Efficacy of Paired Electrochemical Sensors for Measuring Ozone Concentrations

Abstract

Typical low-cost electrochemical sensors for ozone (O₃) are also highly responsive to nitrogen dioxide (NO₂). Consequently, a single sensor’s response to O₃ is indistinguishable from its response to NO₂. Recently, a method for quantifying O₃ concentrations became commercially available (Alphasense Ltd., Essex, UK): collocating a pair of sensors, a typical oxidative gas sensor that responds to both O₃ and NO₂ (model OX-B431) and a second similar sensor that filters O₃ and responds only to NO₂ (model NO2-B43F). By pairing the two sensors, O₃ concentrations can be calculated. We calibrated samples of three NO2-B43F sensors and three OX-B431 sensors with NO₂ and O₃ exclusively and conducted mixture experiments over a range of 0–1.0 ppm NO₂ and 0–125 ppb O₃ to evaluate the ability of the paired sensors to quantify NO₂ and O₃ concentrations in mixture. Although the slopes of the response among our samples of three sensors of each type varied by as much as 37%, the individual response of the NO2-B43F sensors to NO₂ and OX-B431 sensors to NO₂ and O₃ were highly linear over the concentrations studied (R² ≥ 0.99). The NO2-B43F sensors responded minimally to O₃ gas with statistically non-significant slopes of response. In mixtures of NO₂ and O₃, quantification of NO₂ was generally accurate with overestimates up to 29%, compared to O₃, which was generally underestimated by as much as 187%. We observed changes in sensor baseline over 4 days of experiments equivalent to 34 ppb O₃, prompting an alternate method of calculating concentrations by baseline-correcting sensor signal. The baseline-correction method resulted in underestimates of NO₂ up to 44% and decreases in the underestimation of O₃ up to 107% for O₃. Both methods for calculating gas concentrations progressively underestimated O₃ concentrations as the ratio of NO₂ signal to O₃ signal increased. Our results suggest that paired NO2-B43F and OX-B431 sensors permit quantification of NO₂ and O₃ in mixture, but that O₃ concentration estimates are less accurate and precise than those for NO₂.

Acknowledgments

Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention or the Department of Health and Human Services.

Additional information

Funding

This work was supported by the National Institute of Occupational Safety and Health (1R01OH010533 and T42OH008428).