualised using ratios from nearby continuous monitors. And a period-mean from a short campaign should never be presented as an annual mean without that step.

Annual means, short-term objectives and what tubes cannot reach

The annual mean objective for nitrogen dioxide is 40 µg/m³, and the one hour objective is 200 µg/m³ not to be exceeded more than eighteen times in a year. Tubes can address the first and cannot address the second, because a monthly mean carries no information about a single hour.

The working answer from the Defra local air quality management technical guidance is that exceedance of the one hour objective is unlikely where the annual mean is below 60 µg/m³. That threshold does a lot of work in practice. Where your adjusted annual mean at the worst facade sits comfortably below it, tubes have done the job and the short-term objective can be screened out with a cited justification. Where it does not, you need hourly data.

Particulates are the harder gap. Tubes do not measure PM10 or PM2.5 at all, so any scheme where particulate exposure is the live issue needs either local automatic data, a modelled assessment verified against it, or both. Construction dust is a separate exercise again, assessed qualitatively under the Institute of Air Quality Management approach and controlled through a Construction Management Plan rather than measured beforehand.

Correcting a roadside measurement to the facade

A tube on a lamp post at the kerb is not measuring what the people in the flats behind it breathe. Concentrations fall away from the carriageway, steeply at first, and the correction from the monitoring location to the relevant receptor facade uses the Defra fall-off with distance calculator with the local background concentration as an input.

Get the geometry right before you get the arithmetic right. Measure from the kerb to the tube and from the kerb to the facade, on a scaled survey rather than from an aerial image, and record the heights. A tube at 2.5 metres on a post beside a two storey terrace is not representative of a third floor window on a street canyon, and correcting it as though it were produces a number nobody should rely on.

When tube data is enough for an application

Tube data alone will usually carry an assessment where the scheme is not large enough to change traffic flows materially, where the nearest existing monitoring sits close to the site, where the annual mean is the operative metric, and where the conclusion is comfortable rather than marginal. That covers most residential infill, most change of use, and most commercial schemes below the thresholds that trigger a Transport Assessment.

It will not carry an assessment where the outcome turns on a few µg/m³ at a single facade, where the site is in or adjoining an air quality management area and the authority has declared for the one hour objective, where particulates matter, or where you are verifying a dispersion model and need a verification factor you can defend. Model verification derived from a single tube is weak. Derived from several monitors of mixed type across a range of concentrations, it is arguable.

Using the authority's own network rather than your own

Before commissioning anything, read the authority's most recent annual status report. It lists every tube and analyser, the adjusted results for several years, the bias factor used, the data capture, and the reasons behind any air quality management area declaration. That gives you a trend rather than a snapshot, prepared by the body that will be consulted on your application.

Running your own tubes for three months and presenting the period mean as a baseline, when the council has six years of adjusted data two streets away, is the mistake that invites an objection from environmental health. Use their data as the backbone and add your own tubes where the gap is spatial, such as a facade the network does not reach. The parallel with acoustics is exact, and the reasoning behind how long a background noise survey should run applies here too.

Presenting the baseline with its limitations

State the exposure periods and the changeover dates. State data capture as a percentage and say whether annualisation was applied. Name the laboratory and its accreditation, the bias factor, its source and its year. Give the raw and adjusted values separately. Give the distance correction inputs, including the background concentration and its source year. Then state the uncertainty and what it means for the conclusion.

An air quality baseline written that way survives scrutiny even when the numbers are close to the objective, because the reader can see exactly what was done. A full Air Quality Assessment should carry all of it, and a suitably qualified air quality practitioner should prepare or review the work where the conclusion is marginal.

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. For specialist technical reports, qualified professional input may be required.