Sizing of aerosol in gases other than air using a differential mobility analyzer

The differential mobility analyzer (DMA) is a device that sizes aerosol particles based on their electrical mobility. The relationship between particle size and mobility depends, among other factors, on three gas specific parameters, namely, dynamic viscosity, mean free path, and Cunningham slip correction factor C-c. Provided these parameters are known, DMA theory is expected to be valid independent of gas type. The present study demonstrates the sizing accuracy of DMAs for gases other than air using monodisperse polystyrene latex (PSL) spheres with nominal diameters of 60 nm, 149 nm, and 343 nm in He, Ar, H-2, CO2, and N2O. Eliminating possible systematic errors due to uncertainties in DMA geometry and nominal PSL diameter by normalizing the measured PSL diameters to their respective diameters measured in air, the sheath flow rate Q(sh) and C-c are expected to be the main sources for measurement errors. Since C-c data are lacking for PSL spheres in gases other than air, an expression given by Allen and Raabe (1985b) was used to approximate C-c. The experimental results of the present study are consistent with a 2% accuracy of this expression for C-c, which is considerably better than the 5% accuracy estimated by Rader (1990) for a similar expression for oil drops. Finally, we discuss other aspects of operating a DMA with gases other than air, namely, flow meter calibration and dependence of electrical breakdown voltage on gas type. In the present study a thermal mass flow meter (MFM) was used to measure Q(sh). Calibration of this MFM revealed that the gas specific MFM correction factors (K factors) provided by the technical literature can be highly inaccurate (here between -12% and +31%). More accurate K factors are presented.