HIGH-RESOLUTION AERODYNAMIC SPECTROMETRY OF SUBMICRON PARTICLES - SHEATHED VARIABLE-CUT IMPACTORS VS OTHER DEVICES

Aerosol size spectrometers based on single-jet variable-cut inertial impactors are shown to be capable of having fairly high resolution. The maximum size resolving power and the lowest particle size discernible with several well-known aerodynamic spectrometers are discussed first, to put the problem into perspective. Advantages and drawbacks of impactors as well as prior attempts at using them as continuous size spectrometers are reviewed before considering the theoretical limits on their resolving power, which we show may be higher than that of differential mobility analyzers (DMAs) even when the aerosol jet is surrounded with only 50% of combined inner and outer sheaths of clean air. The effect of adding an outer sheath of air of up to 67% to a core aerosol is studied experimentally with a thin-plate orifice nozzle in an impactor running at variable Mach number and fixed Reynolds number Re. At Re = 100, the best resolution observed is comparable to that of the DMA used as monodisperse aerosol generator, and considerably higher than that of all the other aerodynamic size spectrometers considered. Substantial improvements in resolution over that of unsheathed jets are observed at Re = 50. The ability of the present scheme to yield differential spectra at similarly high resolutions is also demonstrated using two impactors in tandem with closely matched cuts. It is concluded that impactors have indeed a remarkable potential for high-resolution sizing of very fine particles, but substantial work will be needed to achieve it fully.