CHARACTERIZATION OF THE BEHAVIOR OF NITROGEN-DIOXIDE AT LOW CONCENTRATIONS IN HUMID AIR WITHIN ADSORBENT CHARCOAL SAMPLING MEDIA

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 cy linder 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 It ambient pressure). NO2 was observed to break through th e adsorbent rapidly but only at trace levels. No further significant i ncrease in NO2 concentration was noted. However, NO was observed to el ute from the adsorbent bed, eventually attaining an eluting concentrat ion equivalent to the NO2 challenge concentration. This conversion pro cess was found to be strongly dependent on the relative humidity of th e test atmosphere, and at low relative humidities (<0.1% RH at 294 K a nd ambient pressure), the NO2 to NO conversion process was strongly in hibited. This behaviour was evaluated in terms of the gas-phase and aq ueous chemistry of NOx. Further, the potential for interference effect s on trace volatile organic compound determinations using a charcoal s ampling medium, with recovery by thermal desorption, arising from NO2 within the sample is highlighted.