CUT SIZE MINIMIZATION AND CLOUD ELEMENT BREAK-UP IN A GROUND-BASED CVI

A ground-based Counterflow Virtual Impactor (CVI) was optimized to achieve nearly complete in situ segregation of cloud droplets and ice crystals (with subsequent evaporation, releasing dissolved gaseous and non-volatile material) from their surrounding carrier gas and interstitial aerosol particles. With a one-dimensional numerical model, the CVI cut size D50 was reduced to 4 mu m from 7 mu m in an earlier design (Anderson et al., 1993). This could be achieved by a velocity increase to 225 m s-1 inside the wind tunnel forming part of the ground-based CVI, and by minimizing all dimensions contributing to the stagnation length Lstag (distance from the wind intersection plane tunnel/CVI to the stagnation plane inside the CVI that cloud elements have to reach to be sampled). CVI and high-speed wind tunnel were designed and constructed according to the modeling results. Subsequent calibrations verified the calculated lower cut sizes D50 and quantified the slope of the collection efficiency curve in terms of cut sharpness Scut. With the new CVI lower cut sizes between 4 and 6 mu m can be achieved. A cloud chamber experiment was performed with CVI measurements supplemented by a Forward Scattering Spectrometer Probe (FSSP). It could be demonstrated that significant drop break-up is caused by wind tunnel velocities well beyond 150 m s-1. For a reduced wind tunnel velocity of 150 m s-1 a reasonable cut size of at least 5 mu m could be maintained, while avoiding break-up. The demonstration of break-up should have consequences for any cloud sampling technique featuring high relative velocities of cloudy air past the inlet. In particular, incloud retrieval of cloud nuclei concentrations on high-speed airborne platforms could be affected to a significant extent.