Under the European Union Daughter Directive for NO₂,⁹ data quality objectives have been set out for the overall accuracy of indicative monitoring techniques (i.e. diffusive monitoring). In the case of diffusion tube monitoring, annual average NO₂ concentration data must comply with these data quality objectives to enable comparison of measurement data with the Daughter Directive for NO₂. For indicative monitoring the data quality objective has been set at ± 25%. Hence, it is recommended that on average, diffusion tube measurements should be within ± 25% of the reference concentration.

This objective has been used within the UK NO₂ Network since 1997 as the data quality objective for the screening of diffusion tube data with unsatisfactory quality. It is broadly consistent with indicators of good laboratory performance used within the WASP proficiency testing scheme for NO₂ diffusion tubes, and also the UK NO₂ Network Laboratory Performance Testing Scheme used from 1996-1999. Applying this criteria to the 1999 field intercomparison exercise, laboratories that are, on average, within approximately ± 25% of the reference value, may be recognised as performing satisfactorily. Conversely, laboratories with an average bias significantly greater than ± 25% will have performed unsatisfactorily.

It must be noted however, that the precision of analytical measurements should also be taken into account, as it is possible to achieve an average bias of less than ± 25% with very imprecise measurements (i.e. purely by chance). Last year, the average precision achieved by all laboratories in the 1998 field intercomparison was 2.5ppb. This value was recommended as a "target" for the precision of diffusion tube measurements. However, as around half of the laboratories will inevitably have precision values greater than the average, this was a rather stringent target. Therefore, in this 1999 study we have taken a figure of 3ppb as an arbitrary guideline for acceptable precision. The average precision achieved in the 1999 study was 2.1ppb.

Using these guidelines, three of the 33 laboratories taking part in this field intercomparison exercise (9%) produced measurement data with average bias >25%, relative to the reference measurement from the chemiluminescent analyser. (In all three cases the bias was negative). Seven laboratories (21%) showed an average precision greater than our arbitrary guideline of 3 ppb. No laboratories produced measurement data with an average bias >25% and an average precision >3ppb.

Under the AQS, local authorities are obliged to assess and review the air quality within their authority¹⁰. Diffusion tube surveys may be used as screening tools within these assessments, as the annual average data derived from NO₂ diffusion tubes may be directly comparable with the AQS air quality objective for the end of 2005 (21 ppb for annual mean NO₂).

In recognition of the evidence for a potential bias in diffusion tube measurement data, and in the absence of a methodology for estimating accuracy as defined by EC Directive 1999/30/EC, information on the bias (relative to the chemiluminescent technique) and precision of diffusion tube measurement data must be presented for the period of monitoring. Where appropriate at Stage 3, scaling factors may also be applied to the diffusion tube measurement data to correct for any systematic bias. Any use of scaling factors must be reported.

In addition to the data presented in this report, performance testing data from the UK NO₂ Network Laboratory Performance Testing Scheme 1996-1999 and the new WASP proficiency scheme are available from laboratories and may be used to provide a further information on the general performance of laboratories.

Chapter 4         Chapter 6