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| Titel |
Sensitivity of high-spectral resolution and broadband thermal infrared nadir instruments to the chemical and microphysical properties of secondary sulfate aerosols in the upper-troposphere/lower-stratosphere |
| VerfasserIn |
Pasquale Sellitto, Bernard Legras |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134239
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| Publikation (Nr.) |
 EGU/EGU2016-14940.pdf |
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| Zusammenfassung |
| The observation of upper-tropospheric/lower-stratospheric (UTLS) secondary sulfate aerosols
(SSA) and their chemical and microphysical properties from satellite nadir observations (with
better spatial resolution than limb observations) is a fundamental tool to better understand
their formation and evolution processes and then to estimate their impact on UTLS chemistry,
and on regional and global radiative balance. Thermal infrared (TIR) observations are
sensitive to the chemical composition of the aerosols due to the strong spectral variations of
the imaginary part of the refractive index in this band and, correspondingly, of
the absorption, as a function of the composition Then, these observations are, in
principle, well adapted to detect and characterize UTLS SSA. Unfortunately, the
exploitation of nadir TIR observations for sulfate aerosol layer monitoring is today very
limited.
Here we present a study aimed at the evaluation of the sensitivity of TIR satellite nadir
observations to the chemical composition and the size distribution of idealised UTLS SSA
layers. The sulfate aerosol particles are assumed as binary systems of sulfuric acid/water
solution droplets, with varying sulphuric acid mixing ratios. The extinction properties of the
SSA, for different sulfuric acid mixing ratios and temperatures, are systematically analysed.
The extinction coefficients are derived by means of a Mie code, using refractive
indices taken from the GEISA (Gestion et Étude des Informations Spectroscopiques
Atmosphériques: Management and Study of Spectroscopic Information) spectroscopic
database and log-normal size distributions with different effective radii and number
concentrations. High-spectral resolution pseudo-observations are generated using forward
radiative transfer calculations performed with the 4A (Automatized Atmospheric
Absorption Atlas) radiative transfer model, to estimate the impact of the extinction of
idealised aerosol layers, at typical UTLS conditions, on the brightness temperature
(BT) spectra observed by satellite instruments. We isolated a marked and typical
spectral signature of these aerosol layers between 700 and 1200 cm−1, due to the
absorption bands of the sulfate and bisulfate ions and the undissociated sulfuric acid,
with the main absorption peaks at 1170 and 905 cm−1 (sulfuric acid vibrational
bands).
The dependence of the residual aerosol spectral BT signature to the sulfuric acid
mixing ratio, and effective number concentration and radius, as well as the role
of interfering parameters like the ozone, sulfur dioxide, carbon dioxide and ash
absorption, and temperature and water vapour profile uncertainties, are analysed and
critically discussed. The information content (degrees of freedom and retrieval
uncertainties) of synthetic satellite observations is estimated for different instrumental
configurations. High spectral resolution (Infrared Atmospheric Sounding Interferometer
(IASI)-like pseudo-observations) and broadband spectral features (Moderate Resolution
Imaging Spectroradiometer (MODIS) and Spinning Enhanced Visible and InfraRed
Imager (SEVIRI)-like pseudo-observations) approaches are proposed and discussed. |
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