Design and Calibration of a Counterflow Virtual Impactor for Sampling of Atmospheric Fog and Cloud Droplets

An instrument is described that samples cloud droplets by removing them from the surrounding air and small unactivated particles through inertial impaction. The sampled droplets are then evaporated, leaving behind the material dissolved or suspended in the droplets as residue particles or gases. The instrument is capable of sampling droplets as a function of their size; it has an adjustable cut size in the range between about 9 and 30 μm in diameter, rejects droplets and particles smaller than the cut size, and captures droplets larger than the cut size. Details of the instrumental design and construction are discussed, as well as a relative calibration of the collection efficiency. Results from the calibration experiments indicate that the counterflow virtual impactor probe behaves in accordance with theoretical predictions using Stoke's number calculations. A complete description of the calibration methodology is presented.