ON THE COLLECTION EFFICIENCY OF A ROTATING ARM COLLECTOR AND ITS APPLICABILITY TO CLOUDWATER AND FOGWATER SAMPLING

A Rotating Arm Collector (RAC) belonging to the class of (virtual) wide stream impactors is designed and operated to sample cloud or fog droplets. The applicability of RACs to sample atmospheric aerosol particles or fog/cloud droplets larger than about 5 mum is demonstrated by discussing their working principle in detail and comparing their sampling characteristics with impactors requiring an inlet. The specific properties of the flow past the sampling units of RACs, especially the variation of the flow velocity along the sampling area results in a dependence of the collection efficiency on the distance to the axis of rotation. This is remarkable when operating a RAC to collect cloud or fog droplets, because drops flow together after sampling. A method is developed which determines the overall collection efficiency of the sampling units experimentally. The resulting collection efficiency curve is compared with simplified theoretical evaluations of the collection efficiency of flat plates (Langmuir and Blodgett, Collected Work of Irving Langmuir 10, pp. 348-393. Pergamon Press, Oxford, 1946) and the collection efficiency of the gutter-formed sampling areas used in this study (Lesnic et al., J. Aerosol Sci. 24, 163-180, 1993). The agreement between experiment and theory is good, showing that the main characteristics of wide stream impaction are not affected by the complex flow field past the sampling units of the RAC and thus the simplified theory is an acceptable approach to describe the real sampling conditions.