Mt. Etna sulfur dioxide flux monitoring using ASTER-TIR data and atmospheric observations

This work is aimed at estimating the sulfur dioxide emission of Mt. Etna volcano (Sicily, Italy) using the thermal infrared images remotely sensed by the Advanced Spacebome Thermal Emission and Reflection Radiometer (ASTER). A new procedure (named FUN) is presented based on approximating functions which represent the atmospheric terms of the radiative transfer equation: transmittance, up-welling and down-welling radiances. The parameters of the approximating functions have been computed through the MODTRAN simulations performed using atmospheric monthly mean profiles measured at Trapani (1989-2003). Trapani (WMO station 16429) is located at the western tip of Sicily and is the closest station to Mt. Etna volcano. With these functions MODTRAN is no longer necessary to compute the SO2 abundance in the plume emitted from Mt. Etna and seen by ASTER. Only an atmospheric radiosounding and a digital elevation model (DEM) registered to the ASTER image are required. This new procedure is compared with the known look-up tables (LUT) procedure proposed by other authors. An interesting aspect of the FUN procedure derives from its combined use with a split window (SW) algorithm. In this case only radiosounding is required to compute the SO2 map. The FUN procedure presented in this paper is a rapid, simple and accurate means of generating SO2 map estimates from the ASTER view of Mt. Etna. The proposed scheme can easily be adapted to another sensor and another volcano. In this paper the results from two ASTER images, 19 July 2003 and 29 July 2001, are presented. In July 2003, a ground truth campaign was deployed, SO2 emission was very low (10-25 kg s(-1); Etna baseline is about 50 kg s(-1)). In July 2001, Mt. Etna erupted and a strong SO2 emission (about 140 kg s(-1)) was measured.