{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,5,9]],"date-time":"2026-05-09T15:20:15Z","timestamp":1778340015851,"version":"3.51.4"},"reference-count":28,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2021,4,29]],"date-time":"2021-04-29T00:00:00Z","timestamp":1619654400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>Lidar observations are very useful to analyse dispersed volcanic clouds in the troposphere mainly because of their high range resolution, providing morphological as well as microphysical (size and mass) properties. In this work, we analyse the volcanic cloud of 18 May 2016 at Mt. Etna, in Italy, retrieved by polarimetric dual-wavelength Lidar measurements. We use the AMPLE (Aerosol Multi-Wavelength Polarization Lidar Experiment) system, located in Catania, about 25 km from the Etna summit craters, pointing at a thin volcanic cloud layer, clearly visible and dispersed from the summit craters at the altitude between 2 and 4 km and 6 and 7 km above the sea level. Both the backscattering and linear depolarization profiles at 355 nm (UV, ultraviolet) and 532 nm (VIS, visible) wavelengths, respectively, were obtained using different angles at 20\u00b0, 30\u00b0, 40\u00b0 and 90\u00b0. The proposed approach inverts the Lidar measurements with a physically based inversion methodology named Volcanic Ash Lidar Retrieval (VALR), based on Maximum-Likelihood (ML). VALRML can provide estimates of volcanic ash mean size and mass concentration at a resolution of few tens of meters. We also compared those results with two methods: Single-variate Regression (SR) and Multi-variate Regression (MR). SR uses the backscattering coefficient or backscattering and depolarization coefficients of one wavelength (UV or VIS in our cases). The MR method uses the backscattering coefficient of both wavelengths (UV and VIS). In absence of in situ airborne validation data, the discrepancy among the different retrieval techniques is estimated with respect to the VALR ML algorithm. The VALR ML analysis provides ash concentrations between about 0.1 \u03bcg\/m3 and 1 mg\/m3 and particle mean sizes of 0.1 \u03bcm and 6 \u03bcm, respectively. Results show that, for the SR method differences are less than &lt;10%, using the backscattering coefficient only and backscattering and depolarization coefficients. Moreover, we find differences of 20\u201330% respect to VALR ML, considering well-known parametric retrieval methods. VALR algorithms show how a physics-based inversion approaches can effectively exploit the spectral-polarimetric Lidar AMPLE capability.<\/jats:p>","DOI":"10.3390\/rs13091728","type":"journal-article","created":{"date-parts":[[2021,4,29]],"date-time":"2021-04-29T10:30:41Z","timestamp":1619692241000},"page":"1728","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["Dual-Wavelength Polarimetric Lidar Observations of the Volcanic Ash Cloud Produced during the 2016 Etna Eruption"],"prefix":"10.3390","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0303-1171","authenticated-orcid":false,"given":"Luigi","family":"Mereu","sequence":"first","affiliation":[{"name":"Department of Information Engineering, Sapienza University of Rome, 00184 Rome, Italy"},{"name":"Center of Excellence CETEMPS, 67100 L\u2019Aquila, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8704-8629","authenticated-orcid":false,"given":"Simona","family":"Scollo","sequence":"additional","affiliation":[{"name":"Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, 95125 Catania, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4996-7648","authenticated-orcid":false,"given":"Antonella","family":"Boselli","sequence":"additional","affiliation":[{"name":"IMAA, Consiglio Nazionale delle Ricerche, 85050 Potenza, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0040-5011","authenticated-orcid":false,"given":"Giuseppe","family":"Leto","sequence":"additional","affiliation":[{"name":"INAF, Osservatorio Astrofisico, 95123 Catania, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6997-0887","authenticated-orcid":false,"given":"Ricardo","family":"Zanmar Sanchez","sequence":"additional","affiliation":[{"name":"INAF, Osservatorio Astrofisico, 95123 Catania, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2368-2193","authenticated-orcid":false,"given":"Costanza","family":"Bonadonna","sequence":"additional","affiliation":[{"name":"Department of Earth Sciences, University of Geneva, 1205 Geneva, Switzerland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5873-204X","authenticated-orcid":false,"given":"Frank Silvio","family":"Marzano","sequence":"additional","affiliation":[{"name":"Department of Information Engineering, Sapienza University of Rome, 00184 Rome, Italy"},{"name":"Center of Excellence CETEMPS, 67100 L\u2019Aquila, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,4,29]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Boselli, A., Scollo, S., Leto, G., Zanmar Sanchez, R., Sannino, A., Wang, X., Coltelli, M., and Spinelli, N. 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