A new scheme for sulphur dioxide retrieval from IASI measurements: application to the Eyjafjallajökull eruption of April and May 2010

Abstract

Abstract.A new optimal estimation algorithm for the retrieval of sulphur dioxide (SO 2 ) has been developed for the Infrared Atmospheric Sounding Interferometer (IASI) using the channels between 1000-1200 and 1300-1410 cm −1 . These regions include the two SO 2 absorption bands centred at about 8.7 and 7.3 µm (the ν 1 and ν 3 bands respectively). The retrieval assumes a Gaussian SO 2 profile and returns the SO 2 column amount in Dobson units and the altitude of the plume in millibars (mb). Forward modelled spectra (against which the measurements are compared) are based on the Radiative Transfer for TOVS (RTTOV) code. In our implementation RTTOV uses atmospheric profiles from European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological data. The retrieval includes a comprehensive error budget for every pixel derived from an error covariance matrix that is based on the SO 2 -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 SO 2 retrieval is not affected by underlying cloud but is affected if the SO 2 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 SO 2 plumes from the Eyjafjallajökull eruption during April and May 2010. From 14 April to 4 May (during Phase I and II of the eruption) the total amount of SO 2 present in the atmosphere, estimated by IASI measurements, is generally below 0.02 Tg. During the last part of the eruption (Phase III) the values are an order of magnitude higher, with a maximum of 0.18 Tg measured on the afternoon of 7 May.

IASI observations of sulfur dioxide (SO2) in the boundary layer of Norilsk

“…The method for retrieving near-surface SO 2 concentrations in Norilsk from IASI measurements is largely based on the one presented in Carboni et al [2012]. It relies on the optimal estimation framework [Rodgers, 2000], which traditionally consists of simultaneous iterative adjustment of the atmospheric parameters of interest and spectrally interfering unknown variables.…”

“…While complete information on these interfering unknowns is not available, often there is some a priori information. To exploit this, a generalized covariance matrix can be constructed from the difference between observed spectra and forward simulated spectra (see Carboni et al [2012] for details). In this way, a mean difference and associated covariance matrix can be calculated, representative for our missing information on the interfering unknowns (these include the IASI noise, forward model errors, errors in the meteorological fields, and errors coming from the lack of knowledge of the parameters affecting radiance spectra).…”

“…These periods correspond to the winter (here taken from November to March), characterized by high negative thermal contrasts and low water vapor content in the boundary layer, the summer (June to August) with low positive thermal contrasts and the highest humidities, and the midseason (April-May, September-October) representing intermediate thermal contrasts and humidity values. Note that it would have been possible to use only one for these three periods but it would have included large variability, which would have caused higher errors on SO 2 retrieved columns [Carboni et al, 2012]. To calculate the for each period, about 20,000 IASI spectra from 2010 and 2011 have been used.…”

Observations of volcanic SO<sub>2</sub> from MLS on Aura

“…Several recent papers (Clarisse et al, 2012(Clarisse et al, , 2014Carboni et al, 2012) have demonstrated the potential of the IASI instrument to provide useful SO 2 measurements. In particular, Clarisse et al (2014) files with plume heights estimated from IASI and find very good agreement.…”

Retrieval of ash properties from IASI measurements

“…In this work the Oxford-RAL Retrieval of Aerosol and Cloud (ORAC) algorithm (Thomas et al, 2009a;Poulsen et al, 2012) and the use of a generalized error covariance matrix, successfully applied to the retrieval of volcanic SO 2 by Carboni et al (2012), are adapted for use with volcanic ash. The method uses an optimal estimation retrieval algorithm to obtain probable values for the ash optical depth (AOD), particle effective radius, plume top height, and effective radiating temperature.…”