Calibration of a Counterflow Virtual Impactor at Aerodynamic Diameters from 1 to 15 ?m

Abstract

Unlike traditional virtual impactors, which suffer from a fixed level of small particle contamination in the large particle sample stream, the counterflow virtual impactor (CVI; Ogren et al., 1985) offers the potential for contamination-free sampling of large aerosol particles. Here we present the design and calibration of a CVI intended for studying aerosol effects on cloud microphysics. Such studies require that a large and well-characterized fraction of total cloud droplet number be sampled and that the interstitial aerosol be excluded. The new CVI provides cut sizes (i.e., 50% collection efficiency diameters) down to 7 μm, which is too small for calibration with previous methods (Noone et al., 1988a). Therefore, a new calibration method has been developed which is capable of measuring both cut size and cut sharpness at small diameters. This new method, which uses a polydispersion of glass beads, represents the first set of CVI collection efficiency results for diameters well below 10 μm. The new CVI is shown to have excellent size-selectivity, comparable to other well-designed impactors, and to be well suited for number-based studies in marine stratiform clouds