Question: Single vs Triplicate Monitoring. How many passive diffusion tubes should be used at each monitoring location?
Local authorities commonly deploy a single passive diffusion tube to monitor nitrogen dioxide (NO2) for LAQM purposes. Diffusion tubes are more frequently deployed in triplicate when co-located with a continuous monitor.
Whilst the deployment of single diffusion tubes has the advantage of increasing spatial coverage in a cost-effective manner, utilising a single diffusion tube can have limitations when reviewing and comparing concentration data.
The principal benefit of deploying tubes in triplicate at a single location is that the variance[1] between each of the recorded tube concentrations can be calculated for each period of deployment (usually 4-5 weeks). This allows outlier concentrations to be more easily identified, which is not possible when only a single tube is deployed. Should the variance between the triplicate set of diffusion tube concentrations be calculated as low, this demonstrates that there is little concentration variability between the tube readings, improving confidence of the concentration data obtained when using passive diffusion tubes (as stated in Paragraph 7.204 in Technical Guidance LAQM.TG(22)).
A further benefit of deploying tubes in triplicate is increased mitigation against data loss. There is the inherent possibility of sample contamination at the stages of preparation, handling, storage, deployment and analysis of diffusion tubes. For example, once deployed, it is possible for diffusion tubes to become contaminated with insects or water droplets and the results may therefore be flagged by the laboratory as potentially compromised. This, and other such issues, can result in missing data for periods of deployment during the year. As such, the use of triplicates can help limit this.
Local authorities may wish to consider fewer monitoring positions with single diffusion tubes, reducing the number of locations where low concentrations have historically been recorded, and install more diffusion tubes in triplicate. This may be considered particularly in areas of concern, or where annual mean concentrations are closer to the air quality objective, so as to improve confidence in the data collected. This should be balanced against the benefits of long-term data trends obtained at existing sites.
Should local authorities require further assistance in identifying appropriate triplicate diffusion tube locations, or concerns regarding diffusion tube data, then please contact the LAQM Helpdesk.
[1] Paragraph 7.219 in LAQM.TG(22) explains the calculation of diffusion tube precision within the Diffusion Tube Data Processing Tool (DTDPT). Good diffusion tube precision applies where the coefficient of variation (CV) of triplicate diffusion tubes for eight or more periods during the year is less than 20%, and the average CV of all monitoring periods is less than 10%. Poor precision applies where the CV of four or more periods is greater than 20% and/or the average CV is greater than 10%.
