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

This paper presents a novel low-cost instrument that uses a sensor based on conductivitychanges of heated tungstic oxide, which is capable of accurately measuring ambientconcentrations of ozone. A combination of temperature steps and air flow-rate steps is used tocontinually reset and re-zero the sensor. A two-stage calibration procedure is presented, inwhich a nonlinear transformation converts sensor resistance to a signal linear in ozoneconcentration, then a linear correlation is used to align the calibration with a referenceinstrument. The required calibration functions specific for the sensor, and control system forair 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 cellphone technology to enable monitoring in remote locations. Data from field trials are presentedhere to demonstrate that both the accuracy and the stability of the instrument over periods ofmonths are within a few parts-per-billion by volume. We show that common failure modes canbe detected through measurement of signals available from the instrument. The combination oflong-term stability, self-diagnosis, and simple, inexpensive repair means that the cost ofoperation and calibration of the instruments is significantly reduced in comparison withtraditional reference instrumentation. These instruments enable the economical constructionand operation of ozone monitoring networks of accuracy, time resolution and spatial densitysufficient to resolve the local gradients that are characteristic of urban air pollution.