Characteristics of the Berner Impactor for Sampling Inorganic Ions

Abstract

The Berner cascade impactor was studied extensively to validate a method for measuring the size distributions of inorganic ionic chemicals in ambient air. Stage efficiencies were calibrated from 0.06- to 10-μm 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.