[1] The analysis for BrO using the technique of differential optical absorption spectroscopy as applied to spectra of light scattered from the zenith sky has historically presented something of a challenge, leading to uncertainty about the accuracy of measurements. This has largely been due to the large sensitivity of the measurement to many analysis parameters and due to the small size of the absorption features being measured. BrO differential slant columns have been measured by six different groups taking part in an intercomparison exercise at Observatoire de Haute-Provence in France from 23 to 27 June 1996. The data are analyzed in a collaborative attempt to improve the overall analysis for BrO through investigation of a series of sources of errors in the instrumentation, calibration, input to the analysis, and the spectral analysis itself. The study included comprehensive sensitivity tests performed using both actual measurements and synthetic data. The latter proved invaluable for assessing several aspects of the spectral analysis without the limitations of spectral quality and instrument variability. The most significant sources of error are identified as the wavelength calibration of several of the absorption cross sections fitted and of the measured spectra themselves, the wavelength region of the fitting, the temperature dependence of the O 3 absorption cross sections, failure to adequately account for the so-called I 0 effect, inadequate offset correction, and inadequate measurement of the individual instrument slit functions. Recommendations for optimal analysis settings are presented, and comparing the results from the analysis of the campaign data shows BrO differential slant column observations from the various groups to be in agreement to within 4% on average between 87°and 90°s olar zenith angle, with a scatter of 16%.

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The use of a tungstic oxide semiconductor as a sensor for ozone at concentration levels relevant to atmospheric monitoring applications is an important advance in attempts to produce cheap, lightweight and reliable instruments. Problems of stability are a possible obstacle to this application. A model that describes the response of these sensors to ozone is proposed here and using it an explanation for the drift of resistance with time at constant concentrations of ozone is given. Consideration of this drift model enables a measurement routine to be employed that compensates for the drift observed experimentally, thus producing a reliable calibration of the sensor.

The behaviour of gas-sensitive resistors based on WO3 towards small concentrations of ozone in air can be understood with a simple model involving the reaction of ozone with surface oxygen vacancies. This model has been validated by comparison with experimental results for the effects of varying oxygen partial pressure on the ozone response. A complete description of the behaviour of devices constructed by printing WO3 as porous layers onto an impermeable substrate requires consideration of the effects of the microstructure of such a device upon its response. A very simple series-parallel equivalent circuit model captures the effects and allows a simple interpretation of the sensor behaviour, including the quadratic limiting steady state resistance response to ozone and the effects of variation of device thickness. An important fact that allows WO3 to be used at rather high temperatures as an effective ozone sensor is that ozone does not decompose at any discernible rate on the oxide surface. Saturation of the oxide surface at ambient temperature with water vapour inhibits the ozone response when the sensor is subsequently heated. The effect can be removed by heating at sufficiently high temperature. Water vapour also gives a high-temperature sensor response, but appears to act at sites different to those that mediate the response to ozone.