Abstract
Abstract. Desert dust aerosols are the most prominent tropospheric aerosols, playing an important role in the earth's climate. However, their radiative forcing is currently not known with sufficient precision to even determine its sign. The sources of uncertainty are multiple, one of them being a poor characterisation of the dust aerosol's vertical profile on a global scale. In this work, we tackle this scientific issue by designing a method for retrieving dust aerosol vertical profiles from Thermal Infrared measurements by Infrared Atmospheric Sounding Interferometer (IASI) instruments onboard the Metop satellite series. IASI offers almost global coverage twice a day, and long (past and future) time series of radiances, therefore being extremely well suited for climate studies. Our retrieval follows Rodger's formalism and is based on a two-step approach, treating separately the issues of low altitude sensitivity and difficult a priori definition. We compare our results for a selected test case above the Atlantic Ocean and North Africa in June 2009, with optical depth data from MODIS, aerosol absorbing index from GOME-2 and OMI, and vertical profiles of extinction coefficients from CALIOP. We also use literature information on desert dust sources to interpret our results above land. Our retrievals provide perfectly reasonable results in terms of optical depth. The retrieved vertical profiles (with on average 1.5 degrees of freedom) show most of the time sensitivity down to the lowest layer, and agree well with CALIOP extinction profiles for medium to high dust optical depth. We conclude that this new method is extremely promising for improving the scientific knowledge about the 3-D distribution of desert dust aerosols in the atmosphere.
Highlights
Windblown dust from arid regions is the most prominent type of aerosols in the troposphere, in global annual average mass burden (Textor et al, 2006), present mainly, but not exclusively, in the tropics
We propose a strategy for the retrieval of airborne desert dust concentration vertical profiles, using thermal infrared (TIR) radiance data collected by the hyperspectral high-resolution Infrared Atmospheric Sounding Interferometer (IASI) instrument onboard Metop-A
We have shown that the altitude sensitivity lies in both the broadband extinction by dust, which occurs with a typical V-shape in the TIR atmospheric window, and in the effect dust has on absorption by tropospheric gases
Summary
Windblown dust from arid regions is the most prominent type of aerosols in the troposphere, in global annual average mass burden (Textor et al, 2006), present mainly, but not exclusively, in the tropics. Current thermal infrared (TIR) dust retrievals using measurements from AIRS (Aqua), IASI (Metop) or SEVIRI (MSG) provide total optical depth (OD, Pierangelo et al, 2004, 2005; Zhang et al, 2006; De Paepe and Dewitte, 2009; Peyridieu et al, 2010, 2012; DeSouza-Machado et al, 2010; Klüser et al, 2011) and in some cases particle size (Pierangelo et al, 2005; Zhang et al, 2006; Peyridieu et al, 2012), mineral composition (Klüser et al, 2012) or an equivalent or radiative altitude, being that of the atmospheric model’s layer that was filled with aerosols for the retrieval (Pierangelo et al, 2004; Peyridieu et al, 2010, 2012; DeSouza-Machado et al, 2010) This altitude is obtained either from look-up tables, or after running the retrieval successively with aerosols located at different levels and selecting the one that leads to the lowest residuals.
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