An ozone monitoring instrument based on the tungsten trioxide (WO3) semiconductor

Novel sensors based on tungsten trioxide (WO3) semiconductors have been found to hold much promise as a cheaper alternative for ozone monitoring. The sensitivity of the metal oxide to ozone is owed to the presence of surface oxygen vacancies. The filling of oxygen vacancies by the decomposing ozone traps free charge carriers with consequent measurable increase in resistance. A Langmuir-type model simulating the fundamental surface reactions has been found to reproduce the measured sensor response. Using a temperature-cycling technique in which the temperature of the sensor is alternated between 600 and 530 °C representing scrubbing and measurement resistances, respectively, ozone measurements have been obtained from several urban sites. Results from eight sensors show that the sensors have adequate sensitivity and response time with measurements comparable to ozone measured by UV absorption spectrometer (r.m.s. of variances up to 7 ppbv). Problems of apparent changes in sensor sensitivity in the course of field measurements have been explained by inadequate ‘burn-in’ period prior to calibration and field deployment. The sensors should be run in ambient conditions for about a week before calibration.