Continuous aerosol size spectrometry with a variable-orifice impactor

An aerosol size spectrometer based on a round-jet impactor of continuously variable geometry is tested using an iris diaphragm as the nozzle. The impactor is run by fixing the nozzle-to-plate distance L, the aerosol mass flow rate m′, and the volumetric pumping capacity Q, which results in a nearly fixed downstream pressure at variable nozzle diameter dn. Tests are carried below 1.3 and 2.5 mm. The size spectrometer has an excellent resolution when the jet Reynolds number and nozzle-to-plate distance are kept within the ranges 175 Re 700; 0.75 L/dn 3.33. The instrument may also be run at atmospheric pressure, with an estimated lower size range of 1.7 μm.