A new simplified approach for simultaneous retrieval of SO<sub>2</sub> and ash content of tropospheric volcanic clouds: an application to the Mt Etna volcano

Abstract

Abstract. 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.

Synergistic use of Lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the Mount Etna eruption of 25–27 October 2013

“…The SO 2 total column retrieval by MODIS is based on the SO 2 wide absorption around 8.6 µm (MODIS channel 29). The retrieval scheme used in this study is based on the volcanic plume removal (VPR) approach developed for the Etna volcano (Pugnaghi et al, 2013). The VPR procedure also takes into account the ash influence on SO 2 estimates by using the channels centred around 11 and 12 µm (MODIS channels 31 and 32) .…”

Real time retrieval of volcanic cloud particles and SO<sub>2</sub> by satellite using an improved simplified approach

“…The transmittance τ a can easily be computed for pumice-type ash particles because a very good correlation exists between τ a,8.7 and τ a,11 (see Fig. 5a, and Pugnaghi et al, 2013).…”

“…For the multispectral sensors the plume mask can be derived from ash detection techniques based on brightness temperature difference (see Prata, 1989b) and successive improvements (see Millington et al, 2012;Pavolonis et al, 2013), principle components analysis (Hillger and Clark 2002a, b), or neural networks . The volcanic cloud height can be obtained from visible/TIR ground-based cameras (Scollo et al, 2014), ground radar (Montopoli et al, 2014;Marzano et al, 2006;, lidar system (Scollo et al, 2012) measurements, or multispectral satellite data using different techniques like "dark pixels" (Prata and Grant, 2001;Corradini et al, 2010), CO 2 slicing (Menzel et al, 1983;Platnick et al, 2003), H 2 O intercept method (Nieman et al, 1993), tracking of volcanic cloud center of mass (Guerrieri et al, 2015), inversion schemes based on optimal estimation (Francis et al, 2012), or parallax-based methods with image pairs collected by LEO-GEO (Zakšek et al, 2013) and GEO-GEO instruments. Knowing the plume height, its temperature can be obtained from the available vertical atmospheric sounding.…”

“…The subsequent steps of the VPR procedure have not been changed and can be found in Pugnaghi et al (2013). Nevertheless, to conclude the theoretical discussion, it is important to note the superposition effect of ash and sulfur dioxide on the radiance measured by the sensor.…”

“…Three test procedures were defined to validate the improved version of the VPR. The first one is, as described in Pugnaghi et al (2013), a comparison between the ash and SO 2 parameters set to perform the MODTRAN simulations and the corresponding values obtained from the new VPR retrievals (Sect. 3.1).…”