Why are Air Quality Assessments for Nitrogen Dioxide (NO2) necessary?
Planning applications often require roadside air quality assessments for NO2. They are most likely when there is a large amount of traffic passing a site or where there is a high chance of congestion at peak times. If the plan is for a residential property, then the concern is that it will introduce people into an area of poor air quality. For this reason, the air quality assessment works out if the air quality at the facades of the proposed homes is likely to be above the UK annual mean objective for NO2.
How do we carry out the test?
Our air quality assessments follow the guidance in the Department for Environment Food & Rural Affairs (DEFRA) ‘Local Air Quality Management Technical Guidance’. Sampling uses Palmes-type diffusion tubes containing a layer of TEA absorbent. These pick up the NO2 in the air at a known flow rate.
Usually, the test needs three sample tubes per test point. We fix these tubes to lamp posts or lights nearby. There may be more test points per site. We position them at a height of between 2 and 2.7m in such a way that the air can circulate freely around them. In the same way, we also ensure that they are not in an area of high air turbulence.
We normally put the tubes out on the dates that the DEFRA diffusion tube calendar specifies and they are then left in place for a period of about a month. Then 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 for the site.
Sources of interference can often affect the tubes and cause a bias (an under or overestimation). Therefore we correct the results by multiplying them by a bias correction factor.
What happens after the assessment?
After the air quality assessments, we provide you with a report that shows the annual mean result for NO2. This shows whether the sample point is likely to exceed the UK Air Quality Objective for NO2 of 40µgm-3.
Further Information is available below
UK Government – how planning can take account of the impact of new development on air quality
Nitrogen Dioxide in the United Kingdom
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