Characterization of the behaviour of nitrogen dioxide at low concentrations in humid air within adsorbent charcoal sampling media

Abstract

The behaviour of nitrogen dioxide (NO₂) in humid air within a charcoal sampling medium is described. Dynamic test atmospheres of controlled NO₂ concentration, flow and relative humidity were generated from a cylinder containing a reference standard of NO₂. These atmospheres were passed through an adsorbent charcoal bed and the elution behaviours of NO₂ and nitric oxide (NO) were investigated. The capacity for NO₂ was found to be independent of concentration over the range studied (3–42 vpm at 294 K ambient pressure). NO₂ was observed to break through the adsorbent rapidly but only at trace levels. No further significant increase in NO₂ concentration was noted. However, NO was observed to elute from the adsorbent bed, eventually attaining an eluting concentration equivalent to the NO₂ challenge concentration. This conversion process was found to be strongly dependent on the relative humidity of the test atmosphere, and at low relative humidities (<0.1% RH at 294 K and ambient pressure), the NO₂ to NO conversion process was strongly inhibited. This behaviour was evaluated in terms of the gas-phase and aqueous chemistry of NO_x. Further, the potential for interference effects on trace volatile organic compound determinations using a charcoal sampling medium, with recovery by thermal desorption, arising from NO₂ within the sample is highlighted.