Retrieving radius, concentration, optical depth, and mass of different types of aerosols from high-resolution infrared nadir spectra

Abstract

We present a sophisticated radiative transfer code for modeling outgoing IR radiation from planetary atmospheres and, conversely, for retrieving atmospheric properties from high-resolution nadir-observed spectra. The forward model is built around a doubling-adding routine and calculates, in a spherical refractive geometry, the outgoing radiation emitted by the Earth and the atmosphere containing one layer of aerosol. The inverse model uses an optimal estimation approach and can simultaneously retrieve atmospheric trace gases, aerosol effective radius, and concentration. It is different from existing codes, as most forward codes dealing with multiple scattering assume a plane-parallel atmosphere, and as for the retrieval, it does not rely on precalculated spectra, the use of microwindows, or two-step retrievals. The simultaneous retrieval on a broad spectral range exploits the full potential of current state-of-the-art hyperspectral IR sounders, such as AIRS and IASI, and should be particularly useful in studying major pollution events. We present five example retrievals of IASI spectra observed in the range from 800 to 1200 cm(-1) above dust, volcanic ash, sulfuric acid, ice particles, and biomass burning aerosols.

Determination of time- and height-resolved volcanic ash emissions and their use for quantitative ash dispersion modeling: the 2010 Eyjafjallajökull eruption

“…Observational constraints on volcanic ash emissions were provided through infrared satellite retrievals of total column airborne ash loadings (Prata, 1989;Prata and Grant, 2001;Clarisse et al, 2010a) using data from two different satellite instruments, the geosynchronous Meteosat Second Generation (MSG) Spin-stabilised Enhanced Visible and Infrared Imager (SEVIRI) and the polar-orbiting MetOp Infrared Atmospheric Sounding Interferometer (IASI). These two instruments combine high temporal coverage (SEVIRI has a sampling time of 15 min but hourly averages were used here), with an enhanced sensitivity to ash (IASI has over 1000 spectral channels which can be used for ash detection).…”

“…Infrared sounding of aerosols has, however, a number of distinct advantages, such as the availability of night time data and the high sensitivity to aerosol morphology. Due to its complexity, a sophisticated method for retrieving radius and mass loadings (Clarisse et al, 2010a), based on optimal estimation which iteratively fits synthetic spectra to an observed spectrum by varying radius and mass loading is not suitable for the treatment of large amounts of data. Another difficulty is that reliable results can only be obtained if the wavenumber-resolved refractive index of the observed aerosols is known.…”

Volcanic ash detection with infrared limb sounding: MIPAS observations and radiative transfer simulations

“…Barton et al, 1992;Wen and Rose, 1994;Prata and Grant, 2001) and its limitations and challenges are discussed by Simpson et al (2000) and . For hyper-spectral instruments new methods are reported by Clarisse et al (2010) and Gangale et al (2010), which also rely on the characteristic spectral behaviour of volcanic ash and ice that is exploited by Prata (1989b).…”

Uncertainty in two-channel infrared remote sensing retrievals of a well-characterised volcanic ash cloud

“…Similar to other retrieval methods, a lognormal distribution is used with a fixed geometric standard deviation. Clarisse et al (2010) demonstrated an alternate method of optimal estimation using the Infrared Atmospheric Sounding Interferometer (IASI) (Clerbaux et al 2009). This method takes advantage of IASI's high spectral resolution in the range 800-1200 cm −1 (8.3-12.5 μm), but again, an assumption of the ash composition and PSD is needed.…”