A two-stage impactor for fog droplet collection: Design and performance

A two-stage fog water impactor was designed for the measurement of size-dependent cloud droplet solute concentrations. The instrument achieves isokinetic sampling conditions by using a variable inlet as well as by using a rotating stand in connection with a wind vane to keep the impactor aligned with the actual mean wind direction. These features are important for reliable results, especially at the high wind velocities which frequently occur at mountain stations. The instrument operated successfully during the Great Dun Fell Cloud Experiment in spring 1993. The results of field measurements, as well as of cloud chamber experiments to test the performance of the instrument are presented.