Air quality baseline monitoring and the twelve month year

An air quality baseline monitoring period of twelve months is the default because the objectives it is measured against are annual means. Nitrogen dioxide and PM10 both carry an annual mean objective of 40 micrograms per cubic metre, and an annual mean cannot be derived honestly from a summer campaign. Run three months and you have a seasonal snapshot that needs annualising before it can be compared with anything. Plan for a full calendar year, or plan to use the authority's own data, and work out which of the two you need before the programme is fixed.

Most schemes end up doing both.

Why an annual mean needs a full year

Roadside nitrogen dioxide swings with the seasons. Winter brings lower mixing heights, more cold starts and more domestic combustion, so concentrations run higher. Summer brings dispersion and holidays. A set of tubes exposed from May to July will understate the annual figure at most urban sites, and an assessment built on it will be challenged.

The national technical guidance for local air quality management deals with this directly. Where a monitoring site has less than nine months of valid data in a year, the results should be annualised using data from nearby continuous monitors with a full data set, and the adjustment has to be reported. Nine months is the practical floor, and twelve is the position with no argument in it.

Data capture matters as much as duration. A continuous analyser is normally expected to achieve at least 75 per cent valid data across the year, and a tube site needs most of its monthly periods to return a valid result. Lost tubes, vandalised brackets and laboratory failures eat into that, so over-provide sites rather than running the minimum.

Diffusion tubes, bias adjustment and co-location

Diffusion tubes measure nitrogen dioxide only. They are passive, low cost to deploy and systematically biased, which is why a bias adjustment factor has to be applied before the data means anything.

Two routes to that factor exist. The national bias adjustment spreadsheet, which draws on co-location studies across the country for each laboratory and tube preparation method, is the usual choice. A local co-location study, where triplicate tubes are exposed alongside a reference continuous analyser for a sufficient number of periods, gives a factor specific to your laboratory and conditions and is the stronger evidence where the margin is tight.

Record all of this in the report:

1. The laboratory, its accreditation and the preparation method used.
2. The exposure periods and the dates each tube was changed.
3. The bias adjustment factor applied, its source, and the year it relates to.
4. Any distance correction applied to relate a tube at the kerb to a receptor facade.
5. Any annualisation, the monitors used for it, and the resulting factor.

Mount tubes at the standard height, away from building faces and corners, and photograph every location with a note of the grid reference. The commonest criticism of a tube survey is not the chemistry. It is that nobody can tell exactly where the tube was.

Using the authority's published monitoring instead

Every authority with air quality duties publishes an annual status report setting out its monitoring network, measured concentrations, any air quality management areas and the progress of its action plan. That report is free, already quality assured, and often covers the roads your scheme affects.

Use it first. For many schemes, the authority's own tube and analyser data within a few hundred metres of the site provides a defensible baseline, and the assessment can be built on it with no new monitoring at all. Where the authority has declared an air quality management area, the declaration order and the supporting data tell you what the problem pollutant is and where.

Three checks before relying on it. Confirm the monitoring year, because a report published this year usually covers last year. Confirm the receptor type each site represents, since a kerbside analyser is not a facade concentration. And confirm whether the bias adjustment in the report has already been applied to the figures quoted.

Short campaigns and the caveats they carry

Sometimes there is no twelve month option, because the site was bought in March and the application has to go in by autumn. A three to six month campaign can still be useful, provided the report is honest about what it is.

State the period, apply annualisation using nearby long-running monitors, and give the adjusted figure with the unadjusted one beside it. Then say in terms what the uncertainty is. An assessment that reports a baseline of 36 micrograms per cubic metre annualised from four months of summer data, against an objective of 40, is not evidence that the objective is met. It is evidence that further work is needed, and an officer or a consultee will say so.

The honest fallback is a worst case assumption. Where the baseline is uncertain and the margin is small, assess against the higher plausible baseline and show the scheme works anyway. That is a stronger position than a precise-looking number nobody can defend. Survey timing of this kind is exactly why planning a survey budget across two financial years is worth doing at the outset.

Verification against continuous monitors

Where the assessment uses dispersion modelling, the model has to be verified against measured data before its predictions are used. The procedure is set out in the national guidance: compare modelled and monitored concentrations at the monitoring sites, derive an adjustment for the road contribution, apply it, and report the residual difference.

Verification needs monitoring sites in the same situation as the receptors being modelled. Verifying a street canyon model against an open roadside site produces an adjustment factor that means little. Where no suitable local site exists, say so and widen the uncertainty rather than quietly using a distant one.

Keep the traffic data and the monitoring year aligned. A model run on traffic counts from one year verified against monitoring from another introduces an error nobody can quantify afterwards.

Programming monitoring ahead of an application

Work backwards from the submission date. Twelve months of tubes, plus roughly a month at each end for installation and for the final laboratory results, plus time for bias adjustment, modelling, verification and reporting. That is fourteen to fifteen months before validation for a scheme that needs its own baseline.

Where that is impossible, the sequence that usually works is this. Use the authority's published data to establish the baseline for the application. Start your own monitoring at the same time. If the application is called in, appealed or amended, the longer record exists by then. Winter constraints affect other survey types in the same way, and the reasoning behind winter ground investigation is a useful comparison.

An illustrative programme for a mid-sized residential scheme in an air quality management area: order tubes in month one, expose from month two to month thirteen, obtain the authority's annual status report for the baseline in parallel, commission traffic counts in a neutral month avoiding August and December, model and verify in months fourteen and fifteen, and submit in month sixteen. Those durations are illustrative and will vary with the laboratory and the consultant.

Reporting the period and its limitations

The methodology section should let a reader reconstruct what you did. Monitoring type, locations with grid references and photographs, exposure or measurement dates, data capture achieved, bias adjustment and its source, annualisation if applied, distance correction if applied, and the objectives the results are compared against.

Then state the limitations in your own words rather than in a standard paragraph. If two tubes were lost, say which and how it was handled. If the baseline year was unusual, say why. An Air Quality Assessment is a report councils usually expect from a suitably qualified author, and professional review is strongly advised, because the bias adjustment and verification steps are where a weak report fails.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. To see which environmental reports a scheme in an air quality management area is likely to need, enter the site postcode on PlanScope.

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