Evaluation of low-cost electro-chemical sensors for environmental monitoring of ozone, nitrogen dioxide, and carbon monoxide

Abstract

Development of an air quality monitoring network with high spatio-temporal resolution requires installation of a large number of air pollutant monitors. However, state-of-the-art monitors are costly and may not be compatible with wireless data logging systems. In this study, low-cost electro-chemical sensors manufactured by Alphasense Ltd. for detection of CO and oxidative gases (predominantly O₃ and NO₂) were evaluated. The voltages from three oxidative gas sensors and three CO sensors were recorded every 2.5 sec when exposed to controlled gas concentrations in a 0.125-m³ acrylic glass chamber. Electro-chemical sensors for detection of oxidative gases demonstrated sensitivity to both NO₂ and O₃ with similar voltages recorded when exposed to equivalent environmental concentrations of NO₂ or O₃ gases, when evaluated separately. There was a strong linear relationship between the recorded voltages and target concentrations of oxidative gases (R² > 0.98) over a wide range of concentrations. Although a strong linear relationship was also observed for CO concentrations below 12 ppm, a saturation effect was observed wherein the voltage only changes minimally for higher CO concentrations (12-50 ppm). The nonlinear behavior of the CO sensors implied their unsuitability for environments where high CO concentrations are expected. Using a manufacturer-supplied shroud, sensors were tested at 2 different flow rates (0.25 and 0.5 Lpm) to mimic field calibration of the sensors with zero air and a span gas concentration (2 ppm NO2 or 15 ppm CO). As with all electrochemical sensors, the tested devices were subject to drift with a bias up to 20% after 9 months of continuous operation. Alphasense CO sensors were found to be a proper choice for occupational and environmental CO monitoring with maximum concentration of 12 ppm, especially due to the field-ready calibration capability. Alphasense oxidative gas sensors are usable only if it is valuable to know the sum of the NO₂ and O₃ concentrations.

Keywords: Air quality; field calibration; gas sensors; monitoring network.

Figure 1.

Low-cost electro-chemical oxidative gas and CO sensors tested in this study: (a) sizing and (b) positioning of the sensors on a custom-built circuit board.

Figure 2.

Schematic of the experimental set-up: (a) evaluation of the oxidative gas sensors and (b) evaluation of the CO sensors.

Figure 3.

Recorded voltages by the electro-chemical oxidative gas sensors at different target O₃ concentrations (error bars indicate standard errors): (a) results of the experiments performed in Mar 2016 with a newly opened sensor and (b) results of the experiments performed on the same sensors in Nov2016.

Figure 4.

Time series of O₃ concentrations measured by different O₃ monitors in comparison to predicted O₃ concentration values according to the recorded voltages by the Alphasense oxidative gas sensors (error bars indicate standard errors): (a) target concentration of 25 ppb; (b) target concentration of 50 ppb; (c) target concentration of 100 ppb;and (d) target concentration of 150 ppb.

Figure 5.

Recorded voltages by the Alphasense oxidative gas sensors at different target NO₂ concentrations (error bars indicate standard errors).

Figure 6.

Recorded voltages by the Alphasense CO sensors at different target CO concentrations (error bars indicate standard errors which may be smaller than the symbol size).

Figure 7.

Time series of CO concentrations measured by different CO monitors for different target CO concentrations of (a) 2 ppm, (b) 5 ppm, (c) 10 ppm, (d) 10 ppm, and (d) 12 ppm, (error bars indicate standard errors which may be smaller than the symbol size), where the CO concentrations for the Alphasense sensor are calculated from the linear regression presented in Figure 6.

Figure 8.

Response time of the Alphasense sensors obtained from the bump test: (a) zero air test of the oxidative gas sensor; (b) span test of the oxidative gas sensor with NO₂ at concentration of 2 ppm; (c) zero air test of CO sensor; and (d) span test of CO sensor with CO at concentration of 15 ppm.