A continuous colorimetric method for measurement of nitrogen dioxide (NO₂) in ambient air, which uses Saltzman type absorbing solutions, was evaluated and then subjected to a collaborative test.

The evaluation shows that dynamic calibration is required to obtain reliable results. Static calibration is not reliable. The method write-up specifies dynamic calibration.

The collaborative test was carried out by having ten collaborators sample ambient air and the same ambient air spiked with a reliable source of NO₂ for four days at a common site in Kansas City, Missouri; NO₂ concentrations of 50 to 370 μg/m³ were sampled.

The results show that, based on 1-h avg concentrations, the within-laboratory standard deviation is 6 percent of the concentration over the range 90 to 370-μg NO₂/m³, and the between-laboratory standard deviation is 14 percent of the concentration over the same range. The results also show that the method has a significant positive bias. This bias cannot be accurately quantitated because different collaborators give widely different biases, that is, the bias is collaborator dependent. The “average” bias ranges from +3 to +15 percent of the NO₂ concentration over the just mentioned range.

The lower detectable limit of the method is 19 μg/m³.