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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 11, 315-328, 2018
https://doi.org/10.5194/amt-11-315-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
15 Jan 2018
Calibration and assessment of electrochemical air quality sensors by co-location with regulatory-grade instruments
David H. Hagan1, Gabriel Isaacman-VanWertz1,2, Jonathan P. Franklin1, Lisa M. M. Wallace3, Benjamin D. Kocar1, Colette L. Heald1,4, and Jesse H. Kroll1,5 1Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
2Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061, USA
3Air Surveillance and Analysis Section, Hawaii State Department of Health, Hilo, HI 96720, USA
4Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
5Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract. The use of low-cost air quality sensors for air pollution research has outpaced our understanding of their capabilities and limitations under real-world conditions, and there is thus a critical need for understanding and optimizing the performance of such sensors in the field. Here we describe the deployment, calibration, and evaluation of electrochemical sensors on the island of Hawai`i, which is an ideal test bed for characterizing such sensors due to its large and variable sulfur dioxide (SO2) levels and lack of other co-pollutants. Nine custom-built SO2 sensors were co-located with two Hawaii Department of Health Air Quality stations over the course of 5 months, enabling comparison of sensor output with regulatory-grade instruments under a range of realistic environmental conditions. Calibration using a nonparametric algorithm (k nearest neighbors) was found to have excellent performance (RMSE < 7 ppb, MAE < 4 ppb, r2 > 0.997) across a wide dynamic range in SO2 (< 1 ppb, > 2 ppm). However, since nonparametric algorithms generally cannot extrapolate to conditions beyond those outside the training set, we introduce a new hybrid linear–nonparametric algorithm, enabling accurate measurements even when pollutant levels are higher than encountered during calibration. We find no significant change in instrument sensitivity toward SO2 after 18 weeks and demonstrate that calibration accuracy remains high when a sensor is calibrated at one location and then moved to another. The performance of electrochemical SO2 sensors is also strong at lower SO2 mixing ratios (< 25 ppb), for which they exhibit an error of less than 2.5 ppb. While some specific results of this study (calibration accuracy, performance of the various algorithms, etc.) may differ for measurements of other pollutant species in other areas (e.g., polluted urban regions), the calibration and validation approaches described here should be widely applicable to a range of pollutants, sensors, and environments.

Citation: Hagan, D. H., Isaacman-VanWertz, G., Franklin, J. P., Wallace, L. M. M., Kocar, B. D., Heald, C. L., and Kroll, J. H.: Calibration and assessment of electrochemical air quality sensors by co-location with regulatory-grade instruments, Atmos. Meas. Tech., 11, 315-328, https://doi.org/10.5194/amt-11-315-2018, 2018.
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Short summary
The use of low-cost sensors for air pollution research has outpaced our understanding of their capabilities and limitations under real-world conditions. Here we describe the deployment, calibration and evaluation of electrochemical sensors on the Island of Hawai‘i. We obtain excellent performance (RMSE < 7 ppb, r2 = 0.997) across a wide dynamic range (1 ppb–2 ppm). We introduce a hybrid regression algorithm which works across a large dynamic range and shows little decay in sensitivity over time.
The use of low-cost sensors for air pollution research has outpaced our understanding of their...
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