Elisa Carboni

Mt. Etna tropospheric ash retrieval and sensitivity analysis using moderate resolution imaging spectroradiometer measurements

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.

Satellite detection, long-range transport, and air quality impacts of volcanic sulfur dioxide from the 2014-2015 flood lava eruption at Bárðarbunga (Iceland)

The 2014–2015 Barðarbunga-Veiðivotn fissure eruption at Holuhraun produced about 1.5 km3 of lava, making it the largest eruption in Iceland in more than 200 years. Over the course of the eruption, daily volcanic sulfur dioxide (SO2) emissions exceeded daily SO2 emissions from all anthropogenic sources in Europe in 2010 by at least a factor of 3. We present surface air quality observations from across Northern Europe together with satellite remote sensing data and model simulations of volcanic SO2 for September 2014. We show that volcanic SO2 was transported in the lowermost troposphere over long distances and detected by air quality monitoring stations up to 2750 km away from the source. Using retrievals from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), we calculate an average daily SO2 mass burden of 99 ± 49 kilotons (kt) of SO2 from OMI and 61 ± 18 kt of SO2 from IASI for September 2014. This volcanic burden is at least a factor of 2 greater than the average SO2 mass burden between 2007 and 2009 due to anthropogenic emissions from the whole of Europe. Combining the observational data with model simulations using the United Kingdom Met Offices Numerical Atmospheric-dispersion Modelling Environment model, we are able to constrain SO2 emission rates to up to 120 kilotons per day (kt/d) during early September 2014, followed by a decrease to 20–60 kt/d between 6 and 22 September 2014, followed by a renewed increase to 60–120 kt/d until the end of September 2014. Based on these fluxes, we estimate that the eruption emitted a total of 2.0 ± 0.6 Tg of SO2 during September 2014, in good agreement with ground-based remote sensing and petrological estimates. Although satellite-derived and model-simulated vertical column densities of SO2 agree well, the model simulations are biased low by up to a factor of 8 when compared to surface observations of volcanic SO2 on 6–7 September 2014 in Ireland. These biases are mainly due to relatively small horizontal and vertical positional errors in the simulations of the volcanic plume occurring over transport distances of thousands of kilometers. Although the volcanic air pollution episodes were transient and lava-dominated volcanic eruptions are sporadic events, the observations suggest that (i) during an eruption, volcanic SO2 measurements should be assimilated for near real-time air quality forecasting and (ii) existing air quality monitoring networks should be retained or extended to monitor SO2 and other volcanic pollutants.

Correcting the record of volcanic stratospheric aerosol impact: Nabro and Sarychev Peak

Since 2010, several papers have been published that reveal a pattern of discrepancies between stratospheric aerosol data from the Optical Spectrograph and Infrared Imaging System (OSIRIS) instrument and other measurements and model simulations of volcanic plumes from Kasatochi, Sarychev Peak, and Nabro volcanoes. OSIRIS measurements show two discrepancies, a posteruption lag in aerosol onset/increase and a low bias in maximum stratospheric aerosol optical depth. Assumed robustness of the OSIRIS data drove various conclusions, some controversial, such as the contention that the June 2011 Nabro plume was strictly tropospheric, and entered the stratosphere indirectly via the Asian monsoon. Those conclusions were driven by OSIRIS data and a Smithsonian Institution report of strictly tropospheric injection heights. We address the issue of Nabros eruption chronology and injection height, and the reasons for the OSIRIS aerosol discrepancies. We lay out the time line of Nabro injection height with geostationary image data, and stratospheric plume evolution after eruption onset using retrievals of sulfur dioxide and sulfate aerosol. The observations show that Nabro injected sulfur directly to or above the tropopause upon the initial eruption on 12/13 June and again on 16 June 2011. Next, OSIRIS data are examined for nonvolcanic and volcanically perturbed conditions. In nonvolcanic conditions OSIRIS profiles systematically terminate 1–4 km above the tropopause. Additionally, OSIRIS profiles terminate when 750 nm aerosol extinction exceeds ∼0.0025 km−1, a level that is commonly exceeded after volcanic injections. Our findings largely resolve the discrepancies in published works involving OSIRIS aerosol data and offer a correction to the Nabro injection-height and eruption chronology.

Measuring volcanic plume and ash properties from space

Abstract The remote sensing of volcanic ash plumes from space can provide a warning of an aviation hazard and knowledge on eruption processes and radiative effects. In this paper new algorithms are presented to provide volcanic plume properties from measurements by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), the Advanced Along Track Scanning Radiometer (AATSR) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). A challenge of remote sensing is to provide near-real-time methods to identify, and so warn of, the presence of volcanic ash. To achieve this, a singular vector decomposition method has been developed for the MIPAS instrument on board the Environmental Satellite. This method was applied to observations of the ash clouds from the eruptions of Nabro and the Puyehue–Cordón Caulle in 2011 and led to a sensitive volcanic signal flag which was capable of tracking changes in the volcanic signal spectra as the plume evolved. A second challenge for remote sensing is to identify the ash plume height. This is a critical parameter for the initialization of algorithms that numerically model the evolution and transport of a volcanic plume. As MIPAS is a limb sounder, the identification of ash also provides an estimate of height provided the plume is above about 6 km. This is complemented by a new algorithm, Stereo Ash Plume Height Retrieval Algorithm, that identifies plume height using the parallax between images provided by Along Track Scanning Radiometer-type instruments. The algorithm was tested on an image taken at 14:01 GMT on 6 June 2011 of the Puyehue–Cordón Caulle eruption plume and gave a height of 11.9±1.4 km, which agreed with the value derived from the location of the plume shadow (12.7±1.8 km). This plume height was similar to the height observed by MIPAS (12 ± 1.5 km) at 02:56 GMT on 6 June. The quantitative use of satellite imagery and the full exploitation of high-resolution spectral measurements of ash depends upon knowing the optical properties of the observed ash. Laboratory measurements of ash from the 1993 eruption of Mt Aso, Japan have been used to determine the refractive indices from 1 to 20 µm. These preliminary measurements have spectral features similar to ash values that have been used to date, albeit with slightly different positions and strengths of the absorption bands. The refractive indices have been used to retrieve ash properties (plume height, optical depth and ash effective radius) from AATSR and SEVIRI instruments using two versions of Oxford-RAL Retrieval of Aerosol and Cloud (ORAC) algorithm. For AATSR a new ash cloud type was used in ORAC for the analysis of the plume from the 2011 Eyjafjallajökull eruption. For the first c. 500 km of the plume ORAC gave values for plume height of 2.5–6.5 km, optical depth 1–2.5 and effective radius 3–7 µm, which are in agreement with other observations. A weakness of the algorithm occurs when underlying cloud invalidates the assumption of a single cloud layer. This is rectified in a modified version of ORAC applied to SEVIRI measurements. In this case an extra model of a cloud underlying the ash plume was included in the range of applied models. In cases where the plume overlay cloud, this new model worked well, showing good agreement with correlative Cloud–Aerosol Lidar with Orthogonal Polarization observations.

