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The simultaneous presence of SO2 and ash in a volcanic plume can lead to asignificant error in the SO2 column abundance retrieval when multispectralThermal InfraRed (TIR) data are used. The ash particles within the plume witheffective radii from 1 to 10 μm reduce the Top Of Atmosphere (TOA)radiance in the entire TIR spectral range, including the channels used forSO2 retrieval. The net effect is a significant SO2 overestimation. In this work the interference of ash is discussed and two correctionprocedures for satellite SO2 volcanic plume retrieval in the TIR spectralrange are developed to achieve an higher computational speed and a betteraccuracy. The ash correction can be applied when the sensor spectral range includes the7.3 and/or 8.7 μm SO2 absorption bands, and the split window bandscentered around 11 and 12 μm required for ash retrieval. This allowsthe possibility of simultaneous estimation of both volcanic SO2 and ash inthe same data set. The proposed ash correction procedures have been appliedto the Moderate Resolution Imaging Spectroradiometer (MODIS) and the SpinEnhanced Visible and Infrared Imager (SEVIRI) measurements. Data collectedduring the 24 November 2006 Mt. Etna eruption have been used to illustratethe technique. The SO2 and ash estimation is carried out by using a bestweighted 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 beencomputed using the MODTRAN 4 Radiative Transfer Model. The results show the importance of the ash correction on SO2 retrievals at8.7 μm, where the corrected SO2 column abundance values are lessthan 50% of the uncorrected values. The ash correction on SO2 retrievalat 7.3 μm is much less important and only significant for low SO2column abundances. Results also show that the simplified and fastercorrection procedure underestimates the ash correction compared with the moretime consuming but more accurate correction procedure. Such underestimationis greater for instruments having better ground pixel resolution, i.e.greater for MODIS than for SEVIRI

atmospheric measurement techniques

Retrieval of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; from thermal infrared satellite measurements: correction procedures for the effects of volcanic ash