Bias adjustment of nitrogen dioxide diffusion tubes

It is widely acknowledged that diffusion tubes have a tendency to over or under read, sometimes by a significant amount. Inaccuracies in the region of ± 20-30% are not uncommon in diffusion tubes. In addition to this, inaccuracies can vary between laboratories that analyse diffusion tubes and even between different individual technicians in the same laboratory. Therefore, to produce accurate data a ‘bias adjustment’ of the raw data is often required.

The official guidance states that a bias adjustment figure should be based on the collocation of triplicate tubes with a nitrous oxide (NOx) continuous analysers and then compare the results of the two methods. In Richmond our bias adjustment site is the roadside site located at Castelnau Library, Barnes.

Diffusion tube bias adjustment calculations

The following results show how the bias adjustment was calculated for the years 2002 - 2005.

• 2002(pdf, 17KB)
• 2003(pdf, 17KB)
• 2004(pdf, 14KB)
• 2005(pdf, 17KB)
• 2006(pdf, 13KB)