Composition and size characterisation of volcanic ash Gemma Prata Supervisors: Tamsin Mather and David Pyle

Clarisse et al. 2010

Refractive index influence on retrievals Francis et al. ran some sensitivity experiments and showed that retrieved quantities are strongly dependent on the choice of ash refractive index data used in the radiative transfer model Plot shows scaled mass extinction coefficient at three wavelengths used by the retrieval scheme as a function of ash effective radius, as well as the ratio of 10.8/12.0 µm (black) Francis et al. 2012, JGR

Eyjafjallajökull, 13th May 2010 Andesite Pollack et al. 1973 Obsidian Pollack et al. 1973 1.5% Hematite dust Balkanski et al. 2007 Volcanic dust (basalt?) Volz 1973 Francis et al. 2012, JGR

Particle size measurement Malvern Mastersizer 2000 Size range: 0.02 – 2000 µm Uses Mie and Fraunhofer scattering Mie: requires sample refractive index Fraunhofer: refractive index not required Need about a teaspoon of sample

Particle size distributions

Effect of size distribution Volume percentage

Bulk composition – XRF

Individual component composition – Microprobe

Size-resolved and internal composition – QEMSCAN

Individual component composition Size-resolved composition Bulk composition Individual component composition Size-resolved composition

X-Ray Fluorescence (XRF) Melt whole sample quick and easy bulk composition assumes no size dependence of composition Quench to glass

Electron Probe Microanalysis (EPMA) gives composition at specific points user selects points beam diameter 10 µm

Glass vs. bulk composition Denser crystals are not normally found in ultra-distal deposits, however Smith et al. 2011 – used biotite crystals to fingerprint ultra-distal deposits from the YTT eruption

QEMSCAN Example from Etna 2006 Colour key: Sulphates (purple) Silicates (green, blue, pink) Sulphur/sulphides (yellow, claret) Martin et al. 2008, JGR

Complex index of refraction: n*  =  n + i · k n – real part k – imaginary part Literature values compiled from Narcisi & Vezzoli, Global and Planetary Change, 1999 P < 0.05

Questions?