Advances in Single Particle Polarimetry for Studies of Clouds, Dust and Volcanic Ash

Polarimetry has been employed by the atmospheric, remote sensing community for many decades in ground based radar and lidar applications and for space borne measurements, as well. In particular, radar polarimetry has been successful in measuring the electromagnetic radiation that is reflected from cloud hydrometeors that also rotate the incident radiation to a degree that is dependent on the size and shapes of the water droplets and ice crystals in the cloud. Likewise, lidar also measures the amount by which an ensemble of aerosol or cloud particles depolarize the incident linearly or circularly polarized light. The intensity of backscattered light and degree of depolarization is used to differentiate spherical from aspherical particles, and subsequently identify liquid and ice layers in thin clouds, or dust and ash layers in a mixture of aerosol particles.

Hence, even though polarimetry has been accepted by the atmospheric community as the means to extract more information from populations of aerosol and cloud particles, remarkably this technique has only recently begun being used to look at individual particles using in situ sensors. There were a number of attempts in the late 1960s and early 1970s to distinguish water droplets from ice crystals using polarization techniques but the community failed to take interest in the technology at that time.

The idea laid dormant until 2007 when several groups began developing polarimetry for use in ice nucleus counters, cloud chambers and in single particle optical spectrometers flown on aircraft.

This presentation will describe, with associated data examples, the polarimetric, single particle sensors that are in active use in aerosol and cloud research: the Cloud and Aerosol Spectrometer with Polarization (CAS-POL), the Aerosol Particle Spectrometer with Polarization Detection (APSPD), the Cloud Particle Spectrometer with Polarization Detection (CPSPD), the Backscatter Cloudprobe with Polarization Detection (BCPD) and the Spectrometer for Ice Nucleation (SPIN).

These five instruments have been deployed on numerous ground based and airborne field experiments since 2010 with results that are beginning to open new avenues in our understanding of aerosol and cloud processes.