The tunnel impactor. A multiple inertial impactor for coarse aerosol

The Tunnel Impactor is a new sampling instrument which was developed to enable the determination of the mass concentration and mass size distribution of coarse aerosol from the ambient atmosphere. The instrument consists of a large wind-directed tube functioning as an aerosol inlet. Further down the tube four impactors are mounted, each with different dimensions (2, 5, 15 and 50 mm wide ribbons), thus offering different cut-off characteristics. A constant air flow through the tube is accomplished by four ventilators, mounted at the rear end, in order to keep impaction conditions constant and well defined. The calibration of the instrument was carried out under atmospheric conditions and involved liquid DEHS aerosol and solid copper sulphate aerosol. The data showed considerable scatter, inherent to atmospheric calibration. Nevertheless the S-shaped mass collection efficiency curves could be established. The results show that the cut-off diameters are in the range from 10 to 60 μm roughly. The instrument was found to perform properly at windspeeds above 1 m s⁻¹, and is expected to function satisfactorily up to wind speeds well above 7.5 m s⁻¹. Experiments during handling of coal in the yard of a steel factory showed good performance of the instrument in practice.