Systematic biases in measurement of urban nitrogen dioxide using passive diffusion samplers
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Date
2000Author
Heal, Mathew R
Kirby, C
Cape, Neil
Metadata
Abstract
Measurement of nitrogen dioxide using passive diffusion tube over 22 months in Cambridge,
U.K. are analysed as a function of sampler exposure time, and compared with NO2 concentrations
obtained from a co-located chemiluminescence analyser. The average ratios of passive sampler
to analyser NO2 at a city centre site (mean NO2 concentration 22 ppb) are 1.27 (n = 22), 1.16 (n
= 34) and 1.11 (n = 7) for exposures of 1, 2 and 4-weeks, respectively. Modelling the generation
of extra NO2 arising from chemical reaction between co-diffusing NO and O3 in the tube gave a
ratio (modelled/measured) of 1.31 for 1-week exposures. Such overestimation is greatest when NO2
constitutes, on average, about half of total NOx (= NO + NO2) at the monitoring locality. Although 4-
week exposures gave concentrations which were not significantly different from analyser NO2, there
was no correlation between the datasets. At both the city-centre site and another semi-rural site (mean
NO2 concentration 11 ppb) the average of the aggregate of four consecutive 1-week sampler exposures
or of two consecutive 2-week sampler exposures was systematically greater than for a single
4-week exposure. The results indicate two independent and opposing systematic biases in measurement
of NO2 by passive diffusion sampler: an exposure-time independent chemical overestimation
with magnitude determined by local relative concentrations of NO and O3 to NO2, and an exposuretime
dependent reduction in sampling efficiency. The impact of these and other potential sources of
systematic bias on the application of passive diffusion tubes for assessing ambient concentrations of
NO2 in short (1-week) or long (4-week) exposures are discussed in detail.