In this work the volcanic ash and SO2 retrievals obtained by applying three different procedures (LUT - Look Up Table, NN - Neural Network and VPR - Volcanic Plume Removal) on MODIS TIR synthetic measurements have been compared. The synthetic measurements are generated using MODTRAN RTM for defined volcanic cloud configurations. The results, presented as the percentage difference between the retrieved values and the true values used for the synthetic data generation, indicate maximum differences of +/- 15% and +/- 10% for all the procedures and for ash and SO2 retrievals respectively. A sensitivity analysis has been also realized to investigate the influence of volcanic cloud altitude and water vapour profile on SO2 retrievals at 7.3 and 8.6 micron. Results confirm the high sensitivity of the 7.3 micron retrieval to the volcanic cloud altitude and show that the SO2 total masses estimated at 7.3 and 8.6 micron separately can be used to improve the information on the volcanic cloud altitude. Finally, the water vapour profile is used to compute the minimum altitude over which the 7.3 m retrieval is effective.

A new procedure is presented for simultaneous estimation of SO2 and ash abundance in a volcanic plume, using thermal infrared (TIR) MODIS data. Plume altitude and temperature are the only two input parameters required to run the procedure, while surface emissivity, temperature, atmospheric profiles, ash optical properties, and radiative transfer models are not necessary to perform the atmospheric corrections. The procedure gives the most reliable results when the surface under the plume is uniform, for example above the ocean, but still produces fairly good estimates in more challenging and not easily modelled conditions, such as above land or meteorological cloud layers. The developed approach was tested on the Etna volcano. By linearly interpolating the radiances surrounding a detected volcanic plume, the volcanic plume removal (VPR) procedure described here computes the radiances that would have been measured by the sensor in the absence of a plume, and reconstructs a new image without plume. The new image and the original data allow computation of plume transmittance in the TIR-MODIS bands 29, 31, and 32 (8.6, 11.0 and 12.0 μm) by applying a simplified model consisting of a uniform plume at a fixed altitude and temperature. The transmittances are then refined with a polynomial relationship obtained by means of MODTRAN simulations adapted for the geographical region, ash type, and atmospheric profiles. Bands 31 and 32 are SO2 transparent and, from their transmittances, the effective ash particle radius (Re), and aerosol optical depth at 550 nm (AOD550) are computed. A simple relation between the ash transmittances of bands 31 and 29 is demonstrated and used for SO2 columnar content (cs) estimation. Comparing the results of the VPR procedure with MODTRAN simulations for more than 200 000 different cases, the frequency distribution of the differences shows the following: the Re error is less than ±0.5 μm in more than 60% of cases; the AOD550 error is less than ±0.125 in 80% of cases; the cs error is less than ±0.5 g m−2 in more than 60% of considered cases. The VPR procedure was applied in two case studies of recent eruptions occurring at the Mt Etna volcano, Italy, and successfully compared with the results obtained from the established SO2 and ash assessments based on look-up tables (LUTs). Assessment of the sensitivity to the plume altitude uncertainty is also made. The VPR procedure is simple, extremely fast, and can be adapted to other ash types and different volcanoes.

