A Novel Approach for Volcanic Ash and SO2 Retrieval and Flux Reconstruction using SEVIRI Data.
Merucci, Luca1; Corradini, Stefano2; Pugnaghi, Sergio3; Guerrieri, Lorenzo3; Arvani, Barbara3
1Istituto Nazionale di Geofisica e Vulcanologia, ITALY; 2INGV, ITALY; 3University of Modena and Reggio Emilia, ITALY

During volcanic eruptions large amounts of ash and gases are injected into the atmosphere. Depending on eruptive intensity the volcanic ash can reach different altitudes in the atmosphere and its residence time depends on particle size. Volcanic ash falls cause respiratory problems, damage to agricultural and industrial activities, while the suspended ash impact the climate and represent a severe threat to aviation security.
Volcanic SO2 has an impact on the environment and when injected at high altitudes can be oxidized to form sulphates capable of reflecting solar radiation then causing surface cooling. Observations of the volcanic degassing also yield insights into the magmatic processes which control volcanic activity during both quiescent and eruptive phases.
Since volcanoes are globally distributed and the volcanic clouds could have large horizontal spread, the satellite measurements offer a unique and practical platform from which to make observations. In particular the high repetition time measurements of the geostationary satellite instruments, permit to entirely follow the volcanic eruption evolution.

We present here the application to SEVIRI thermal infrared (TIR) data of a novel procedure for the simultaneous retrieval of SO2 and ash abundances in a volcanic plume formerly described for MODIS images. This procedure is simple, extremely fast and requires as inputs only the plume altitude and temperature.
The new procedure is based on the volcanic plume removal (VPR): the background radiance obtained by linear interpolation of the radiance measured in the area surrounding the plume is first calculated and then subtracted to the original SEVIRI image. This difference highlights the plume area and allows the computation of the plume transmittances in the TIR-SEVIRI bands affected by SO2 and ash absorption, i.e. the bands centered at 8.6, 11.0 and 12.0 µm.
The plume transmittances are calculated in two steps: (1) by modeling a simple single layer plume of fixed altitude and thickness, neglecting the atmosphere above it; (2) refining this first approximation with polynomial relationships that include the geographical region, the local meteorology and the ash type. From the SEVIRI bands centered at 11.0 and 12.0 ìm (SO2 transparent bands) transmittances, the ash particle effective radius (Re) and the aerosol optical depth at 550 nm (AOD550) are retrieved. Then, the SO2 columnar content is retrieved by means of a simple relation between the transmittances of the ash and SO2 absorbing bands. The VPR procedure can be applied to different volcanoes as well as extended to other ash types by re-computing the polynomial parameters. The results of VPR procedure retrieval will be used for the reconstruction of SO2 and ash flux time series of Mt.Etna sample eruptions. By exploiting the high repetition cycle of SEVIRI (15 minutes) the volcanic emitted flux can be measured, allowing the study of short-term variations of gas and ash emissions from space-based data. While being a novel tool per se, the volcanic flux time series reconstruction lays the foundation for further innovative investigations and outcomes.