AIR-POLLUTION MONITORING WITH A SEMICONDUCTOR GAS SENSOR ARRAY SYSTEM

An array system of semiconductor gas sensors based on differently doped SnO2 thin films was used to monitor the air pollution in a city atmosphere (Oulu, Finland). Silver and palladium were the main constituents used as dopants in stannic oxide thin films which were made by the RF cathode sputtering technique on alumina substrates (3 x 3 mm2) consisting of screen-printed heating resistors and gold electrodes. The sensors doped with silver were very sensitive to H2S, while the palladium-doped sensors were very sensitive to hydrogen, for instance, but not at all sensitive to H2S. The sensor array system was installed at a city air pollution monitoring station where the H2S and SO2 concentrations were being simultaneously recorded by commercial analyzing equipment based on coulometric titration together with the signals from the semiconductor gas sensor array. Together with the H2S concentration peaks from the commercial equipment, resistance of the silver-doped sensors decreased drastically. In some cases, a relatively high response of the palladium-doped sensors (insensitive to H2S) together with the response of the silver-doped sensors proved that H2S emission was accompanied also with the emission of other gases. In some cases, the response of the palladium-doped sensors was present without any response of the silver-doped sensors.