Tin dioxide gas sensor as a tool for atmospheric pollution monitoring: Problems and possibilities for improvements

Abstract

The monitoring of atmospheric pollution needs network of gas sensors in order to increase the number of measuring points. For this goal, tin dioxide sensors have been evaluated. Their potentials and limitations (selectivity and stability), are presented via on-site experiments of urban pollution control. The study shows that some global information indicating mainly the traffic pollution can be obtained. The general problem of long-term stability and selectivity of SnO2 sensors is discussed. Some specific solutions are proposed, for example, a chemical treatment in the case of instability due to SO2. The problem of the dual response to oxidising and reducing gases is discussed, especially in regard to CO/NO2 detection. To solve this problem, the use of active filters is proposed. For example, a thin film of rhodium deposited above the sensing material allows to filter NO2. The use of a platinum filter has also been applied to improve the selectivity for the reducing gases. The results point out the difficulty to control this type of device in thin film technology. On the contrary, with thick films obtained by screen-printing, the possibility to separate HC from VOC (CH4 from CO and C2H5OH) is demonstrated. Some preliminary results concerning the use of a MnO2 filter are also presented in order to control the ozone reaction. The possibilities offered by signal processing are finally discussed using a multi-variable approach with 90 days aged sensors. AC measurements are exploited to improve the selectivity for the oxidizing gases NO2 and O3. The results obtained with a model built with atmospheric air as carrier gas appear promising.