The following recommendations are proposed to improve the quality data within the UK NO₂ Network and for further work to better define potential uncertainties:

Laboratories displaying a negative bias are strongly encouraged to review their procedures for preparing diffusion tubes and extracting nitrite after exposure; the most likely cause of under-read is incomplete extraction of absorbed NO₂ into the analysed sample and/or insufficient loading of the TEA absorbent onto the diffusion tube.

The extraction efficiency of a laboratory may be effectively tested and monitored on a monthly basis using the WASP programme for NO₂ diffusion tubes. As low extraction efficiency is likely to be the largest individual source of negative bias in diffusion tube measurements, it is strongly recommended that checks are made to ensure that laboratory performance in the WASP scheme is within ±25% of the target concentration.

Laboratory based tests investigating the effect of diffusion tube preparation technique should be performed, with a view to standardisation of the preparation techniques used within the UK NO₂ Network.

 

Chapter 7         Chapter 9

---

Report and site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions