Surface acoustic wave NO2 sensing using attenuation as the measured quantity

In this paper, we present a new method for gas sensing using the attenuation of surface acoustic waves (SAWs) as the measured quantity. For a NO2-sensor employing this technique, simple delay lines on LiNbO3 substrates were coated with copper or lead phthalocyanine as sensitive layer. The sensor signal is a change of the attenuation originating from surface conductivity changes induced by the adsorbed NO2. The advantage of monitoring attenuation is that the amplitude of a surface wave is much less sensitive to temperature fluctuations of the device than the customarily employed SAW velocity. Therefore, high stability and sensitivity of the sensor to atmospheric NO2 concentrations in the ppb range could be achieved. For field measurements, a compact electronic system was developed. Comparisons of the SAW sensor with other techniques like optical absorption spectroscopy or chemoluminiscence in several ambient air measurement campaigns were carried out. The results of the comparative measurements were found to agree well.