Retrieval of volcanic ash particle size, mass, optical depth and mass of sulfur dioxide from multispectral data using neural networks Alessandro Piscini 1 , Matteo Picchiani 2 , Stefano Corradini 1 , Marco Chini 1 , Luca Merucci 1 and Fabio Del Frate 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy 2 Earth Observation Laboratory – Tor Vergata University, Rome, Italy 7-9 December 2011 ESA/ESRIN, Frascati, Italy