Abstract. A new procedure for the simultaneous estimation of SO2 and ash abundances in a volcanic plume using thermal infrared (TIR) MODIS data is presented. Plume altitude and temperature are the only two input parameters needed to run the procedure, while surface emissivity, atmospheric profiles and radiative transfer models are not required to perform the atmospheric corrections. The proposed space-based retrievals are simple, extremely fast and can be easily extended and applied to any volcano. By linearly interpolating the radiances of the edges of the detected volcanic plume, the Volcanic Plume Removal (VPR) procedure here described, computes the radiances that would have been measured at the sensor if the plume was missing and reconstructs a new image without the plume. The comparison of the new image with the original data containing the plume highlights the plume presence and allows the computation of the plume transmittance in three TIR-MODIS bands: 29, 31 and 32 (8.6, 11.0 and 12.0 μm). The procedure results are very good when the surface under the plume is rather uniform, as it is often the case with plume widths of few tens of kilometers. As a consequence it works very well when the plume is above the sea, but still produces fairly good estimates in more challenging and not easily modeled conditions, such as images with land or uniform cloud layers under the plume. In the aforementioned bands the plume transmittances are derived in two steps: (1) using a simple model with the plume at a fixed altitude and neglecting the layer of atmosphere above it; (2) refining the first result with a polynomial relationship obtained by means of MODTRAN simulations adapted for the geographical region, the ash type and the atmospheric profiles. Bands 31 and 32 are SO2 transparent and, from their transmittances, the ash particle effective radius (Re) and the aerosol optical depth at 550 nm (AOD550) are computed. A simple relation between the ash transmittances of bands 31 and 29 is demonstrated and used for the SO2 columnar content estimation. Comparing the results of the VPR procedure with the MODTRAN simulations for more than 200 thousands different cases, the frequency distribution of the differences says that: the Re error is less than ±0.5 μm in more than the 60% of the cases; the AOD550 error is less than ±0.125 in the 80% of the cases; the SO2 error is less than ±0.5 g m−2 in more than the 60% of the considered cases. The VPR procedure has been applied in two case studies of recent eruptions occurred at Mt. Etna volcano, Italy and successfully compared with the results obtained with the well known SO2 and ash retrievals based look-up tables (LUTs). By recomputing the parameters of the polynomial relationship, the VPR procedure can be easily extended to other ash types and applied to different volcanoes.

Aerosol optical properties, shortwave (SW) and longwave (LW) irradiances, were measured at the island of Lampedusa during a very intense Saharan dust event in March 2010. The dust optical depth at 500 nm reached the 10 year record value of 1.9 on 25 March. Outgoing radiative fluxes from the Clouds and the Earth's Radiant Energy System were used to derive the dust radiative effect at the top of the atmosphere (TOA). The SW and LW radiative forcings (RFs) over the sea were derived by combining irradiances measured during the dust event and on a pristine day, and radiative transfer calculations. At the satellite overpass (solar zenith angle of about 35 degrees) the SW instantaneous RF was -209 W m(-2) at the surface, -116 W m(-2) at TOA, and +93 W m(-2) in the atmosphere; the diffuse SW downward irradiance increased by up to 376 W m(-2) with respect to the dust-free case. The LW RF was as large as +41.5 W m(-2) at the surface, +20 W m(-2) at TOA, and -22 W m(-2) in the atmosphere. The LW forcing offsets about 20% of the SW instantaneous forcing at the surface, about 17% at TOA, and 24% in the atmosphere. It is estimated that on a daily basis the LW radiative forcing offsets 49% of the SW effect at the surface, 35% at TOA, and about 77% in the atmosphere, thus compensating for a large fraction of the SW heating.

In this paper, a sensitivity analysis and procedure development for volcanic-plume sulfur dioxide and ash retrievals using ground thermal infrared camera have been carried out. The semiconductor device camera, considered as a reference, has a spectral range of 8-14 ¿m with noise equivalent temperature difference that is better than 100 mK at 300 K. The camera will be used to monitor and assess the hazards of Mt. Etna volcano to mitigate the risk and impact of volcanic eruptions on the civil society and transports. A minimum number of filters have been selected for sulfur dioxide (SO2) and volcanic ash retrievals. The sensitivity study has been carried out to determine the SO2 and volcanic ash minimum concentration detectable by the system varying the camera geometry and the atmospheric profiles. Results show a meaningful sensitivity increase considering high instrument altitudes and low camera-elevation angles. For all geometry configurations and monthly profiles, the sensitivity limit varies between 0.5 and 2 g ·m-2 for SO2 columnar abundance and between 0.02 and 1 for ash optical depth. Two procedures to detect SO2 and ash, based on the least square fit method and on the brightness temperature difference (BTD) algorithm, respectively, have also been proposed. Results show that high concentration of atmospheric water vapor columnar content significantly reduces the ash-plume effect on the BTD. A water vapor-correction procedure introduced improves the ash retrievals and the cloud discrimination in every season, considering all the camera geometries.

