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–NOx,
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/NOx ∼ 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.