Abstract A new experimental system was recently developed for the rapid acquisition of data for aerosol sampler aspiration efficiency, and was applied in this latest work to the experimental study of thin-walled probes oriented at 90° to the freestream. Previous experimental studies from the 1980s, backed up by a physical model, suggested that aspiration efficiency for this orientation ( A 90 ) may be uniquely described as a function of St R 1 / 2 (where St is the Stokes number for particle motion in the region of the sampler entry, and R is the ratio of the freestream air velocity to the average air velocity across the plane of the sampler inlet). But in this new work the experimental system was sufficiently selective as to allow detection of an additional dependency on R not revealed by the previous work. It is proposed that this additional dependency is associated with effects associated with the non-uniformity of the airflow distribution across the plane of the test sampler at such extreme orientations, derived from the inertia of the air motion as it approaches the sampler. One result of such non-uniformity is to reduce the effective cross-sectional area of the inlet as R increases. The experimental results suggest that A 90 is now better described as a unique function of St R 1 / 4 (instead of St R 1 / 2 ). Such new insights into the basic physical behavior of a simple aerosol sampling scenario may be useful in helping to explain some aspects of the performances of more complicated aerosol samplers like those used in practical occupational and environmental air sampling situations.