Abstract

Two studies at three sites in the UK provided confirmation that systematic positive bias in NO₂ diffusion tube measurement occurred because of changes to “within-tube” chemistry, rather than eddy diffusion at the mouth of the tube. In the first study in Cambridge, UK, sampler overestimation for 1 and 2 week exposures was compared to corresponding time-averaged monitor measurements (NO–NO₂–NO_x, O₃) and weather variables. Nonlinearity between sampler and monitor NO₂ measurements was interpreted in terms of spatial and temporal variations in relative and absolute availability of NO, NO₂ and O₃ at the site. A maximum overestimation occurred for an exposure mean NO₂/NO_x ∼ 0.5. The separate contributions of reduced NO₂ photolysis and eddy diffusion were compared in Study II using samplers of two materials, acrylic and quartz, and of different lengths (40, 55, 71 and 120 mm) at three sites: Norwich background, Cambridge intermediate, London kerbside. For compared sites, NO₂ measured by acrylic samplers was significantly higher than for equivalent quartz samplers. For quartz samplers [NO₂]_mean was only just above the monitor at Norwich and London; sampler/monitor NO₂ = 1.04 (P = 0.59) and 1.01(P = 0.76), respectively. For acrylic samplers the order of [NO₂]_mean was 40 mm > 120 mm > 71 mm ≥ 55 mm. Excepting 40 mm samplers, this accords with a chemical bias where co-diffusing NO and O₃ molecules in longer tubes have more time to react to form excess NO₂. Bias in 40 mm samplers is discussed. Eddy diffusion is negligible for standard samplers because [NO₂]_mean was equivalent for 55 mm and 71 mm acrylic samplers and close to monitor NO₂ for 71 mm quartz tubes. Both studies showed that sampler accuracy was dependent on location. Significantly, overestimation was greatest (∼3–4 ppb) where the NO₂ annual mean was ∼20 ppb, close to the UK and EU air quality standard of 21 ppb.