Measurement of nitrogen dioxide diffusive sampling rates for Palmes diffusion tubes using a controlled atmosphere test facility (CATFAC)

- Palmes NO2 diffusion tubes were modified and studied under controlled conditions.
- Modifications consisted of meshes and filters secured over the tube sampling end.
- The 28-day sampling rate for tubes was determined for each design tested.
- The determined sampling rates were applied to tubes deployed in the field.
- Certain modifications improved the repeatability of tube measurements in the field.

We report measurements of the 28 day NO2 diffusive sampling rates for seven designs of Palmes diffusion tubes (PDTs), which were exposed in a controlled atmosphere test facility (CATFAC) containing traceable concentrations of nitrogen dioxide, nitric oxide and water vapour under defined conditions of temperature (20 °C) and wind speed. One of the aims of the work was to implement low cost modifications to the conventional open tube PDT design, using either meshes or filters. This would potentially reduce some of the undesirable bias effects due to wind, which may lead to an over estimation of the NO2 concentration. Exposure tests in the CATFAC were carried out over a wide concentration range applicable to ambient monitoring, and also over a range of wind speeds at a constant concentration. For a given PDT design, the measured NO2 diffusive sampling rates were found to be effectively constant over the conditions tested. These rates were then applied to NO2 field measurements carried out at a monitoring station in central London, and three of the modified PDT designs were found to deliver improved repeatability and consequently reduced measurement uncertainty over the conventional open tubes.