Incoming and rebounding particle velocities were measured to within several particle diameters of the impaction surface using laser Doppler velocimetry. Impacts occurred normal to the surface and ranged from 1 m / s , near the threshold for particle hounce, to 100 m/s, well into the plastic damage regime. Monodisperse ammonium fluorescein spheres, 2.6-6.9 p m in diameter, impacted target surfaces including polished molybdenum and silicon, cleaved mica, and a fluorocarbon polymer. The incident kinetic energy recovered on rebound depended on particle size and target composition at low velocity ( < 20 m/s), where the adhesion surface energy is important. No dependence on target composition was found at higher velocities where up to half of the impact energy was lost to plastic deformation. Plastic deformation was a significant component of energy loss even at impact velocities near critical velocity. Critical velocities for the onset of hounce decreased with a stronger power-law dependence on particle diameter than expected from classical adhesion theory or the elastic flattening model proposed by Dahneke. This is consistent with kinetic energy loss contributions from both plastic deformation and surface forces. Auxiliary experiments conducted with and without continuous discharge of the impact surface indicated the absence of a significant electrostatic contribution to particle adhesion.

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The Berner cascade impactor was studied extensively to validate a method for measuring the sue distributions of inorganic ionic chemicals in ambient air. Stage efficiencies were calibrated from 0.06-to 10-pm particle diameters; the monodisperse submicron test aerosol was solid ammonium fluorescein coated with oleic acid. Sampled alone, laboratory-generated liquid aerosol suffers blowoff from the last two stages, but this effect is absent with ambient aerosol. Stage cutoffs are sharp and wall losses small. A fluorocarbon grease compatible with acid and ion analysis was shown to prevent particle bounce effectively. Ammonium sulfate particles are sized properly at high relative humidity, ruling out possible growth by condensation during jet expansion. The volatilization loss of submicron ammonium nitrate particles is less than 10% under conditions that produce up to 95% loss from Teflon filters. On the basis of loading experiments, this result is ascribed mainly to the small surface-to-mass ratio of the deposits. A data inversion program was developed, based on a Smooth-Twomey algorithm by Markowski. An example of the size distribution of the principal inorganic ions in ambient air is presented.