| Popis: |
An automated thermodenuder system is developed for the measurement of NH3 concentrations in ambient air. The method is based on collection of NH3 by a denuder coated with vanadium pentoxide, desorption and conversion of NH3 to NOx at 700°C and detection of the resulting NOx by a NOx monitor. Advantages are an easily applied and stable coating, an annular denuder approach, which allows a sampling flow of 10l min−1 and direct NH3-to-NOx conversion on the heated V2O5-coating of the denuder. At ambient air concentrations interferences of HNO3, PAN and volatile amines are insignificant, due to their low concentrations and in the case of HNO3 due to its low collection efficiency by the acidic V2O5-coating. The detection limit is 0.1 μgm−3NH3, obtained with a sampling time of 10 min. The reproducibility is 5% in the range of ambient air concentrations. Intercomparison of measurements in ambient air by the thermodenuder system, a phosphoric acid coated denuder, a wet annular denuder system and diode-laser spectroscopy show good agreement. The system is automated by a simple multichannel time-switch resulting in a compact instrument for field experiments, monitoring networks and dry deposition measurements by the gradient method. |
