Why Use Diffusion Tubes
Diffusive samplers (as described in paragraphs 7.197 – 7.210 of the Technical Guidance LAQM.TG(22)) are widely used for indicative monitoring of ambient nitrogen dioxide (NO2) in the context of Review and Assessment.
Diffusion tubes are particularly useful:
- When simple, indicative techniques will suffice.
- To give an indication of longer-term average NO2 concentrations.
- For indicative comparison with the Air Quality Strategy Objectives based on the annual mean.
- For highlighting areas of high NO2 concentration (particularly when dealing with sources such as traffic emissions, which do not change much from day to day).
- Where installation of an automatic analyser is not feasible.
Interpretation of the Data
Diffusion tubes take samples over an approximate 1 month period. As such they are useful for assessing the annual objective of 40 µg/m3, but cannot be used to assess the number of hours greater than 200 µg/m3. As they are not the reference method, and passive diffusion typically results in a low accuracy, it is necessary to bias correct the results based upon local or national co-location studies with chemiluminescent analysers.
Local authorities using NO2 diffusion tubes in their Review and Assessment are required to quantify the “bias” of their diffusion tube measurements and apply an appropriate bias adjustment factor to the annual mean if required. Local authorities can either:
- Carry out their own co-location study (in which the accuracy of the diffusion tubes is quantified by exposure alongside an automatic chemiluminescence analyser) and use the results to calculate a local bias adjustment factor. For more information please see: Locally-derived bias adjustment factors.
- Use a combined bias adjustment factor, based on the result of many co-location studies (using the same laboratory and tube preparation method). For further information on combined bias adjustment factors, please see: National bias adjustment factors.
The decision of whether to use the results of a local co-location study or the average of as many co-location studies as possible (for a particular laboratory and tube preparation method) depends on a number of factors. Please refer to Box 7-13 of the Technical Guidance LAQM.TG(22) for more details.
It is also necessary to calculate the data capture, and if this is less than 75%, the results should be annualised. The Diffusion Tube Data Processing Tool can be used to calculate both the locally-derived bias adjustment factor and carry out annualisation to derive an annual mean concentration.
Diffusion tube monitoring in the context of Review and Assessment is designed to align with specific calendar dates. Recommended exposure dates for diffusion tubes are provided within the Diffusion Tube Monitoring Calendar.
Please also see the the Practical Guidance page for further information.