Sulphur dioxide (SO₂) fluxes of active degassing volcanoes are routinely measured with ground-based equipment to characterize and monitor volcanic activity. SO₂ of unmonitored volcanoes or from explosive volcanic eruptions, can be measured with satellites. However, remote-sensing methods based on absorption spectroscopy generally provide integrated amounts of already dispersed plumes of SO₂ and satellite derived flux estimates are rarely reported. Here we review a number of different techniques to derive volcanic SO₂ fluxes using satellite measurements of plumes of SO₂ and investigate the temporal evolution of the total emissions of SO₂ for three very different volcanic events in 2011: Puyehue-Cordón Caulle (Chile), Nyamulagira (DR Congo) and Nabro (Eritrea). High spectral resolution satellite instruments operating both in the ultraviolet-visible (OMI/Aura and GOME-2/MetOp-A) and thermal infrared (IASI/MetOp-A) spectral ranges, and multispectral satellite instruments operating in the thermal infrared (MODIS/Terra-Aqua) are used. We show that satellite data can provide fluxes with a sampling of a day or less (few hours in the best case). Generally the flux results from the different methods are consistent, and we discuss the advantages and weaknesses of each technique. Although the primary objective of this study is the calculation of SO₂ fluxes, it also enables us to assess the consistency of the SO₂ products from the different sensors used.

A retrieval of tropospheric volcanic ash from Mt Etna has been carried out, using measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS). The NASA-MODIS satellite instrument acquires images in the 0.4 to 14 μm spectral range with a spatial resolution of 1 km at nadir. The eruption which occurred on 24 November 2006 is considered as a test case in this work. In order to derive the ash plume optical thickness, the particle effective radius and the total mass, the Brightness Temperature Difference procedure has been applied to MODIS channels 31 (centered at 11 μm) and 32 (centered at 12 μm). Channel 5 (centered at 1.24 μm) has been used to refine the cloud discrimination, exploiting the distinct reflectivity of meteorological and volcanic clouds in the near infrared spectral range. The detection of volcanic ash pixels has been significantly improved by applying an atmospheric water vapor correction to MODIS data. This procedure doubles the number of pixels identified as containing volcanic ash compared to the original method. The retrieved mean ash optical thickness at 0.55 μm, mean particle effective radius and the total ash mass in the plume are 0.4, 3.5 μm and 3620 tons, respectively. A detailed sensitivity analysis has been carried out to investigate errors in the retrieval caused by the uncertainty in various parameters: surface temperature and emissivity, plume geometry (altitude and thickness), ash type and atmospheric water vapor. Results show that the largest contributions to retrieval errors are from uncertainty in surface parameters, aerosol type and atmospheric water vapor. The total tropospheric volcanic ash retrieval errors are estimated to be 30%, 30% and 40% for mean AOT, mean effective radius and total mass retrieval, respectively.

In this paper, we use calibrated images collected by the video-surveillance system of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, to retrieve the height of the eruption column during the recent Etna explosive activity. The analysis is carried out on nineteen lava fountains from the New South East Crater dataset. The novel procedure described in this work is achieved in three main steps: at first we calibrated the camera, then we selected the images which recorded the maximum phase of the eruptive activity, and finally we applied an appropriate correction to account for the plume projection on the camera line of sight due to the wind. The results show that the column altitudes range between 6 and 9 km (upper limit of the camera system). The comparison with the plume height values estimated from the analysis of several SEVIRI and MODIS satellite images, show a good agreement. Finally, for nine events we also evaluated the thickness of the volcanic plumes in the umbrella region which ranges between 2 and 3 km.

During explosive eruptions, emergency responders and government agencies need to make fast decisions that should be based on an accurate forecast of tephra dispersal and assessment of the expected impact. Here, we propose a new operational tephra fallout monitoring and forecasting system based on quantitative volcanological observations and modelling. The new system runs at the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo (INGV-OE) and is able to provide a reliable hazard assessment to the National Department of Civil Protection (DPC) during explosive eruptions. The new operational system combines data from low-cost calibrated visible cameras and satellite images to estimate the variation of column height with time and model volcanic plume and fallout in near-real-time (NRT). The new system has three main objectives: (i) to determine column height in NRT using multiple sensors (calibrated cameras and satellite images); (ii) to compute isomass and isopleth maps of tephra deposits in NRT; (iii) to help the DPC to best select the eruption scenarios run daily by INGV-OE every three hours. A particular novel feature of the new system is the computation of an isopleth map, which helps to identify the region of sedimentation of large clasts (≥5 cm) that could cause injuries to tourists, hikers, guides, and scientists, as well as damage buildings in the proximity of the summit craters. The proposed system could be easily adapted to other volcano observatories worldwide.

The simultaneous presence of SO₂ and ash in a volcanic plume can lead to a significant error in the SO₂ column abundance retrieval when multispectral Thermal InfraRed (TIR) data are used. The ash particles within the plume with effective radii from 1 to 10 μm reduce the Top Of Atmosphere (TOA) radiance in the entire TIR spectral range, including the channels used for SO₂ retrieval. The net effect is a significant SO₂ overestimation. In this work the interference of ash is discussed and two correction procedures for satellite SO₂ volcanic plume retrieval in the TIR spectral range are developed to achieve an higher computational speed and a better accuracy. The ash correction can be applied when the sensor spectral range includes the 7.3 and/or 8.7 μm SO₂ absorption bands, and the split window bands centered around 11 and 12 μm required for ash retrieval. This allows the possibility of simultaneous estimation of both volcanic SO₂ and ash in the same data set. The proposed ash correction procedures have been applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Spin Enhanced Visible and Infrared Imager (SEVIRI) measurements. Data collected during the 24 November 2006 Mt. Etna eruption have been used to illustrate the technique. The SO₂ and ash estimation is carried out by using a best weighted least squares fit method and the Brightness Temperature Difference (BTD) procedures, respectively. The simulated TOA radiance Look-Up Table (LUT) needed for the SO₂ column abundance and the ash retrievals have been computed using the MODTRAN 4 Radiative Transfer Model. The results show the importance of the ash correction on SO₂ retrievals at 8.7 μm, where the corrected SO₂ column abundance values are less than 50% of the uncorrected values. The ash correction on SO₂ retrieval at 7.3 μm is much less important and only significant for low SO₂ column abundances. Results also show that the simplified and faster correction procedure underestimates the ash correction compared with the more time consuming but more accurate correction procedure. Such underestimation is greater for instruments having better ground pixel resolution, i.e. greater for MODIS than for SEVIRI.