Improvements in Calibration of Black Carbon Mass Instruments using the Centrifugal Particle Mass Analyzer-Electrometer Reference Mass Standard (CERMS)

Calibration of mass instruments, particularly those measuring black carbon (BC) mass concentrations, is a challenge. Most instruments infer the BC mass concentration using some variation of light absorption or a surrogate thereof (incandescence), leading to some discrepancies between the measurements. To unify the response of these instruments, thermal optical analysis (TOA) is commonly used as a reference method. TOA involves collecting particles onto filters, which are then analyzed to quantify the elemental and organic carbon fractions. However, filter collection becomes impractical for calibrations at low concentrations, requiring long sampling times. Moreover, TOA was shown to be the largest contributor to the uncertainty associated with the measurement of BC mass, on the order of 17%.
The centrifugal particle mass analyzer (CPMA)–electrometer reference mass standard (CERMS) has been proposed as an alternative calibration technique for mass-based instruments. This method enables rapid, real-time calibration that is independent of source concentration. Particles undergo unipolar charging and are classified by their mass-to-charge ratio (mass setpoint) based on the balance of centrifugal and electrostatic forces. This classification results in a proportionality between the total charge and the total particle mass concentration in the aerosol stream.
Previously, we have demonstrated an overall reproducibility of 11% across three independent CERMS systems, identifying the CPMA as the least reproducible component. Here, we show the results of several investigations aimed to improve CERMS calibration procedure, accuracy and uncertainty. First, we developed an effective CPMA calibration procedure using mass-certified polystyrene latex spheres obtained from the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Modeling of the CERMS response indicates that the lowest errors occur at mass setpoints corresponding to the highest particle concentrations. Based on these findings, we performed a comparison of three CERMS systems operated under optimal conditions and automated procedure by introducing a diluter upstream of each system. This work was performed using three Artium LII 300 instruments and one AVL MSS as the device under test, investigating the reproducibility of the calibration across multiple instruments.