DRASTIC IMPROVEMENT OF THE RESOLUTION OF AEROSOL-SIZE SPECTROMETERS VIA AERODYNAMIC FOCUSING - THE CASE OF VARIABLE-PRESSURE IMPACTORS

Due to their finite inertia,particles carried in a gas follow trajecto ries that differ from fluid streamlines. They may thus be size-segregr ated, though only with the modest resolving power allowed by the depen dence of their trajectories on the fluid streamline Psi in which they are initially seeded. This limitation is greatly reduced in the presen t work via aerodynamic focusing, a phenomenon discovered by Israel and Friedlander and briefly reviewed here. McMurry and his colleagues hav e recently shown that passage through a succession of coaxial convergi ng nozzles brings virtually into the axis of symmetry (Psi = 0) all pa rticles within a relatively wide size range. Considerable gains in ins trument resolution may thus be achieved by inserting a series of focus ing nozzles right upstream of an aerosol size spectrometer. This gener al principle is demonstrated here for the case of variable-pressure im pactors, which are ideally suited for coupling to the focusing lenses. The resolution improvements found are impressive, particularly in the viscous and compressible regimes.