Photometric Observations of Aerosol Optical Properties and Emission Flux Rates of Stromboli Volcano Plume during the PEACETIME Campaign

The characterisation of aerosol emissions from volcanoes is a crucial step towards the
assessment of their importance for regional air quality and regional-to-global climate. In this paper
we present, for the first time, the characterisation of aerosol emissions of the Stromboli volcano,
in terms of their optical properties and emission flux rates, carried out during the PEACETIME
oceanographic campaign. Using sun-photometric observations realised during a near-ideal full
plume crossing, a plume-isolated aerosol optical depth of 0.07–0.08 in the shorter-wavelength visible
range, decreasing to about 0.02 in the near infrared range, was found. An Ångström exponent of
1.40 0.40 was also derived. This value may suggest the dominant presence of sulphate aerosols
with a minor presence of ash. During the crossing, two separate plume sections were identified, one
possibly slightly affected by ash coming from a mild explosion, and the other more likely composed
of pure sulphate aerosols. Exploiting the full crossing scan of the plume, an aerosol emission flux rate
of 9–13 kg/s was estimated. This value was 50% larger than for typical passively degassing volcanoes,
thus pointing to the importance of mild explosions for aerosol emissions in the atmosphere.