Theory and Practice of the use of Metal Oxide Semiconductor Pollution Sensors

The purpose of this work is to improve measurement capabilities for urban pollution monitors, making use of small, rapidly deployable and low-cost sensing solutions. A complete pollution monitoring instrument was designed and introduced in the course of this project, capable of supporting a wide variety of sensors. In this work, Metal Oxide Semiconductor gas sensors were used to measure concentrations of NO2 and O3 at an urban background site on University campus. These two gases are closely linked to traffic pollution and harmful to human health. The sensors were extensively tested in multiple experiments:

• Over one month the instrument achieved a residual standard error compared to the AURN reference (a chemiluminescence/UV fluorescence monitor) of 23.6 =/- 3:9 μgm-3 for NO2 and 23.3 =/- 3:9 μgm-3 for O3.

• Analysis techniques were introduced to allow comparison of multiple calibrations over a long period. These could then be used to discard poor-quality calibrations, improving performance to a residual standard error of 22.9 =/-5:3 μgm-3 for NO2 and 20.0 =/- 5:1 μgm-3 for O3.

• Analysis techniques using separate calibrations for day and night were attempted, but produced very marginal results. Using multiple calibrations produced an improvement in residual standard error for NO2 of 3.4 =/- 6:2 μgm-3 and 1.72 =/- 1:85 μgm-3 for O3. The effects of wind on sensor calibration were demonstrated, but an improvement in sensor performance through analysis could not be practically achieved.

• Using these analysis techniques, the effects of instrument design, such as active air flow, sensor \warm-up" time after power-up, and manufacturing variation in the sensors were investigated and two experiments using the unique characteristics of the instrument platform to examine the variation of pollutant gas on a small scale are described.

Some of these data analysis procedures are not speci?c to instruments making use of Metal Oxide Semiconductor gas sensors and may prove generally useful for other types of instrument.