Comparison of NO2 diffusion tube measurement methods and related uncertainties

Conventional Palmes Diffusion Tubes (PDTs) are extensively employed by UK Local Authorities for measuring NO₂ in air quality monitoring studies. These devices are known to suffer from biases due to from the effects of wind speed. Modified PDTs with wind protective filters have also been developed for deployment in the UK Urban NO₂ Network (UUNN) with an improved measurement accuracy and repeatability. We report the performance of the two designs and also when enclosed in additional shelters. The comparison was carried out against simultaneous reference measurements and was evaluated through a statistical and modelled uncertainty. The model incorporated the individual components of the measurement uncertainty to provide an estimate of the total measurement uncertainty and identified which elements could be reduced across mean values of multiple measurements.

We found that conventional PDTs could be adversely affected by wind speed and that the incorporation of shelters delivered improved repeatability and better accuracy across multiple diffusion tube measurements. The UUNN style diffusive samplers were more accurate than the PDTs and had better repeatability. The additional use of shelters with UUNN style samplers made no discernible difference to the measurements.