Sensitive NO2 detection with surface acoustic wave devices using a cyclic measuring technique

The detection of NO2 in ambient air requires sensors that are capable of measuring concentrations in the low ppb range at high resolution. Using a cyclic measuring technique, a resolution below 1 ppb has been attained at NO2 concentrations below 10 ppb with a surface acoustic wave (SAW) sensor. This technique requires that the SAW sensor is first put into a defined initial state by a combination of heating and evacuation, where it is almost totally freed from residual NO2 adsorbates. From this state, NO2 concentration is determined from the temporal change of the SAW attenuation, which is proportional to the NO2 coverage. Advantages of this technique are that previously adsorbed test gas is rapidly removed and that drift of the sensor is eliminated. An increase of the lifetime of the sensor in ambient air was found to be possible by using an ozone filter.