Retrieval of desert dust aerosol vertical profiles from IASI measurements in the TIR atmospheric window

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 dust aerosols vertical profile on a global scale. In this work, we tackle this scientific issue by designing a method for retrieving dust aerosols vertical profiles from Thermal Infrared measurements by IASI instruments onboard the Metop satellite series. IASI offers almost global coverage twice a day, and long (past and future) time series of radiances, being therefore extremely well-suited for climate studies. Our retrieval follows Rodger's formalism and is based on a two-steps approach, treating separately the issues of low altitude sensitivity and of 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

Three‐dimensional distribution of a major desert dust outbreak over East Asia in March 2008 derived from IASI satellite observations

“…The overall results of AEROIASI show that the DOF or the number of independent pieces of information in the retrieval of dust profiles varies during the iterative procedure, a typical value of ~1.4 being used during the first iterations designed to determine the shape of the dust extinction profiles. This is similar to the DOF of ~1.5 obtained with the approach of Vandenbussche et al [], which also derives dust vertical profiles from IASI. In order to obtain numerically stable results, DOFs are limited to ~1 during the last iterations near convergence (using the auto‐adaptive scaling factor γ ).…”

“…Note that we use AERONET‐derived radius and width averaged over East Asia observed during the dust outbreak case study described in section 4. According to test cases presented by Pierangelo et al [] and Vandenbussche et al [], IASI thermal infrared spectra are expected to be significantly sensitive to dust altitudes and AODs, as brightness temperatures may differ by ~10 to ~12 K (depending on wave number) both for changes in the AOD at 10 µm from 0.75 to 2.5 or in the dust altitude by 3 km. On the other hand, a lower sensitivity is found with respect to particle size, as brightness temperatures change considering two typical dust size distributions at most by ~1 K and ~2 K (both around 1100 cm −1 ) for a dust layer respectively located at 1–2 km and 4–5 km.…”

“…OMI [ Torres et al , ], SeaWIFS (Sea‐Viewing Wide Field‐of‐View Sensor) [ Diner et al, ; Hsu et al, ], POLDER (Polarization and Directionality of the Earth's Reflectances) [ Su et al , ], MISR (Multi‐angle Imaging Spectroradiometer) [ Diner et al, ], and SEVIRI [ Carrer et al , ; Banks et al , ]. Recently, AODs associated with the horizontal distribution of Saharan dust plumes have also been derived on a daily basis using TIR hyperspectral measurements from AIRS (Atmospheric Infrared Sounder) [ DeSouza‐Machado et al , ] and IASI (Infrared Atmospheric Sounder Interferometer) [ Klüser et al , ; Vandenbussche et al , ]. These Infrared observations have also been used to detect and retrieve optical and microphysical properties of different types of aerosols [ Clarisse et al, , ] and also coarse dust effective size on a monthly basis [ Pierangelo et al ., ; Peyridieu et al ., ; Capelle et al ., ].…”

“…Monthly mean altitudes of dust layers have been retrieved from AIRS [ Pierangelo et al, ] and IASI [ Peyridieu et al , ; Capelle et al , ] over the ocean. Daily observations of the vertical distribution of Saharan dust plumes have also been derived so far mostly over ocean and for a limited fraction of cloud‐free pixels over land, both in terms of layer top heights from AIRS [ DeSouza‐Machado et al , ] and vertical profiles of dust concentration from IASI [ Vandenbussche et al , ]. However, until now, there existed no satellite‐based approach allowing for the observation of the continuous 3D distribution of dust plumes over all surfaces (both over land and ocean) with full cloud‐free coverage and on a daily basis.…”

A Decadal Data Set of Global Atmospheric Dust Retrieved From IASI Satellite Measurements

“…With an effective radius of 1.66 μm, this distribution can be considered to be a coarse mode distribution, even when most of the particles are in the accumulation mode (smaller than 1 μm). While dust particle size distributions can vary greatly in space and time (Mahowald et al, ; Reid et al, ), the effect of the size distribution on the retrieved DOD has been shown to be of second‐order importance (Capelle et al, ; Vandenbussche et al, ), and hence we have neither considered other size distributions nor attempted to retrieve an effective radius.…”