Validation of low-cost ozone measurement instruments suitable for use in an air-quality monitoring network

Validation of low-cost ozone measurement instruments suitable for use in an air-quality monitoring network
Williams et al (2013)
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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