Volcanic eruptions distribute ash in the atmosphere that may be transported over great distances. These particles impact atmospheric radiation transfer by absorbing, emitting and scattering electromagnetic radiation. While in the atmosphere, dust particles may interfere with aviation activities causing considerable economic losses. Remote detection and global monitoring of ash clouds is therefore of great interest.
Volcanic dust particles are irregularly shaped and can be substantially porous. However, when modeling optical properties, simpler model geometries may provide adequate performance. We are using ellipsoidal model particles to recreate the measured scattering properties of volcanic dust. This approach has been previously shown to work well for the mineral dust [1, 2]. Porosity is accounted for by using the effective medium approximation. To validate the approach we use laboratory measurements from several volcanic dust samples as a reference: Pinatubo (Philippines), Eyjafjalla (Iceland), Puyehue (Chile), St. Helens (USA) , Lokon (Indonesia), Spurr (USA) and Redoubt (USA). The laboratory data consist of the full scattering phase matrix, the size distribution, estimate for the refractive index and some electron microscope images of the particles .
In Figure 1 the fitting results are shown for the phase matrix (P11) of the Eyjafjalla volcano. Even though the fitting is performed by optimizing all of the six independent phase matrix elements simultaneously, instead fitting only the P11 element, good agreement with the measured P11 is obtained. In particular, the results show a considerable improvement to the still often used spherical model particles.
The purpose of this modelling exercise is to derive a generic shape distribution of ellipsoidal model particles that would closely mimic scattering by volcanic dust particles and could be used in retrievals where the shape distribution cannot be optimized. We then plan to test this approach in AATSR satellite retrieval algorithm by comparing the results based on Mie spheres and ellipsoids.
 L. Bi, P. Yang, G. W. Kattawar, and R. Kahn, Single-scattering properties of triaxial ellipsoidal particles for a size parameter range from the rayleigh to geometric-optics regimes, Appl. Optics48(1), 114126 (2009).
 S. Merikallio, T. Nousiainen, M. Kahnert, and A.-M. Harri, "Light scattering by the Martian dust analog, palagonite, modeled with ellipsoids," Opt. Express 21, 17972-17985 (2013) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-15-17972
 O. Muñoz, O. Moreno, D. D. Dabrowska, H. Volten, and J. W. Hovenier, The Amsterdam-Granada light scattering database, J. Quant. Spectrosc. Radiat. Transfer113, 565574 (2012).
Figure 1. Angular distribution of scattered intensity for Eyjafjalla volcano dust particles at 647 nm wavelength. Measurements are shown in red. Simulation results based on model spheres are shown with dashed blue line and the corresponding result for the best-fit shape distribution of ellipsoids is shown in black. The fitting is based on optimizing the whole matrix simultaneously, in-stead of only this scattering matrix element, P11. The peach colored shading spans the area covered by all ellipsoids, excluding spheres.
Supplementary URL: http://www.iaa.es/scattering/