A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO<sub>2</sub> Content

Prata, G. S.; Ventress, Lucy J.; Carboni, Elisa; Mather, Tamsin A.; Grainger, R. G.; Pyle, David M.

doi:10.1029/2018jd028679

Cited by 30 publications

(24 citation statements)

References 51 publications

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“…For this sample, we used the empirical relation obtained in Ref. [31] between the real part of the CRI and the weight percent of SiO 2 to obtain the a priori value of the anchor point. In the next section we come back to this relation.…”

Section: Research Articlementioning

confidence: 99%

“…The majority of satellite studies that report measurements of volcanic ash currently rely on CRI measurements made in the 1970s on bulk material [26,27,28]. It is well-known, however, that such measurements underestimate the contribution of scattering in aerosol [29][30][31]. In addition, it has been shown that the spectra observed by satellites cannot be reproduced by forward models relying on these indices, especially when the optical depth is large [32].…”

Section: Introductionmentioning

confidence: 99%

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Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

et al. 2020

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Very fine silicate-rich volcanic ash, generated by explosive volcanic eruptions, can efficiently be traced downwind with infrared satellite sounders. Their measurements can also be used to derive physical parameters, such as optical depths and effective radii. However, one of the key requirements for accurate retrievals is a good knowledge of the complex refractive index (CRI) of the ash under investigation. In the past, the vast majority of the studies used the CRIs from Pollack et al. [Icarus 19, 372 (1973)], which are based on measurements of thin slices of volcanic rock, and therefore are not representative for airborne volcanic ash particles. Here, we report measurements of the CRI of volcanic ash in suspension, generated from samples collected from recent high-impact eruptions in Chile (Puyehue-Cordón Caulle, Calbuco, and Chaitén), Iceland (Eyjafjallajökull and Grímsvötn), and Italy (Etna). The samples cover a wide range of SiO 2 content (46% to 76%) as confirmed by an X-ray fluorescence analysis. In the experimental setup, volcanic ash was suspended in nitrogen through mechanical agitation. Extinction spectra were recorded in the infrared, visible, and ultraviolet spectral regions. The particle size distribution within the airflow was also recorded. An iterative algorithm allowed us to obtain fully consistent CRIs for the six samples, compatible with the observed extinction spectra and the Kramers-Krönig relations. While a good agreement is found with other recently reported CRIs in the UV/Vis, larger differences are found in the longwave infrared spectral region.

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Section: Research Articlementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

et al. 2020

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“…This is important because the main methods for detecting and quantifying ash in volcanic clouds rely on an infrared signature due to SiO 2 . With knowledge of the SiO 2 content or the ratio of non-bridging oxygen to the tetrahedrally-coordinated cations (NBO/T) and the glass and crystal amounts it is possible to select appropriate refractive indices to use for the scattering calculations needed in order to retrieve ash properties from infrared satellite measurements [40].…”

Section: Refractive Index and Compositionmentioning

confidence: 99%

Passive Earth Observations of Volcanic Clouds in the Atmosphere

Prata

Lynch

2019

Atmosphere

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Current Earth Observation (EO) satellites provide excellent spatial, temporal and spectral coverage for passive measurements of atmospheric volcanic emissions. Of particular value for ash detection and quantification are the geostationary satellites that now carry multispectral imagers. These instruments have multiple spectral channels spanning the visible to infrared (IR) wavelengths and provide 1 × 1 km2 to 4 × 4 km2 resolution data every 5–15 min, continuously. For ash detection, two channels situated near 11 and 12 μ m are needed; for ash quantification a third or fourth channel also in the infrared is useful for constraining the height of the ash cloud. This work describes passive EO infrared measurements and techniques to determine volcanic cloud properties and includes examples using current methods with an emphasis on the main difficulties and ways to overcome them. A challenging aspect of using satellite data is to design algorithms that make use of the spectral, temporal (especially for geostationary sensors) and spatial information. The hyperspectral sensor AIRS is used to identify specific molecules from their spectral signatures (e.g., for SO2) and retrievals are demonstrated as global, regional and hemispheric maps of AIRS column SO2. This kind of information is not available on all sensors, but by combining temporal, spatial and broadband multi-spectral information from polar and geo sensors (e.g., MODIS and SEVIRI) useful insights can be made. For example, repeat coverage of a particular area using geostationary data can reveal temporal behaviour of broadband channels indicative of eruptive activity. In many instances, identifying the nature of a pixel (clear, cloud, ash etc.) is the major challenge. Sophisticated cloud detection schemes have been developed that utilise statistical measures, physical models and temporal variation to classify pixels. The state of the art on cloud detection is good, but improvements are always needed. An IR-based multispectral cloud identification scheme is described and some examples shown. The scheme is physically based but has deficiencies that can be improved during the daytime by including information from the visible channels. Physical retrieval schemes applied to ash detected pixels suffer from a lack of knowledge of some basic microphysical and optical parameters needed to run the retrieval models. In particular, there is a lack of accurate spectral refractive index information for ash particles. The size distribution of fine ash (1–63 μ m, diameter) is poorly constrained and more measurements are needed, particularly for ash that is airborne. Height measurements are also lacking and a satellite-based stereoscopic height retrieval is used to illustrate the value of this information for aviation. The importance of water in volcanic clouds is discussed here and the separation of ice-rich and ash-rich portions of volcanic clouds is analysed for the first time. More work is required in trying to identify ice-coated ash particles, and it is suggested that a class of ice-rich volcanic cloud be recognized and termed a ‘volcanic ice’ cloud. Such clouds are frequently observed in tropical eruptions of great vertical extent (e.g., 8 km or higher) and are often not identified correctly by traditional IR methods (e.g., reverse absorption). Finally, the global, hemispheric and regional sampling of EO satellites is demonstrated for a few eruptions where the ash and SO 2 dispersed over large distances (1000s km).

