An assessment of an andersen mark-II cascade impactor

Abstract

An Andersen Mark-II cascade impactor and pre-separator have been calibrated with both monodisperse methylene blue and polymer latex particles. Collection efficiency curves agreed closely with the manufacturer's data, provided that the collection plates were coated with a thin layer of silicone grease. The collection efficiency curve for the pre-separator was shallower and displaced to smaller sizes than expected, almost overlapping with the equivalent curve for the first impaction stage (stage 0). A detailed study of the distribution of wall losses for different sizes of methylene blue particles revealed that most deposition occurred on the walls and near to jet entrances immediately before the stage where most of the particles impacted. These wall losses were dependent on particle size, increasing above 20% of the total mass collected by the impactor when the aerodynamic diameter of the particles exceeded 10 μm. The effect of wall losses on the shape of the collection efficiency curves was insignificant except at efficiencies greater than 90% and effective cut-off diameters (ECDs) were therefore unaffected. There were signs that wall losses in this impactor were increased when the moisture content of the aerosols was high but the effect could not be quantified with the current aerosol measurement techniques.