Efficacy of Paired Electrochemical Sensors for Measuring Ozone Concentrations

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.