The following conclusions may be drawn from this field intercomparison exercise:

30 of the 33 laboratories (91%) performed to the data quality objectives set out in the proposed EU Daughter Directive for NO₂, that is, bias within ± 25%.

26 laboratories (79%) displayed an average measurement precision within our arbitrary guideline of £ 3 ppb.

The range in bias of measurement data, relative to the average of all the diffusion tube measurements in the study, was -30.7% to +24.4%. This represents an improvement relative to the 1998 intercomparison in which the range in bias was -39% to +58%.

The average bias associated with the national monthly and annual average measurement data reported by the UK NO₂ Network is estimated at - 6.8% relative to the chemiluminescent technique. This is a larger negative bias than that obtained in the 1998 intercomparison and may reflect the reduced occurrence of large positive bias, while large negative bias continues to be observed.

The average precision associated with the measurements in this study was 2.1 ppb.

Local authorities are recommended to ensure that the performance for their analytical laboratory is within approximately ± 25% (for bias) and 3ppb (for precision). It is also recommended that additional information on performance testing and validation exercises be utilised in assessing laboratory performance where this is available. Full details on the performance of individual laboratories are available direct from the laboratory.

Further evidence of the effect of diffusion tube preparation technique upon the performance of diffusion tubes was identified. Tubes prepared using the TEA in acetone method typically exhibited a positive bias, while tubes prepared using the TEA in water method typically exhibited a negative bias.

Chapter 6         Chapter 8