A novel methodology to determine volcanic aerosols optical properties in the UV and NIR and Ångström parameters using Sun photometry

Abstract

Remote volcanic aerosol optical depth (AOD) observations of Mount Etna summit and distal bulk plume have been carried out between 14 and 20 July 2016 in the framework of the EPL-RADIO project. Ultraviolet (UV) and near-infrared (NIR) AODs were measured using a Microtops-II Ozone Monitor (MIIOM) Sun photometer, using a Langley plot (LP) instrumental calibration routine. Ozone-corrected UV AODs at 320 nm are derived for the first time with a Microtops, thus extending the exploitable spectral band range of portable photometers to shorter wavelengths. The new UV AODs have theoretical uncertainties <±0.035 (±12%), dominated by LP calibration errors. Using UV and NIR AODs, the Ångström coefficients have been derived. The UV AODs and Ångström exponents have been compared, at background conditions, to colocated Cimel Sun photometer observations. A root-mean-square deviation of 0.03 (13%) for the UV AOD is found for this comparison, thus in agreement with estimated theoretical uncertainties. The MIIOM Ångström exponent estimations are found consistent with Cimel observations, even if with an average overestimation of 17.5%, mainly due to negative biases (−0.02/−21%) of NIR AODs. Results of quasi-simultaneous characterization of proximal and distal plume (7 km from craters), for 20 July 2016, are shown. During the measurements, brownish ash puffs were visible. While proximal and distal plumes were observed within approximately 1 h, their Ångström exponent varied significantly (mean values: −0.30±0.22 and 1.16±0.33, for the proximal and distal plumes). These results indicate quick sedimentation of ash particles and show the potential of this new retrieval technique to characterize volcanic aerosols.