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“…These are ideal test cases, which do not include other aerosols or aqueous cloud. These spectra include six different atmospheres: high latitude, mid-latitude day and night, tropical daytime, and polar summer and winter (including atmospheric profiles created for the Michelson Interferometer for Passive Atmospheric Sounding, MIPAS; Remedios et al, 2007). The spectra were modelled using the refractive indices of samples of volcanic ash from the Eyjafjallajökull eruption in 2010 (Peters, 2010): the main eruption considered in this study.…”

Section: The Infrared Atmospheric Sounding Interferometermentioning

confidence: 99%

“…Acknowledgements. Five of the atmospheric profiles used for the channel selection are reference spectra for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS; Remedios et al, 2007). The IASI spectra used in this study are available from the Centre for Environmental Data Analysis (EUMETSAT, 2009).…”

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confidence: 99%

An adaptation of the CO<sub>2</sub> slicing technique for the Infrared Atmospheric Sounding Interferometer to obtain the height of tropospheric volcanic ash clouds

et al. 2019

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Ash clouds are a geographically far-reaching hazard associated with volcanic eruptions. To minimise the risk that these pose to aircraft and to limit disruption to the aviation industry, it is important to closely monitor the emission and atmospheric dispersion of these plumes. The altitude of the plume is an important consideration and is an essential input into many models of ash cloud propagation. CO 2 slicing is an established technique for obtaining the top height of aqueous clouds, and previous studies have demonstrated that there is potential for this method to be used for volcanic ash. In this study, the CO 2 slicing technique has been adapted for volcanic ash and applied to spectra obtained from the Infrared Atmospheric Sounding Interferometer (IASI). Simulated ash spectra are first used to select the most appropriate channels and then demonstrate that the technique has merit for determining the altitude of the ash. These results indicate a strong match between the true heights and CO 2 slicing output with a root mean square error (RMSE) of less than 800 m. Following this, the technique was applied to spectra obtained with IASI during the Eyjafjallajökull and Grímsvötn eruptions in 2010 and 2011 respectively, both of which emitted ash clouds into the troposphere, and which have been extensively studied with satellite imagery. The CO 2 slicing results were compared against those from an optimal estimation scheme, also developed for IASI, and a satellite-borne lidar is used for validation. The CO 2 slicing heights returned an RMSE value of 2.2 km when compared against the lidar. This is lower than the RMSE for the optimal estimation scheme (2.8 km). The CO 2 slicing technique is a relatively fast tool and the results suggest that this method could be used to get a first approximation of the ash cloud height, potentially for use for hazard mitigation, or as an input for other retrieval techniques or models of ash cloud propagation.

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A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO₂ Content

Cited by 30 publications

References 51 publications

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

Passive Earth Observations of Volcanic Clouds in the Atmosphere

An adaptation of the CO<sub>2</sub> slicing technique for the Infrared Atmospheric Sounding Interferometer to obtain the height of tropospheric volcanic ash clouds

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A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO2 Content

Cited by 30 publications

References 51 publications

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet

Passive Earth Observations of Volcanic Clouds in the Atmosphere

An adaptation of the CO&lt;sub&gt;2&lt;/sub&gt; slicing technique for the Infrared Atmospheric Sounding Interferometer to obtain the height of tropospheric volcanic ash clouds

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A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO₂ Content

An adaptation of the CO<sub>2</sub> slicing technique for the Infrared Atmospheric Sounding Interferometer to obtain the height of tropospheric volcanic ash clouds