Air Quality Assessments – Nitrogen Dioxide

Why are Air Quality Assessments necessary ?

Planning applications often require Roadside Air Quality assessments for nitrogen dioxide. They are most likely when there is a large amount of traffic passing a site and there is a high chance of congestion at peak times. If the planning is for residential developments, the concern is that the development will introduce people into areas of poor air quality. For this reason, the air quality assessment determines whether the air quality at the facades of the proposed residential buildings is likely to exceed the UK annual mean objective for NO2.

How do we carry out the Nitrogen Dioxide Air Quality Assessment ?

The sampling is conducted in accordance with the guidance published by the Department for Environment Food & Rural Affairs (DEFRA) ‘Local Air Quality Management Technical Guidance’, April 2016. Sampling uses Palmes-type diffusion tubes containing TEA absorbent. These absorb the nitrogen dioxide at a known flow rate.

Usually the sampling involves installing three sample tubes, on a lamp post (or similar) at the sampling point. We position them at a height of between 2 and 2.7m in such a way that air can circulate freely around them. Similarly, we also ensure that they are not in an area of high air turbulence.

Synergy normally put the tubes out on the dates that the DEFRA diffusion tube calendar specifies. They are then left them in place for a period of about a month. We remove them on the next date that the diffusion tube calendar specifies. We repeat this process over a number of months. A laboratory with UKAS accreditation then analyses the tubes. When we receive the results we adjust them to calculate an annual average.

Sources of interference can often affect the tubes and cause a bias (an under or over estimation). Therefore we correct the results by multiplying them by a bias correction factor.

What Happens after the Nitrogen Dioxide Air Quality Assessment? 

We provide you with a report that shows the calculated annual mean for Nitrogen Dioxide. This shows whether the sample point is likely to  exceed the UK Air Quality Objective for nitrogen Dioxide (NO2) of 40µgm-3. 

Further reading

Defra Website

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