Accurate nitrogen dixide (NO2) monitoring is necessary for detailed assessments such as an Updating and Screening Assessment or where an Air Quality Management Area has been declared. Continuous monitoring of NOx should be carried out using a chemiluminescent NOx analyser as defined by the EU as the reference method for NO2.
Due to the expense of automatic analysers local authorities use low cost NO2 diffusion tubes to expand their monitoring network. However, a number of environmental factors can affect diffusion tube performance resulting in concentrations being greatly under or overestimated, this inaccuracy is often referred to as ‘bias’.
To investigate diffusion tube accuracy ‘bias’ a co-location study is undertaken. The study is carried out by exposing diffusion tubes in triplicate alongside an automatic chemiluminescence analyser, and comparing the results of the two techniques. The comparison produces a ‘bias adjustment factor’ which is then applied to annual mean concentrations to correct diffusion tube measurements. There are two types of bias adjustment factor:
If local authorities have their own co-location studies they may calculate their own bias adjustment factor. A specially designed spreadsheet is used to calculate the precision and accuracy (bias) of individual co-location studies and gives 95% confidence intervals in the bias adjustment calculations.
A combined bias adjustment factor, based on the result of many co-location studies which local authorities can use to correct their diffusion tube measurements. The national database is updated throughout the year the next version of the National Bias Adjustment Spreadsheet is due in March 2016.
More information and bias adjustment spreadsheets are available on the Defra website.
The precision and accuracy of the laboratories which supply and analyse the diffusion tubes are also assessed. Laboratories take part in an independent Workplace Analysis Scheme for Proficiency (WASP) to assess their performance, the results of which are available on the Defra QA QC Framework page.
A useful overview of the topics discussed here is provided in the following document Diffusion Tubes for Ambient NO2 Monitoring: Practical Guidance for Laboratories and Users.