Oxford-RAL Aerosol and Cloud (ORAC): aerosol retrievals from satellite radiometers

This chapter describes an optimal estimation retrieval scheme for the derivation of the properties of atmospheric aerosol from top-of-atmosphere (TOA) radiances measured by satellite-borne visible-IR radiometers. The algorithm makes up part of the Oxford-RAL Aerosol and Cloud (ORAC) retrieval scheme (the other part of the algorithm performs cloud retrievals and is described in detail elsewhere [by Watts et al.] [37]).

A Multi-Sensor Approach for Volcanic Ash Cloud Retrieval and Eruption Characterization: The 23 November 2013 Etna Lava Fountain

Volcanic activity is observed worldwide with a variety of ground and space-based remote sensing instruments, each with advantages and drawbacks. No single system can give a comprehensive description of eruptive activity, and so, a multi-sensor approach is required. This work integrates infrared and microwave volcanic ash retrievals obtained from the geostationary Meteosat Second Generation (MSG)-Spinning Enhanced Visible and Infrared Imager (SEVIRI), the polar-orbiting Aqua-MODIS and ground-based weather radar. The expected outcomes are improvements in satellite volcanic ash cloud retrieval (altitude, mass, aerosol optical depth and effective radius), the generation of new satellite products (ash concentration and particle number density in the thermal infrared) and better characterization of volcanic eruptions (plume altitude, total ash mass erupted and particle number density from thermal infrared to microwave). This approach is the core of the multi-platform volcanic ash cloud estimation procedure being developed within the European FP7-APhoRISM project. The Mt. Etna (Sicily, Italy) volcano lava fountaining event of 23 November 2013 was considered as a test case. The results of the integration show the presence of two volcanic cloud layers at different altitudes. The improvement of the volcanic ash cloud altitude leads to a mean difference between the SEVIRI ash mass estimations, before and after the integration, of about the 30%. Moreover, the percentage of the airborne “fine” ash retrieved from the satellite is estimated to be about 1%–2% of the total ash emitted during the eruption. Finally, all of the estimated parameters (volcanic ash cloud altitude, thickness and total mass) were also validated with ground-based visible camera measurements, HYSPLIT forward trajectories, Infrared Atmospheric Sounding Interferometer (IASI) satellite data and tephra deposits.

Aerosol maps from GOME data

In this paper, we present a methodology to calibrate the surface reflectance seen by satellite and validate the aerosol optical properties retrieved by the GOME instrument. Data are also visualized in maps by a tool properly developed, named GOMEView. The validation procedure is based on ground measurements obtained by sunphotometers. Results show that calibration of the surface reflectance is crucial to obtain the best results, i.e. in agreement with the ground measurements. Aerosol data have also been classified on the basis of their optical properties evidencing for instance, the presence of desert aerosol over the sea along the west coast of Sahara. Cloud retrievals were also analyzed in terms of their occurrence and amount.

Stereoscopic Estimation of Volcanic Ash Cloud-Top Height from Two Geostationary Satellites

The characterization of volcanic ash clouds released into the atmosphere during explosive eruptions includes cloud height as a fundamental physical parameter. A novel application is proposed of a method based on parallax data acquired from two geostationary instruments for estimating ash cloud-top height (ACTH). An improved version of the method with a detailed discussion of height retrieval accuracy was applied to estimate ACTH from two datasets acquired by two satellites in favorable positions to fully exploit the parallax effect. A combination of MSG SEVIRI (HRV band; 1000 m nadir spatial resolution, 5 min temporal resolution) and Meteosat-7 MVIRI (VIS band, 2500 m nadir spatial resolution, 30 min temporal resolution) was implemented. Since MVIRI does not acquire data at exactly the same time as SEVIRI, a correction procedure enables compensation for wind advection in the atmosphere. The method was applied to the Mt. Etna, Sicily, Italy, eruption of 23 November 2013. The height of the volcanic cloud was tracked with a top height of ~8.5 km. The ash cloud estimate was applied to the visible channels to show the potential accuracy that will soon be achievable also in the infrared range using the next generation of multispectral imagers. The new constellation of geostationary meteorological satellites will enable full exploitation of this technique for continuous global ACTH monitoring.

Theoretical study on SO2 and ash volcanic plume retrievals using ground TIR camera. Sensitivity analysis and retrieval procedure developments

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.

Theoretical Study on Volcanic Plume

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.