Development of instruments to distinguish between water and ice

Like in mixed-phase clouds in the atmosphere in some of our experiments on ice nucleation super-cooled water droplets and ice crystals may co-exist. In these cases, detectors are needed with the ability to distinguish between these two phases besides counting and sizing these particles. This was the motivation to develop detectors with these abilities with two different approaches: Imaging followed by classification and depolarization. The two concepts and the resulting detectors are briefly described below.

The Holographic Imager for Microscopic Objects HOLIMO

The HOLIMO detector is a digital in-line holographic microscope built around a flow cell, which can be attached to different instruments in our lab or used stand-alone with an inlet system in the field for ambient measurements. The setup is relatively simple: A laser is coupled into a fibre. The fibre itself serves as a point source of coherent light illuminating the sample volume of the flow cell. Opposite to the light source, a digital camera chip is placed which records the interference pattern of the light source. In presence of any object – droplets or ice crystals – in the observing volume, the interference pattern is modified and contains information about the shape and position of these objects. The real image of the object can later be numerically reconstructed at any position within the observing volume. In contrast to classical optical microscopy, this method does not depend on any optical components, is always focused and has less aberrations.

The digital image of the detected objects can further be analyzed and classified .e.g for different shapes. In this way, a distinction between droplets and ice crystals with perceivable distinct shapes is possible.

The Ice Optical Detector IODE

The IODE detector utilizes a concept commonly found in LIDAR instruments: Depolarization of a polarized laser beam by non-spherical particles. Ice crystals cause a change in the polarization state of backscattered light while spherical droplets do not cause any change. In our IODE instrument, a continuous-wave laser is sent through the bottom of the chamber of either ZINC, PINC or the Collision chamber in an angle of 3 degrees relative to the horizontal plane. In this plane, the detection optics collect light which was scattered by particles and analyzes the different polarization planes. The signals are further analyzed with a peak detection algorithm to measure the depolarization on a particle-by-particle basis. As a result, the frozen fraction of a mixture of droplets and ice crystals can be derived.