A review of the sampling efficiency of rotating‐arm impactors used in aerobiological studies

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Abstract
Correct sampling of airborne biological particles is the most critical step in aerobiological studies. It is common not to take account of the difference between the true atmospheric concentration, and the apparent concentration, as measured with rotating‐arm impactors. The difference can be quantified by the sampling efficiency (E5). I demonstrate that such differences can lead to substantial errors in many fields of aerobiology. Because of the continuing mis‐use of such aerobiological samplers I present a review of concepts and an examination of previous experimental and theoretical attempts to ascertain Es. I take the example of the widely‐used rotorod sampler. Commonly used theories cannot fully account for demonstrated variations in sampling efficiency when particle diameter and ambient wind speed vary. This is caused by: the complicated air flow patterns that develop around the rotating I‐rods of a rotorod sampler; lack of particle retention; and, the varying densities and shapes of aerobiological particles. Other considerations indicate that, when used outdoors, atmospheric turbulence may also affect sampling efficiency. To resolve these problems, a pragmatic approach is proposed wherein the rotorod is calibrated over a range of wind speeds, particle sizes and possibly turbulent intensities. A repeated measures design would allow a determination of sampler precision. The determination of precision, in addition to accuracy, is viewed as a crucial consideration that aerobiologists should consider in their work. This approach to calibration should, ideally, be enacted through international aerobiological standard measures or be done at calibration centres, and instrument calibrations should be made traceable to these standards.