Nitrogen dioxide sampler study

Conventional Palmes diffusion tubes (PDTs) are widely employed in the UK by local authorities and others for making indicative measurements of nitrogen dioxide (NO₂). However, certain factors in the current design can introduce undesirable biases in these measurements, including from wind turbulence at the open end of the PDT, which can lead to an overestimation of NO₂ concentrations. 

To minimise the measurement problems due to wind, we are implementing simple low-cost changes to the conventional PDT design in collaboration with the two leading manufacturers of PDTs in the UK. The research involved validation studies with our controlled atmosphere test facility (CATFAC) to determine the NO₂ diffusive sampling rates for several modified PDT designs (Figure 1 (left)), together with NO₂ field measurements in central London (Figure 2 (right)).

One design, developed with Gradko, uses an amorphous polyethylene filter at the open end. This has recently been implemented in Defra’s new UK Urban NO₂ Network (UUNN) at over 200 roadside sites and is delivering improved repeatability and reduced measurement uncertainty for this pollutant and could also be considered for future introduction at sites covered by the Local Air Quality Management (LAQM).

CEN TC264 WG11- Ambient Air Quality – Diffusive Samplers for the Determination of Gases and Vapours – Requirements and Test Methods, which has had significant input from NPL, is currently revising BS EN 16339:2013 to consider recent advances in sensor designs, and whether measurements with PDTs can demonstrate equivalence with the chemiluminescence reference method.