Abstract
This work focuses on the evaluation of Aerosol Optical Thickness (AOT) in Mt. Etna volcano area starting from the analysis of MIVIS VIS images. MIVIS images and ancillary data (atmospheric profiles, photometric measurements, atmospheric infrared radiances, surface temperatures, ground reflectances, SO2 abundances) were collected during the «Sicily 97» campaign. Data elaboration was performed with extensive use of 6S radiative transfer model, determining optical thickness with an inversion algorithm that uses atmospheric vertical profile, ground reflectance data and radiance measured by the first MIVIS spectrometer (channels 1-20; range 0.44-0.82 n). Ground reflectance is the most problematic parameter for the algorithm. In order to have a low and uniform surface reflectance, only pixels located at an altitude between 2000-3000 m a.s.l. were analysed. At this altitude,AOT is very low during non-eruptive periods: at Torre del Filosofo (2920 m a.s.l.) on June 16th 1997, during one MIVIS flight, AOT at 0.55 n was 0.19. The uncertainty about ground reflectance produces significant errors on volcanic background AOT, and in some cases the error is up to 100%. The developed algorithm worked well on volcanic plume, allowing us to determine the plume related pixelsAOT. High plume AOT values minimize the problems deriving from reflectance uncertainty. Plume optical thickness shows values included in a range from 0.5 to 1.0. The plume AOT map of Mt. Etna volcano, derived from a MIVIS image of June 16th 1997, is presented.
Highlights
MIVIS is a modular instrument formed by four spectrometers (Daedalus, 1996), with 102 channels ranging from VIS to TIR
Some observations: 1) for ashes, FieldSpec gives very low reflectance, with a small spectral variation, while MIVIS/6S gives higher values; 2) for lava, FieldSpec reflectances are highest in the graph, while MIVIS/6S values are very low, with an opposite effect compared with ashes; 3) lines shape for FieldSpec and MIVIS/6S is clearly different
High plume Aerosol Optical Thickness (AOT) values minimize the problems deriving from reflectance uncertainty, which entails high relative error outside plume
Summary
Mt. Etna is the largest volcano in Europe and one of the most active. Etna is the largest volcano in Europe and one of the most active It is characterised by significant lava flow and periodically strong eruptive activity. Mt. Etna volcano inject in atmosphere great quantity of gases (more than 10% of global volcanic injection for H2O, CO2 and SO2) and aerosol, during non-eruptive periods. Aerosol characteristics can be estimated using remote sensing, in addition to ashes ground precipitation measurements and sampling after eruptions. Aerosol with 1-10 nm diameter can be identified from AVHRR or similar satellite radiometers, using TIR channels brilliance temperature difference (Prata, 1989; Wen and Rose, 1994; Ackerman, 1997; Schneider et al, 1999; Remitti et al, 2006), while liquid or solid aerosol with smaller diameter is not recognisable with such a procedure (Simpson et al, 2000)
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