Temporal changes in field calibration relationships for Aeroqual S500 O_3 and NO_2 sensor-based monitors

摘要

Sensor-based monitors are increasingly used to measure air pollutant concentrations, but require calibration under field conditions. We made intermittent comparisons (6 times over a 6-month period) between ozone and nitrogen dioxide concentrations measured by Aeroqual gas-sensitive semiconductor (O3) and electrochemical (NO2) sensors (two of each) and reference analysers in the UK Automatic Urban and Rural Network. Each deployment period was split into equal (n = 48×1-hour) training and test datasets, to derive and test calibration equations respectively. We observed significant bivariate linear relationships betweenAeroqual O3andReference O3concentrations, and significant multiple linear relationships betweenAeroqual NO2and bothReference NO2andAeroqual O3concentrations. Changes in monitor responses over time (including apparent baseline drift in O3sensor output, and discrepancies between the 2 Aeroqual NO2sensors) resulted in relatively inaccurate concentrations estimates (cf.reference concentrations) from calibration equations derived in the first training period and applied to subsequenttestdeployments (e.g.NO2RMSE = 47.2 μg m−3(n = 286) for a dataset of alltestperiods combined, for one of the two monitor pairs). Substantial improvements in accuracy of estimated concentrations were achieved by combination of repeated intermittent training data into a single calibration dataset (NO2RMSE = 8.5 μg m−3for sametestdataset described above). This latter approach to field calibration is recommended.