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

Open Access Atmospheric Measurement Techniques Atmos. Meas. Tech., 6, 2577–2591, 2013 www.atmos-meas-tech.net/6/2577/2013/ doi:10.5194/amt-6-2577-2013 © Author(s) 2013. CC Attribution 3.0 License. Retrieval of desert dust aerosol vertical profiles from IASI measurements in the TIR atmospheric window S. Vandenbussche, S. Kochenova, A. C. Vandaele, N. Kumps, and M. De Mazière Belgian Institute for Space Aeronomy, Brussels, Belgium Correspondence to: S. Vandenbussche () Received: 19 April 2013 – Published in Atmos. Meas. Tech. Discuss.: 22 May 2013 Revised: 20 August 2013 – Accepted: 2 September 2013 – Published: 7 October 2013 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. 1 Introduction 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. Dust particles absorb and scatter the incoming and reflected solar light: this is the short-wave (SW) direct effect. In addition to that, these aerosols have a longwave (LW) direct effect: they absorb and scatter the thermal emission from the surface and the atmosphere, and they emit thermal radiation themselves. These effects are often assessed in terms of radiative forcing (RF), which is the net (incoming minus outgoing) change in energy at a defined altitude (e.g. the top-of-atmosphere, tropopause or surface), due to a change in a climatic parameter (e.g. the aerosol size, optical properties, total atmospheric load, etc.). Following the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Forster et al., 2007), the anthropogenic direct dust RF at the top-of-atmosphere would be slightly negative, with a 90 % confidence interval including both positive and negative values. The uncertainty in the sign of this total forcing arises from the fact that the SW and LW global effects are competing, being respectively negative and positive, and are not yet quantified accurately enough to allow the determination of the dominant effect. This is of course also true for the direct RF due to naturally occurring atmospheric dust. It has been shown (Perrone et al., 2012; Zhang and Christopher, 2003; Claquin et al., 1998; Liao and Seinfeld, 1998) that dust direct RF mainly depends on total optical depth (SW and LW), aerosol refractive index (mostly but not only SW), aerosol mean particle size (SW and LW), aerosol vertical distribution (LW, SW only for cloudy scenes), water vapour and temperature vertical profiles (only LW), and Published by Copernicus Publications on behalf of the European Geosciences Union. 2578 S. Vandenbussche et al.: Desert dust vertical profiling using IASI surface albedo (only SW). Amongst all these parameters, the dust vertical distribution is currently poorly characterised on a global scale. Indeed, a very recent paper by Winker et al. (2013) reports for the first time a global 3-D monthly climatology of tropospheric aerosols, but there is no data set with higher temporal coverage. In addition to its direct radiative effect, air-borne dust exerts an indirect effect on the climate, through its interaction with clouds. Dust particles act as cloud condensation or ice nuclei, modifying the droplets/ice particles size distribution and therefore altering the optical properties of clouds, their horizontal and vertical extent, their lifetime and the amount of rain (e.g., Bangert et al., 2012; Lee, 2011; Li et al., 2011; Koehler et al., 2010). This indirect effect mainly depends on the aerosol particles number density (Bangert et al., 2012) and the relative vertical location of cloud and aerosol layers (Quijano et al., 2000), again pointing to the necessity of a better characterisation of the 3-D distribution of dust aerosols in the atmosphere. In recent years, a growing effort has been devoted to improve the knowledge of dust characteristics: physical and optical properties, atmospheric load, sources, transport, etc. A number of recent field campaigns, the two SAharan Mineral dUst experiMents (SAMUM) in 2006 and 2008 (Ansmann et al., 2011) and the Geostationary Earth Radiation Budget Intercomparisons of Long-wave and Short-wave radiation (GERBILS) in 2007 (Christopher et al., 2009), have provided valuable information on dust particle size distribution (PSD), shape and refractive index. These microphysical and optical properties are then used for obtaining dust load in the atmosphere on a global scale by retrievals from satellite remote sensing measurements (e.g., Carboni et al., 2012). The currently most advanced aerosol retrievals from satellite sensors are based on UV, visible or near-infrared wavelengths, for example, GOME (ERS-2), GOME2 (Metop), OMI (Aura), MODIS (Aqua and Terra), MISR (Terra), MERIS (Envisat), Polder (PARASOL), or CALIOP (CALIPSO). Short-wave aerosol properties are therefore characterised on an operational basis using data from many different instruments and measurement techniques. Retrievals of aerosol long-wave optical properties are less well developed but have received great attention these last years. These properties cannot be inferred from short-wave measurements, because short-wave and longwave spectral windows are not sensitive to the (...truncated)