How do I bias adjust my diffusion tube data if I change supplier and/or preparation method during the course of a year?
In situations where the supplier and/or preparation method for NO2 diffusion tubes has been changed during a calendar year, the process of bias adjustment is more complex than that detailed within Chapter 7 of Technical Guidance LAQM.TG(22). A single bias adjustment factor, applicable to one laboratory in terms of using a National Factor, or applicable to a single Local Factor derived from a co-location study completed for the calendar year of monitoring.
In this scenario, a single bias adjustment factor, either locally or nationally derived, is no longer applicable. It is instead best practice to apply two separate bias factors across the relevant periods of exposure for each laboratory/preparation method, i.e. if Lab X has been used for four months of the year, apply a Lab X bias factor to those four months, and if Lab Y has been used for the next eight months of the year, use a Lab Y bias factor for those eight months. A weighted average relative to the two exposure periods should then be calculated to determine the final annual mean NO2 concentration.
Following the above example, assuming data capture is 100% for the calendar year, the calculation would be:
Annual mean NO2 concentration = (4-month period average * 4-month bias factor * 4/12) + (8-month period average * 8-month bias factor * 8/12)
Please Note: If the data capture is less than 100% for the year the weighted average should be amended appropriately, i.e. if the data capture is 75% the weighted division would be 9 instead of 12.
In terms of whether a National Factor or Local Factor is used within the calculation, this is dependent on whether a co-location study has been completed by the Local Authority area and the considerations as detailed in Box 7-13 of Technical Guidance LAQM.TG(22) taken into account:
- Where there is not a current co-location study, then the bias adjustment factors for Lab X and Lab Y should be ascertained from the current Diffusion Tube Bias Adjustment Spreadsheet.
- Where a co-location study has been completed, two separate bias calculations should be made using the Local Bias Adjustment Factor Spreadsheet to define Local Factors for Lab X and Lab Y based upon the monitoring results from the two sets of diffusion tubes from the two separate suppliers. The two derived Local Factors should only be used if the resulting tube precision and data capture meet the requirements as per paragraph 7.219 of Technical Guidance LAQM.TG(22). If these conditions are not met, than National Factors should be used within the calculations.
If data capture for the calendar year is less than 75% at any of monitoring locations, annualisation should then be additionally completed in line with Chapter 7 of Technical Guidance LAQM.TG(22).
Worked Example
During 2018, a Local Authority used Supplier A diffusion tubes for January to March inclusive, then changed to diffusion tubes prepared by Supplier B for the remainder of the year (April to December inclusive). The mean concentration for the 3-month period was 30µg/m3, whilst a mean concentration of 25µg/m3 was obtained for the 9-month period. In line with the national bias adjustment database, bias factors of 0.83 and 0.92 were derived for Supplier A and Supplier B respectively for 2018.
The annual mean NO2 concentration at this site is therefore calculated as follows:
Annual mean NO2 concentration = (30µg/m3 * 0.83 * 3/12) + (25µg/m3 * 0.92 * 9/12)
= 23.5µg/m3
