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

A small-scale field trial in Umea, 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 NOx better applicable to the Swedish conditions and climate. The concentrations of NO2 and NOx 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, NOx 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 NOx 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 NOx. 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 NOx.