Practical Field Calibration of Portable Monitors for Mobile Measurements of Multiple Air Pollutants

To reduce inaccuracies in the measurement of air pollutants by portable monitors it is necessary to establish quantitative calibration relationships against their respective reference analyser. This is usually done under controlled laboratory conditions or one-off static co-location alongside a reference analyser in the field, neither of which may adequately represent the extended use of portable monitors in exposure assessment research. To address this, we investigated ways of establishing and evaluating portable monitor calibration relationships from repeated intermittent deployment cycles over an extended period involving stationary deployment at a reference site, mobile monitoring, and completely switched off. We evaluated four types of portable monitors: Aeroqual Ltd. (Auckland, New Zealand) S500 O-3 metal oxide and S500 NO2 electrochemical; RTI (Berkeley, CA, USA) MicroPEM PM2.5; and, AethLabs (San Francisco, CA, USA) AE51 black carbon (BC). Innovations in our study included: (i) comparison of calibrations derived from the individual co-locations of a portable monitor against its reference analyser or from all the co-location periods combined into a single dataset; and, (ii) evaluation of calibrated monitor estimates during transient measurements with the portable monitor close to its reference analyser at separate times from the stationary co-location calibration periods. Within the 7 month duration of the study, combined' calibration relationships for O-3, PM2.5, and BC monitors from all co-locations agreed more closely on average with reference measurements than individual' calibration relationships from co-location deployment nearest in time to transient deployment periods. Individual' calibrations relationships were sometimes substantially unrepresentative of the combined' relationships. Reduced quantitative consistency in field calibration relationships for the PM2.5 monitors may have resulted from generally low PM2.5 concentrations that were encountered in this study. Aeroqual NO2 monitors were sensitive to both NO2 and O-3 and unresolved biases. Overall, however, we observed that with the combined' approach, indicative' measurement accuracy (+/- 30% for O-3, and +/- 50% for BC and PM2.5) for 1 h time averaging could be maintained over the 7-month period for the monitors evaluated here.