Efficacy of Paired Electrochemical Sensors for Measuring Ozone Concentrations.

Typical low-cost electrochemical sensors for ozone (O) are also highly responsive to nitrogen dioxide (NO). Consequently, a single sensor's response to O is indistinguishable from its response to NO. Recently, a method for quantifying O concentrations became commercially available (Alphasense Ltd., Essex, UK): collocating a pair of sensors, a typical oxidative gas sensor that responds to both O and NO (model OX-B431) and a second similar sensor that filters O and responds only to NO (model NO2-B43F). By pairing the two sensors, O concentrations can be calculated. We calibrated samples of three NO2-B43F sensors and three OX-B431 sensors with NO and O exclusively and conducted mixture experiments over a range of 0-1.0 ppm NO and 0-125 ppb O to evaluate the ability of the paired sensors to quantify NO and O 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 NO and OX-B431 sensors to NO and O were highly linear over the concentrations studied (R ≥ 0.99). The NO2-B43F sensors responded minimally to O gas with statistically non-significant slopes of response. In mixtures of NO and O, quantification of NO was generally accurate with overestimates up to 29%, compared to O, which was generally underestimated by as much as 187%. We observed changes in sensor baseline over 4 days of experiments equivalent to 34 ppb O, prompting an alternate method of calculating concentrations by baseline-correcting sensor signal. The baseline-correction method resulted in underestimates of NO up to 44% and decreases in the underestimation of O up to 107% for O. Both methods for calculating gas concentrations progressively underestimated O concentrations as the ratio of NO signal to O signal increased. Our results suggest that paired NO2-B43F and OX-B431 sensors permit quantification of NO and O in mixture, but that O concentration estimates are less accurate and precise than those for NO.