Experimental determination of the phase function and degree of linear polarization of El Chichón and Pinatubo volcanic ashes

Abstract

We present measurements of the phase function and degree of linear polarization for unpolarized incident light as a function of the scattering angle for El Chichón and Pinatubo volcanic ashes at 633 nm. The results are combined and compared with those for Pinatubo at 442 nm and Lokon volcanic ash at 442 and 633 nm [Volten et al., 2001]. The three samples of volcanic ashes consist of micron sized, irregularly shaped particles in random orientation. The results are presented in the form of figures and tables for all three samples. Similarities and differences between the results for the three samples are discussed. The measured phase functions and angular distributions of the polarization can be used to evaluate models used to calculate scattering properties of volcanic ash particles, to constrain scattering properties of volcanic ashes used in radiative transfer calculations, and to retrieve characteristics of (clouds of) volcanic ash particles in the atmosphere from remote sensing data.