Measuring volcanic degassing of SO2 in the lower troposphere with ASTER band ratios

We present a new method for measuring SO2 with the data from the ASTER (Advanced Spaceborne Thermal
Emission and Reflectance radiometer) orbital sensor. The method consists of adjusting the SO2 column
amount until the ratios of radiance simulated on several ASTER bands match the observations. We present a
sensitivity analysis for this method, and two case studies. The sensitivity analysis shows that the selected
band ratios depend much less on atmospheric humidity, sulfate aerosols, surface altitude and emissivity than
the raw radiances. Measurements with b25% relative precision are achieved, but only when the thermal
contrast between the plume and the underlying surface is higher than 10 K. For the case studies we focused
on Miyakejima and Etna, two volcanoes where SO2 is measured regularly by COSPEC or scanning DOAS. The
SO2 fluxes computed from a series of ten images of Miyakejima over the period 2000–2002 is in agreement
with the long term trend of measurement for this volcano. On Etna, we compared SO2 column amounts
measured by ASTER with those acquired simultaneously by ground-based automated scanning DOAS. The
column amounts compare quite well, providing a more rigorous validation of the method. The SO2 maps
retrieved with ASTER can provide quantitative insights into the 2D structure of non-eruptive volcanic
plumes, their dispersion and their progressive depletion in SO2.

R.C. was supported by a grant from F.R.I.A (Fond pour la Recherche
Industrielle et Appliquée). GGS acknowledges a PhD grant funded by
the project “Sviluppo di sistemi di monitoraggio” funded by
Dipartimento di Protezione Civile della Regione Sicilia, INGV (Istituto
Nazionale di Geofisica e Vulcanologia, sezione di Catania—Italy) and
NOVAC (Network for Observation of Volcanic and Atmospheric
Change) EU-funded FP6 project no. 18354. P-F. C. is research associate
with FRS-FNRS and benefited from its financial support (F.4511.08).

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