Optical modeling of vesicular volcanic ash particles

We present a realistic shape model for nonspherical, vesicular particles and use the model to derive single-scattering properties of volcanic fine-ash particles. Light-scattering computations with discrete-dipole approximation reveal that, qualitatively, scattering by the model particles resembles that of the measured, real volcanic ash particles. Comparison of compact and vesicular ash shows that porosity promotes positive degree of linear polarization and decreases the depolarization ratio for both large and small vesicles. Yet, the single-scattering properties of ash particles with large vesicles are found to be surprisingly similar to those of compact ash particles. A comparison with Mie computations of equal-volume spheres indicates that for small size parameters, the spherical shape underestimates the asymmetry parameter of volcanic ash particles; whereas, for larger size parameters, it is overestimated.

Research highlights► Realistic shape model presented for volcanic ash particles. ► Porosity enhances positive polarization and decreases depolarization. ► Similarities are found in scattering by porous ash particles and compact dust. ► Spheres give too low or high asymmetry parameters, depending on size parameter.