Photometric observations of Mt. Etna's different aerosol plumes

We have investigated Mt. Etna's summit crater's plumes using a visible and NIR sun-photometer during eight days in July 1999. After removal of the background optical depth, we have applied the Angstrom equation and a King-type inversion to the volcanic aerosol spectral optical depths to retrieve (1) the Angstrom coefficients alpha and beta, (2) the particle size, surface area and volume spectra and (3) the effective radius. Plumes supposed to contain ash have a larger effective radius (1.48 +/- 0.28 mum vs. 0.68 +/- 0.18 mum), a smaller Angstrom exponent alpha (- 0.02 vs. 1.63) and a more uni-modal Junge-type size distribution. Inversions of the spectral optical depth to size distributions are observed to be little sensitive to small changes in the assumed value of the refractive index. Plumes measured further downwind seem to have higher effective radii (1.68 vs. 1.48 IJ-m) and a more distinct bi-modal type distribution, but the difference lies within experimental error. (C) 2001 Elsevier Science Ltd. All rights reserved.