The depolarization version of the cloud aerosol spectrometer (CAS-DPOL) is a third-generation instrument. The first-generation multi-angle aerosol spectrometer (MASP) was developed in 1992 (Baumgardner, 1996), and was followed by the second-generation cloud aerosol spectrometer (CAS) that is part of the cloud aerosol and precipitation spectrometer (CAPS) developed in 2000 (Baumgardner et al., 2001). The primary difference between the CAS-DPOL and the CAS is the addition of a method to measure the amount of depolarization caused by aspherical particles so that the instrument can more sharply differentiate water from ice in clouds or dust and biological particles from other aerosol particles.

The following sections summarize the theoretical underpinnings of the CAS-DPOL design and then describe how they are implemented in the optical configuration. The methodology of calibrating the CAS-DPOL follows in section 3.0 and then examples are discussed to show how the measurements can be evaluated to estimate the asphericity.