Atmos. Chem. Phys. Discuss., 12, 11861-11897, 2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
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A new scheme for sulphur dioxide retrieval from IASI measurements: application to the Eyjafjallajökull eruption of April and May 2010
E. Carboni1, R. Grainger1, J. Walker1, A. Dudhia1, and R. Siddans2
1AOPP, Physic department, University of Oxford, Oxford, UK
2Rutherford Appleton Laboratory, Didcot, UK

Abstract. A new algorithm for the retrieval of sulphur dioxide (SO2) from IASI data has been developed based on optimal estimation theory. It uses the IASI channels between 1000–1200 and 1300–1410 cm−1. These regions include the two SO2 absorption bands (the ν1 and ν3 bands) centred at about 7.3 and 8.7 microns, respectively. The retrieval assumes a Gaussian distribution for the vertical SO2 profile and returns the SO2 column amount in Dobson units (DU) and the altitude of the plume in millibar (mb). Radiative transfer computations that generate the forward modelled spectra (against which the measurements are compared) are based on RTTOV and ECMWF meteorological data. The retrieval includes a comprehensive error budget for every pixel. This is derived from an error covariance matrix that is based on the SO2-free climatology of the differences between the IASI and forward modelled spectra. The IASI forward model includes the ability to simulate a cloud or ash layer in the atmosphere. This feature is used to illustrate that: (1) the SO2 retrieval is not affected by underling cloud but is affected if the SO2 is within or below a cloud layer; (2) it is possible to discern if ash (or other atmospheric constituents not considered in the error covariance matrix) affects the retrieval using quality control based on the fit of the measured spectrum by the forward modelled spectrum. In this work the algorithm is applied to follow the behaviour of SO2 plumes in atmosphere for the Eyjafjallajökull eruption during April and May 2010 in terms of SO2 amount. From 14 April to 4 May (during phase I and II of the eruption) the total amount of SO2 present in atmosphere, estimated by IASI measurements, is generally below 0.2 Tg. During the last part of the eruption (phase III) the values are one order of magnitude higher, with a maximum of 0.14 Tg measured on 9 May with the afternoon orbits.

Citation: Carboni, E., Grainger, R., Walker, J., Dudhia, A., and Siddans, R.: A new scheme for sulphur dioxide retrieval from IASI measurements: application to the Eyjafjallajökull eruption of April and May 2010, Atmos. Chem. Phys. Discuss., 12, 11861-11897, doi:10.5194/acpd-12-11861-2012, 2012.
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