Droplets, Ice Crystal or Ash? Real time detection using the next generation Backscatter Cloud Probe

The backscatter cloud probe (BCP) has been providing measurements of cloud size distributions since its implementation as part of IAGOS since July, 2011 and is now operating on six A-340 passenger aircraft. Like the majority of single particle optical spectrometers, the BCP measures the equivalent optical diameter (EOD) of particles but is unable to identify the type of particles, e.g. if it is a water droplet, ice crystal, dust or volcanic ash. In addition, because of the close proximity of the sensitive sample volume to the aircraft skin, many of the ice crystals that are counted and sized could be fragments of ice crystals that have impacted the fuselage in front of the BCP but cannot be distinguished from natural crystals.

To address the shattering issue and to expand the capability of the BCP to differentiate different types of atmospheric particles, a next generation BCP has been developed that incorporates the technology that has already been implemented in the DMT CAS-POL that can separate ice crystals from ash and water droplets. A prototype Backscatter Cloud probe with Polarization Detection (BCPD) has been tested in the laboratory, cloud chamber, wind tunnel and on aircraft. Preliminary results indicate that the polarized scattering patterns from spherical and aspherical particles contain information that will potentially allow differentiation of water droplets, ice crystals and various types of aerosols like dust and volcanic ash. In addition, because it measures the interarrival times of particles, some rejection of fragments of shattered ice crystals is possible. An added feature is the processing of the particle transit times and shapes of the transit signal to provide some information on particle morphology.

The design and operating principles of the BCPD will be presented and some examples shown that demonstrate its capabilities.