Effect of the Uncertainty in Density of Polystyrene Latex (PSL) Spheres on the Calibration of Aerosol Instruments

JULIE PONGETTI, Chris Nickolaus, Jonathan Symonds, Cambustion Ltd

     Abstract Number: 306
     Working Group: Instrumentation and Methods

Abstract
Calibrated polystyrene latex (PSL) nano- and microspheres are commonly used by both manufacturers and users to check the accuracy and precision of aerosol sizing instruments. This has worked well traditionally for instruments that use electrical mobility or light scattering as a measurement principle, but there is now an increasing trend towards the use of alternative sizing metrics (e.g., mass, aerodynamic diameter) in the aerosol community.

When calibrated PSL spheres are used as a reference for instruments measuring mass and aerodynamic diameter, the density of the material must also be known along with the particle size to calculate the metric of interest. However, most PSL manufacturers only certify the particle diameter, and the quoted density is that of the bulk material.

This work further investigates the typical variations in the density of PSL particles, which have previously been reported to be of the order of 1-2% above the bulk material density of 1.05 g/cm3 (Ehara et al, 2006). This has implications in the context of instrument calibration and accuracy verification, and the expected uncertainties need to be revisited in this light. For mass-based instruments, the “calibrated” mass which can be obtained using PSL particles shows large uncertainties due to the combined effect of diameter cubed and density; while the influence of density is stronger for instruments based on aerodynamic diameter due to the linear dependency on the particle size.

The relative effect of the uncertainty in the density of PSL opens the question of whether different methods should be preferred for routine testing of instruments measuring something other than mobility diameter. As an example, DMA-classified droplets of a low-volatility liquid are used to test an Aerodynamic Aerosol Classifier (AAC) and the achieved uncertainty in aerodynamic diameter is compared to that of calibrated PSL particles.

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