Abstract
Abstract. Monitoring upper-tropospheric–lower-stratospheric (UTLS) secondary sulfate aerosols and their chemical and microphysical properties from satellite nadir observations is crucial to better understand their formation and evolution processes and then to estimate their impact on UTLS chemistry, and on regional and global radiative balance. Here we present a study aimed at the evaluation of the sensitivity of thermal infrared (TIR) satellite nadir observations to the chemical composition and the size distribution of idealised UTLS sulfate aerosol layers. The extinction properties of sulfuric acid/water droplets, 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. IASI (Infrared Atmospheric Sounding Interferometer) 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 spectra observed by this satellite instrument. We found 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. The dependence of the aerosol spectral 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 (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.
Highlights
Secondary sulfate aerosols are sulfate-containing aqueous solution droplets, generally of submicron size, produced from gas-to-particle conversion processes involving sulfurcontaining gaseous precursors (Hamill et al, 1997)
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
In this paper we have presented sensitivity analyses of the optical properties and brightness temperature (BT) signatures of secondary sulfate aerosols in the upper troposphere/lower stratosphere (UTLS), based on thermal infrared (TIR) IASI pseudoobservations obtained with the 4A/OP radiative transfer model
Summary
Secondary sulfate aerosols are sulfate-containing aqueous solution droplets, generally of submicron size, produced from gas-to-particle conversion processes involving sulfurcontaining gaseous precursors (Hamill et al, 1997). A peculiar aerosol signature (increasing absorption between about 700 and 1300 cm−1) has been observed in the past in those conditions, e.g. by Clarisse et al (2013) for the eruption of the Sarychev volcano, by Baran et al (1993); Grainger et al (1993); Ackerman and Strabala (1994) for the eruption of the Pinatubo volcano and by Karagulian et al (2010) for the eruption of the Kasatochi volcano These signatures have been attributed to the secondary sulfate aerosols formed in the volcanic plume, without linking them to the specific spectroscopic features of the different sulfur-containing species (e.g. undissociated sulfuric acid molecules, sulfate and bisulfate ions) contained in the droplets and their chemical and microphysical characterisation.
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