Evaluation and Collaborative Testing of a Continuous Colorimetric Method for Measurement of Nitrogen Dioxide in Ambient Air

Abstract

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.