Influence of wind-speed on short duration NO₂ measurements using Palmes and Ogawa passive diffusion samplers

We assessed the precision and accuracy of nitrogen dioxide (NO₂) concentrations over 2-day, 3-day and 7-day exposure periods measured with the following types of passive diffusion samplers: standard (open) Palmes tubes; standard Ogawa samplers with commercially-prepared Ogawa absorbent pads (Ogawa[S]); and modified Ogawa samplers with absorbent-impregnated stainless steel meshes normally used in Palmes tubes (Ogawa[P]). We deployed these passive samplers close to the inlet of a chemiluminescence NO₂ analyser at an urban background site in Glasgow, UK over 32 discrete measurement periods. Duplicate relative standard deviation was < 7% for all passive samplers. The Ogawa[P], Ogawa[S] and Palmes samplers explained 93%, 87% and 58% of temporal variation in analyser concentrations respectively. Uptake rates for Palmes and Ogawa[S] samplers were positively and linearly associated with wind-speed (P < 0.01 and P < 0.05 respectively). Computation of adjusted uptake rates using average wind-speed observed during each sampling period increased the variation in analyser concentrations explained by Palmes and Ogawa[S] estimates to 90% and 92% respectively, suggesting that measurements can be corrected for shortening of diffusion path lengths due to wind-speed to improve the accuracy of estimates of short-term NO₂ exposure. Monitoring situations where it is difficult to reliably estimate wind-speed variations, e.g. across multiple sites with different unknown exposures to local winds, and personal exposure monitoring, are likely to benefit from protection of these sampling devices from the effects of wind, for example by use of a mesh or membrane across the open end. The uptake rate of Ogawa[P] samplers was not associated with wind-speed resulting in a high correlation between estimated concentrations and observed analyser concentrations. The use of Palmes meshes in Ogawa[P] samplers reduced the cost of sampler preparation and removed uncertainty associated with the unknown manufacturing process for the commercially-prepared collection pads.