Assessing the accuracy of commercially available gas sensors for the measurement of ambient ozone and nitrogen dioxide

Abstract

The objective of the National Institute for Occupational Safety and Health (NIOSH) accuracy criterion is to ensure that measurements from monitoring devices are within ±25% of the true concentration of the analyte with 95% certainty. To determine whether NO₂ and O₃ sensors meet this criterion, three commercially available units (Cairclip O₃/NO₂, Aeroqual NO₂, and Aeroqual O₃ sensors) were co-located three times with validated instruments (NO_x chemiluminescence [NO_2mon] and photometric O₃ analyzers [O_3mon]) at an outdoor monitoring station. As cofactors of sensor performance such as temperature (T) and relative humidity (RH) potentially influence the response of NO₂ and O₃ sensors, corrections for cofactors were made by using T, RH, and the sensor measurements to predict measurements made by NO_2mon and O_3mon during the first co-location period (training dataset). The developed models were tested in the merged data obtained from the second and third co-location periods (testing dataset). In the training and testing datasets, the mean NO₂ as measured by NO_2mon was 4.6 ppb (range = 0.4-35 ppb) and 9.4 ppb (range = 1-37 ppb), respectively. The mean O₃ in the training and testing datasets as measured by O_3mon was 38.8 ppb (range = 1-65 ppb) and 35.7 ppb (range = 1-61 ppb), respectively. None of the sensor measurements in the training dataset were within the NIOSH accuracy criterion (mean error ≥25%). After correcting for cofactors of sensor performance, the accuracy of the Cairclip O₃/NO₂ and the Aeroqual O₃ sensors considerably improved when tested with the testing dataset (mean error = -1% and 14%, respectively). However, the Aeroqual NO₂ sensor had an error that was not within ±25%. Raw measurements from the tested sensors may be unsuitable for assessing workers' exposure to NO₂ and O₃. Corrections for cofactors of Cairclip O₃/NO₂ and Aeroqual O₃ sensor performance are required for more accurate occupational exposure assessment.