PERFORMANCE OF A HYPERSONIC IMPACTOR WITH SILVER PARTICLES IN THE 2 NM RANGE

The performance of a hypersonic impactor is studied in the diameter ra nge down to 2 nm with condensation particles from silver vapors. Parti cles of a single mobility are selected with a differential mobility an alyzer, and sized from the measured mobility on the assumption that th e usual size-mobility relations hold in the nanometer range. The infer red ''aerodynamic density'' is between 2 and 3 times smaller than that of crystalline Ag, so that the particles are either non-spherical or non-pure crystalline Ag, or the usual size-mobility relations fail in this size range. Yet, this aerosol provides a source for the smallest particles yet analyzed aerodynamically. New results are that: (a) the resolving power of the instrument does not diminish at decreasing part icle size, at least down to 2 nm; (b) the minimum capture efficiency e ta(min) observed below the critical Stokes number (6.5-8%) is nearly i ndependent of particle diameter in all the range above 2 nm and (c) th e characteristic response curves of the impactor do not improve upon s heathing a core aerosol flow with clean air. A provisional conclusion is that the origine of eta(min) is probably turbulent diffusion.