Baseline monitoring in an Air Quality Assessment

Air quality baseline monitoring exists to answer one question before any dispersion model is run: what are concentrations at the places people are actually exposed, now, without the development. Get that wrong and every predicted concentration downstream is wrong by the same margin, because the model output is added to the baseline rather than replacing it. The baseline section is also the part of an Air Quality Assessment that a council's environmental health officer reads first.

Most weak assessments fail here, not in the modelling.

Why the baseline comes first

Dispersion models predict the process contribution from the scheme, meaning the increment attributable to its traffic, its plant or its construction. The predicted environmental concentration is that increment added to an existing background.

So the baseline does three jobs. It establishes whether the objectives are already being met at existing receptors, which decides whether the scheme is adding to a problem or creating one. It provides the data against which the model is verified, because an unverified model is an assertion. And it sets the context in which a small increase is judged, since the same increment is trivial in a rural village and material beside a road that already sits above the annual mean objective.

The annual mean objectives in England are 40 micrograms per cubic metre for nitrogen dioxide and for PM10, with short-term objectives alongside them, and a separate national target regime applies to fine particulate matter. Those are the numbers the baseline is measured against, and the current values should be checked against the published regulations rather than an older report.

Published background maps against local monitoring

Defra publishes modelled background concentration maps on a one kilometre grid, by pollutant and by year, with a base year that is periodically updated and projections forward. They are the starting point for almost every assessment in England.

They are also the source of the most common error in this section. The grid square value is an average across a square kilometre, so it sits below the true figure beside a busy road and above it in the quiet part of the same square. Where the site is within a few tens of metres of a heavily trafficked route, the background map alone cannot describe the baseline, and the assessment needs local monitoring.

Two further points on using the maps. Take the correct base year and the correct projection year for the assessment year, and say which you used. And check whether the council's own review and assessment reports adjust or comment on the mapped values for the area, because the environmental health team will be reading their own data alongside yours.

Diffusion tube surveys and what they can and cannot do

Passive diffusion tubes are the workhorse of local nitrogen dioxide monitoring: low cost to deploy, easy to site in triplicate, and capable of giving a usable annual mean where they are run for a full calendar year and corrected properly.

They also have real limits. They measure nitrogen dioxide only, so they tell you nothing about particulate matter. They give a monthly average rather than any short-term value, so the hourly objective has to be inferred through a published relationship rather than measured. They are affected by exposure period, laboratory and preparation method. And they are vulnerable to interference, vandalism and the enthusiastic council contractor who removes the pole.

Siting decides their value. Tubes belong at the height and position of relevant exposure, which for the annual mean objective means a façade where people live or work, with a small number placed at roadside for model verification and at least one at a background location. Record the exact coordinates, the height, the distance from the kerb and the start and end dates of every exposure period, because an assessment that cannot state those cannot be checked.

Bias adjustment and annualisation

Raw diffusion tube results are not the answer. Two corrections normally apply, and both belong in the report with their arithmetic shown.

Bias adjustment corrects for the systematic over- or under-reading of tubes against a co-located continuous analyser. The usual route is the national bias adjustment factor spreadsheet, selected by laboratory and preparation method for the relevant year. Where a local co-location study exists, and it is properly run, a local factor is preferred. State which you used and why, and give the factor to two decimal places.

Annualisation applies where monitoring has run for less than a full calendar year. The published procedure uses concurrent data from nearby long-running monitors to scale the short period to an annual equivalent, and it needs several reference sites rather than one. A survey of three summer months, annualised against a single urban background analyser, is weak evidence and should be described as such.

Distance correction is the third adjustment, used to relate a roadside measurement to a receptor set back from the kerb. Use the published calculator, state the road type and the distances, and do not extrapolate beyond the range the tool was built for.

Choosing receptor locations

Relevant exposure is the concept that decides where the objectives apply, and it turns on the averaging period.

Annual mean objectives apply where members of the public are likely to be regularly present over a period relevant to that average, which means residential façades, gardens in some circumstances, schools, hospitals and care homes. They do not apply at a pavement, a kerbside, a car park or the middle of a carriageway. The one hour objective applies where people might reasonably be expected to spend an hour or more, which brings in bus stations, shopping streets and outdoor seating.

Select receptors to represent the worst case rather than the average, and include existing receptors, proposed receptors within the development itself, and any sensitive receptor identified by the council. Proposed receptors matter, because a scheme that brings new residents to a location already above the objective raises a different question from one that increases concentrations elsewhere. A Transport Assessment usually supplies the traffic data behind the modelling, and receptor selection should be coordinated with it rather than done separately.

Air quality management areas and what changes inside one

Where a council finds that an objective is unlikely to be met, Part IV of the Environment Act 1995 requires it to declare an air quality management area and produce an action plan.

Inside a declared area, three things change for an applicant. The council's own evidence base becomes the reference point, and a report that ignores the latest annual status report will be sent back. The action plan's measures become material, so a scheme that frustrates a measure in it attracts an objection. And the threshold for what counts as a material increase is lower, because the receiving environment is already failing.

Declarations are made for specific pollutants and specific boundaries, and both matter. A site 40 metres outside the boundary of a nitrogen dioxide area is still in the same air, and the assessment should say so rather than treating the line on the map as a wall.

Reporting uncertainty honestly

Every baseline carries uncertainty, and the professional move is to quantify it rather than bury it.

State the data capture rate for each monitoring location and exclude locations that fall below the accepted threshold. Give the bias adjustment factor and its source. Say how many reference sites were used in any annualisation. Report the model verification outcome, including the adjustment factor applied and its magnitude, because a verification factor far from unity tells the reader something about the inputs. And where a judgement has been made, name it as a judgement.

Councils usually expect an author with relevant experience for this work, and professional review is strongly advised before submission. An Air Quality Assessment that shows its workings on the baseline is far harder to argue with than one that presents a conclusion, and the same discipline applies to the dust and construction phase material that sits alongside it in a Construction Management Plan. You can see how both fit with the rest of the evidence base.

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.