Transmission Efficiency of an Aerodynamic Focusing Lens System: Comparison of Model Calculations and Laboratory Measurements for the Aerodyne Aerosol Mass Spectrometer

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).