Field validation of the Ogawa diffusive sampler for NO2 and NOx in a cold climate

Abstract

A small-scale field trial in Umeå, Sweden with Ogawa samplers and a chemiluminescence instrument indicated that the NO₂ concentration was underestimated with respect to the reference monitor, if calculated according to the manufacturer's Ogawa sampling protocol. By co-locating Ogawa samplers and reference monitors at six sites in two Swedish cities, uptake rates were determined for NO₂ and NO_x better applicable to the Swedish conditions and climate. The concentrations of NO₂ and NO_x calculated according to the instruction manual of the sampler and using the field-determined uptake rates were compared with values derived from chemiluminescence monitors for each week over which samples were taken. When calculated according to the manufacturer's suggested protocol, the Ogawa sampler underestimated the NO₂ concentrations by 9.1% on average for all samples (N = 53), with respect to the reference monitor. In contrast, NO_x concentrations were overestimated by a mean value of 15% for all samples (N = 45). By using the field determined uptake rates for the calculation of NO₂ and NO_x a better estimation of the concentrations was obtained. The ratio between concentrations determined with the Ogawa samplers and chemiluminescence monitors was then 1.02 for all measurements of NO₂ and 1.00 for NO_x . Precision, expressed as the mean coefficient of variation, was 6.4% for six, 6-replicate measurements of NO₂ and 3.7% for five, 6-replicate measurements of NO_x .