A detailed investigation into the role of bias adjustment for nitrogen dioxide diffusion tubes has been carried out for the Scottish Government. The project was led by Prof. Duncan Laxen on behalf of Air Quality Consultants Ltd, with support from Prof. Mat Heal of the School of Chemistry at the University of Edinburgh.
Diffusion tubes are widely used by local authorities to monitor nitrogen dioxide concentrations. The results, however, have to be ‘bias adjusted’ to make them as accurate as possible. This is because tubes can over- or under-read, when compared to co-located chemiluminescence ‘reference’ monitors, due to a wide range of factors, including the laboratories supplying and analysing the tubes, and exposure factors that influence the uptake of nitrogen dioxide in the tube. A detailed review of the literature was provided as part of the project.
Concerns had been raised that some bias adjustment factors had been falling year on year, resulting in a reported improvement in air quality. The Scottish Government wanted to know if these improvements were real or not, or an artefact of the bias adjustment approach. To address this, the study examined diffusion tube and automatic monitoring data for all co-location sites in Scotland over the period 2008 to 2017.
The study demonstrated significant and systematic between-site differences in overall bias, even for the same types of tube. A number of possible explanations for this were identified. Between-lab differences were also identified. The downward trend in the overall bias adjustment factors identified by the Scottish Government appears to be mainly an artefact of the changes in the sites that have been used to derive the overall factor, together with some chemistry-driven downward trends due to the declining NO2 and NOx concentrations.
A number of recommendations are made for Scottish local authorities to improve the consistency of diffusion tube results.
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