Abstract
The size-dependent particle transmission efficiency of the aerodynamic lens system used in the Aerodyne Aerosol Mass Spectrometer (AMS) was investigated with computational fluid dynamics (CFD) calculations and experimental measurements. The CFD calculations revealed that the entire lens system, including the aerodynamic lens itself, the critical orifice which defines the operating lens pressure, and a valve assembly, needs to be considered. Previous calculations considered only the aerodynamic lens. The calculations also investigated the effect of operating the lens system at two different sampling pressures, 7.8 × 104 Pa (585 torr) and 1.0 × 105 Pa (760 torr). Experimental measurements of transmission efficiency were performed with size-selected diethyl hexyl sebacate (DEHS), NH4NO3, and NaNO3 particles on three different AMS instruments at two different ambient sampling pressures (7.8 × 104 Pa, 585 torr and 1.0 × 105 Pa, 760 torr). Comparisons of the measurements and the calculations show qualitative agreement, but there are significant deviations which are as yet unexplained. On the small size end (30 nm to 150 nm vacuum aerodynamic diameter), the measured transmission efficiency is lower than predicted. On the large size end (> 350 nm vacuum aerodynamic diameter) the measured transmission efficiency is greater than predicted at 7.8 × 104 Pa (585 torr) and in good agreement with the prediction at 1.0 × 105 Pa (760 torr).
Acknowledgments
The experiments at the University of Wyoming were performed in the aerosol laboratory developed with support from the W.M. Keck Foundation and were partially supported by NSF grant # ATM-0441836. Work performed at Aerodyne Research, Inc. was supported by the NSF Small Business Innovation Research Program, contract #0216220. Work at MIT was supported by the Singapore-MIT Alliance and by USEPA grant #RD-83107701-0. K. Moore was supported by the USDOE GCEP SURE program. The Aerodyne authors acknowledge valuable discussions with T. S. Bates.