GPS observations, distances from satellites to receivers and meteorological conditions in neutral atmosphere are known to obey a constraint, which provides a residual or in other words a quality index. A method is discussed which provides a residual epoch by epoch in near real time. In general, distribution of residuals during several consecutive epochs belonging to the same satellites, allows estimates of a mean and a standard deviation of mean. Under normal meteorological conditions distribution of residuals appears to be consistent with zero mean as expected. However, consecutive residuals sometimes appear to have a mean different from zero by more than three standard deviations of mean. Such signifi cant consecutive epochs provide a warning of existing inconsistencies among GPS observations, distances from satellites to receivers as obtained by orbital information, meteorological conditions above receivers (as obtained by ground measurements or by extrapolation of meteorological analysis). A procedure has been set up which warns about these inconsistencies in near real time.

From 16 to 26 July 2003 an extensive field campaign was carried out around Mt Etna, Sicily, Italy. During the campaign a new airborne system, Fire Airborne Spectral Analyzer (FASA), was tested. The main instrument of the payload is a high resolution Michelson Interferometer with Rotating Retroreflector (MIROR), which operates in the nominal infrared 600-3000 cm(-1) spectral range with a high resolution of 0.12 cm(-1). This work investigates the feasibility of using MIROR data to retrieve the surface temperature, the hyper-spectral emissivity and the SO2 volcanic plume abundance. The results for the surface parameters show a good agreement with those obtained by satellite data and on ground measurements. In particular, the spectral emissivity shows a meaningful minimum, around 1040 cm(-1), consistent with the basaltic properties of Mt Etna's surface. A sensitivity study has also been carried out to show the difficulties of SO2 columnar abundance retrieval due to the too low SO2 volcanic emission and the too high instrumental noise.

The Spinning Enhanced Visible And Infra Red Imager (SEVIRI) radiometer, on board on Meteosat Second Generation(MSG) geostationary satellite, collects, each 15 minutes, images of the underneath part of the globe in 12 spectral bands with a spatial resolution of 3 km. In this work the Aerosol Optical Thickness (AOT) retrieval over land using SEVIRI data is presented. AOT at 0.55 mu m is estimated minimizing the difference between measured and computed radiances in the visible channel centered at 0.6 mu m by means Look-Up Tables (LUT) obtained using 6S radiative transfer code. The 0.6 pm surface reflectance has been computed using different procedures based on SEVIRI channels 3 and 4 centered respectively around 1.6 and 3.9 mu m. For the 0.6 pm surface reflectance retrieval using the 1.6 mu m procedure, the measurements of five automatic sun-photometers of the Aerosols Robotic Network (AERONET) located in the Mediterranean area (Avignon, Laegeren, Modena, Rome and Lecce) has been used. The procedures show encouraging results in case of 1.6 pm procedure retrieval and the inadequacy of 3.9 pm procedure. An AOT map of the Po Valley (Italy), obtained from an MSG image taken during a typical winter polluted day, is shown in the paper and compared with MODIS retrieval.

This paper focuses on three eruptive events of the Mt. Etna volcano: July 22nd 1998, April 26th 2000 and the recent eruption of July-August 2001. Such eruptions may be a severe threat to aircraft safety, as in the April 2000 event. From the AVHRR visible images the height of the top of the clouds is estimated, using geometrical methods, knowing both NOAA satellite and Sun positions. The results are then compared with information derived from radio-sounding data etc.. The volcanic ash particles with diameters of 1-10 micron are not detectable by aircraft radar but they may be remotely sensed using thermal infrared data. The well-known algorithm, based on the AVHRR channel 4 and channel 5 brightness temperatures difference [Prata, 1989; Schneider et al. 1994], is here applied to highlight the ash clouds of Mt. Etna volcano. Even though it was typically used to detect and follow the volcanic clouds of stratospheric eruptions, here it is succesfully tested for tropospheric plume too. Some good results of this technique are presented together with some basic problems. This work points out that it could be useful to prepare a procedure to monitor Mt. Etna eruption clouds analysing TIR data. Such a procedure should automatically alert ( in real time, using the new Meteosat Second Generation satellite) and indicate the cloud direction on the basis of atmospheric radio-sounded and/or predicted data.