This paper presents a novel low-cost instrument that uses a sensor based on conductivity changes of heated tungstic oxide, which is capable of accurately measuring ambient concentrations of ozone. A combination of temperature steps and air flow-rate steps is used to continually reset and re-zero the sensor. A two-stage calibration procedure is presented, in which a nonlinear transformation converts sensor resistance to a signal linear in ozone concentration, then a linear correlation is used to align the calibration with a reference instrument. The required calibration functions specific for the sensor, and control system for air flow rate and sensor temperature, are housed with the sensor in a compact, simple-to-exchange assembly. The instrument can be operated on solar power and uses cell phone technology to enable monitoring in remote locations. Data from field trials are presented here to demonstrate that both the accuracy and the stability of the instrument over periods of months are within a few parts-per-billion by volume. We show that common failure modes can be detected through measurement of signals available from the instrument. The combination of long-term stability, self-diagnosis, and simple, inexpensive repair means that the cost of operation and calibration of the instruments is significantly reduced in comparison with traditional reference instrumentation. These instruments enable the economical construction and operation of ozone monitoring networks of accuracy, time resolution and spatial density sufficient to resolve the local gradients that are characteristic of urban air pollution.

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Aerosol assisted chemical vapour deposition (AACVD) reactions of tungsten hexacarbonyl, [W(CO) 6 ], in acetone, methanol, acetonitrile and a 50 : 50 mixture of acetone and toluene resulted in the deposition of blue partially reduced WO 32x films which showed preferred orientation along the (0 1 0) direction. Films deposited solely from toluene, however, were composed of a mixture of tungsten metal and W 3 O. All films could be annealed to yellow randomly orientated crystalline monoclinic WO 3 . The films deposited from methanol had a morphology comprising of a network of randomly orientated needles, which is strikingly different to the agglomeration of spherical particles observed with the other solvents. The WO 3 films functioned as gas sensitive resistors for the detection of NO 2 . Responses were recorded at minimum concentrations of 1.03 ppm of NO 2 , significantly exceeding those of commercial screen printed sensors.

The use of the APCVD reaction of WCl 6 with ethanol to produce tungsten oxide films (3600 -6700 nm-thick) for use as gas sensors is presented. The response of these films to varying concentrations of ethanol and NO 2 at varying temperatures has been examined. A comparison of the CVD sensors to a thick film (~ 60 mm) screen printed sensor has shown the CVD sensors have a faster rate of response, a stable sensing response and faster return to baseline with no drift at the conditions examined. Short titleGas sensing properties of WO 3-x thin films deposited via APCVD