Particle size distributions of Mount Etna's aerosol plume constrained by Sun photometry

Near simultaneous instrument calibration and volcanic aerosol measurement were performed near the summit of Mount Etna, Sicily, in October 1997 using an eight channel Sun‐tracking photometer. Three objectives were achieved using this methodology: calibration of the instrument, measurement of the background atmosphere, and measurement of the volcanic aerosol plume. Out‐of‐plume calibration measurements were used to extrapolate extraterrestrial solar voltages for the five nonpolarized visible and near‐infrared channels using a Langley calibration routine. The Langley coefficients were then used to calculate the optical depth of the background atmosphere and subsequently the volcanic plume using the Beer‐Bouger‐Lambert law. Coefficients were calculated from the Ångström equation with means of α = 1.67 (0.13 < α < 2.42) and β = 0.06 (0.001 < β < 0.14). A constrained linear inversion of the spectral optical depth of the plume was performed using Mie code to produce particle number, surface area and volume spectra, and the effective scattering radius. The volume spectra indicate a trimodal distribution consisting of particles of radius < 0.1 μm (nucleation mode), a possible second fine mode (< 1.0 μm), and large particles of radius >1.0 μm (acid droplets) with minima at 0.5 and 1.5 μm. The mean effective radius was determined to be at 0.83 μm within the range 0.35 < r < 1.6 μm, and the total aerosol mass flux was estimated as 4.5–8.0 kg s −1 with the smaller radius mode contributing 6–18% of the total mass flux.