Abstract
Abstract. The simultaneous presence of SO2 and ash in a volcanic plume can lead to a significant error in the SO2 column abundance retrieval when multispectral Thermal InfraRed (TIR) data are used. The ash particles within the plume with effective radii from 1 to 10 μm reduce the Top Of Atmosphere (TOA) radiance in the entire TIR spectral range, including the channels used for SO2 retrieval. The net effect is a significant SO2 overestimation. In this work the interference of ash is discussed and two correction procedures for satellite SO2 volcanic plume retrieval in the TIR spectral range are developed to achieve an higher computational speed and a better accuracy. The ash correction can be applied when the sensor spectral range includes the 7.3 and/or 8.7 μm SO2 absorption bands, and the split window bands centered around 11 and 12 μm required for ash retrieval. This allows the possibility of simultaneous estimation of both volcanic SO2 and ash in the same data set. The proposed ash correction procedures have been applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Spin Enhanced Visible and Infrared Imager (SEVIRI) measurements. Data collected during the 24 November 2006 Mt. Etna eruption have been used to illustrate the technique. The SO2 and ash estimation is carried out by using a best weighted least squares fit method and the Brightness Temperature Difference (BTD) procedures, respectively. The simulated TOA radiance Look-Up Table (LUT) needed for the SO2 column abundance and the ash retrievals have been computed using the MODTRAN 4 Radiative Transfer Model. The results show the importance of the ash correction on SO2 retrievals at 8.7 μm, where the corrected SO2 column abundance values are less than 50% of the uncorrected values. The ash correction on SO2 retrieval at 7.3 μm is much less important and only significant for low SO2 column abundances. Results also show that the simplified and faster correction procedure underestimates the ash correction compared with the more time consuming but more accurate correction procedure. Such underestimation is greater for instruments having better ground pixel resolution, i.e. greater for MODIS than for SEVIRI.
Highlights
Volcanic eruptions are by their nature unpredictable and can send large amounts (Tg) of gas and particles high into the troposphere and sometimes into the stratosphere
The plume ash particles tend to reduce the Top Of Atmosphere (TOA) radiance in the entire Thermal InfraRed (TIR) spectral range (7–14 μm), including the channels used for the SO2 retrieval
The characteristics of the MODTRAN simulations and the details of the ash correction procedures applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) and Spin Enhanced Visible and Infrared Imager (SEVIRI) measurements are presented
Summary
Volcanic eruptions are by their nature unpredictable and can send large amounts (Tg) of gas and particles high into the troposphere and sometimes into the stratosphere. These methods generally rely on using data in the infrared window region between 7–14 μm to determine column abundance of SO2 and infrared optical depth, particle radius and mass concentration for ash particles. Further work (Wen and Rose, 1994; Prata and Grant, 2001) showed that by utilizing microphysical and radiative transfer models it was possible to retrieve fine-ash (1–10 μm, radius) particle size, optical depth and subsequently mass concentrations. As SO2 and ash are often emitted by an erupting volcano simultaneously, and as the winds can transport these substances together, a measurement of SO2 can be used, in some circumstances, as a proxy for ash This is important for aviation, when the volcanic debris has been transported over long distances and the satellite-based ash detection signal is weak.
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