Atmospheric Measurement Techniques
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10.5194/amt-2-177-2009
http://www.atmos-meas-tech.net/2/177/2009/
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http://www.atmos-meas-tech.net/2/177/2009/amt-2-177-2009.pdf
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191
2009-05-26
Retrieval of SO<sub>2</sub> from thermal infrared satellite measurements: correction procedures for the effects of volcanic ash
S. Corradini
corradini@ingv.it
L. Merucci
A. J. Prata
Istituto Nazionale di Geofisica e Vulcanologia,Via di Vigna Murata 605, 00143 Roma, Italy
Norwegian Institute for Air Research, Instituttveien 18 Kjeller, 2027, Norway
The simultaneous presence of SO<sub>2</sub> and ash in a volcanic plume can lead to a
significant error in the SO<sub>2</sub> 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
SO<sub>2</sub> retrieval. The net effect is a significant SO<sub>2</sub> overestimation.
<br><br>
In this work the interference of ash is discussed and two correction
procedures for satellite SO<sub>2</sub> volcanic plume retrieval in the TIR spectral
range are developed to achieve an higher computational speed and a better
accuracy.
<br><br>
The ash correction can be applied when the sensor spectral range includes the
7.3 and/or 8.7 μm SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> column abundance and the ash retrievals have been
computed using the MODTRAN 4 Radiative Transfer Model.
<br><br>
The results show the importance of the ash correction on SO<sub>2</sub> retrievals at
8.7 μm, where the corrected SO<sub>2</sub> column abundance values are less
than 50% of the uncorrected values. The ash correction on SO<sub>2</sub> retrieval
at 7.3 μm is much less important and only significant for low SO<sub>2</sub>
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.
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