Biases in the Measurement of Ambient Nitrogen Dioxide (NO2) by Palmes Passive Diffusion Tube: A Review of Current Understanding

Abstract

Palmes-type passive diffusion tubes (PDTs) are widely used to measure levels of nitrogen dioxide (NO2) in air quality studies. Molecules of NO2 diffuse down the concentration gradient established in the tube by their reactive conversion into nitrite (NO2−) with triethanolamine (TEA) absorbent at the inner end. The relatively low uptake rate for the tube geometry means that exposure-averaged NO2 concentration can be calculated from first principles using the diffusion coefficient, D, for NO2 in air. This review provides a critical assessment of the current understanding of sources and extent of potential bias in NO2 PDT measurements in each of the following methodological stages: preparation of the absorbent; quantification of the absorbed NO2−; deployment in the field; calculation of the exposure-average NO2 concentration from the absorbed NO2−; and assessment of PDT bias through comparison against a chemiluminescence NO2 analyser. The review has revealed strong evidence that PDT measurement of NO2 can be subject to bias from a number of sources. The most significant positive biases are ambient wind flow at the entrance of the tube potentially leading to bias of tens of percent, and within-tube chemical reaction between NO and O3 causing bias up to ~25% at urban background locations, but much less at roadside and rural locations. Sources of potentially significant negative bias are associated with deployment times of several weeks in warm and sunny conditions, and deployments in atmospheres with relative humidities <~75% which causes incomplete conversion of NO2 to NO2−. Evidence suggests that biases (positive or negative) can be introduced by individual laboratories in the PDT preparation and NO2− quantification steps. It is insufficiently acknowledged that the value of D is not accurately known—some controlled chamber experiments can be interpreted as indicating that the value of D currently used is too low, giving rise to a positive bias in PDT-derived NO2 concentration. More than one bias may be present in a given PDT deployment, and because the biases act independently the net effect on PDT NO2 determination is the linear sum of individual biases acting on that deployment. The effect of net bias can be reduced by application of a local “bias adjustment” factor derived from co-locations of PDTs with a chemiluminescence analyser. When this is carried out, the PDT is suitable as an indicative measure of NO2 for air quality assessments. However, it must be recognised that individual PDT deployments may be subject to unknown variation in the bias adjustment factor for that deployment.