Cut size minimization and cloud element break-up in a ground-based CVI

摘要

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 intersitial aerosol particles. With a one-dimensional numerical model, the CVI cut size D_50 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 ot 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 L_stag (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 D_50 and quantified the slope of the collection efficiency curve in terms of cut sharpness S_cut. 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 SDpectrometer 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 featureing high relative velocities of cloudy air past the inlet. In particular, in-cloud retrieval of cloud nuclei concentrations on high-speed airborne platforms could be affected to a signficant extent.