A synergy between CALIOP-CALIPSO and GRASP-POLDER retrievals towards advancing monitoring capabilities of dust aerosols across North Africa and Middle East
Authors: Moustaka Anna , Gkikas Antonis , Proestakis Emmanouil , Lopatin Anton , Kazadzis Stelios , Dubovik Oleg , Tourpali Kleareti , Zerefos Christos , Amiridis Vassilis 
Published: July 2024

Dust aerosols constitute one of the key climatic drivers, playing also a vital role in various environmental processes. Additionally, they can have adverse impacts on air quality, thereby affecting human health. &#160;Worldwide, the most active dust sources are encompassed across North Africa and in the Middle East emitting massive dust loads which are transported in nearby and distant areas. Such widespread phenomena are well captured by spaceborne instruments. The present study exploits synergies between satellite sensors deploying passive and active remote sensing techniques towards upgrading mineral particles monitoring. Our key objective is to improve the quality of the mid-visible (532nm) dust optical depth (DOD) obtained by the CALIOP lidar, mounted on the polar-orbiting CALIPSO satellite. CALIOP is equipped with a depolarization channel that enables an accurate detection of non-spherical mineral particles. Nevertheless, CALIOP tends to underestimate the columnar optical depth and one of the contributing factors is the misrepresentation of the predefined lidar ratio (LR) in the retrieval algorithms. In elastic lidars (such as CALIOP), the definition of LR is crucial for converting the backscatter to extinction coefficient and subsequently deriving the columnar optical depth (i.e., integration of the extinction throughout aerosol layers). Here, the latter parameter is constrained by GRASP aerosol retrievals utilizing as inputs multidirectional polarimetric observations acquired in the visible spectrum (9 spectral channels) by the POLDER instruments. Evaluation studies have justified the high-accuracy of the GRASP-POLDER aerosol products over the &#8220;challenging&#8221; (in terms of retrievals performance) bright deserts. We are analyzing atmospheric scenes in the Sahara Desert and the Arabian Peninsula only when dust aerosols are probed by CALIOP throughout the period when the CALIOP/POLDER instruments were flying in tandem in the A-Train constellation. By employing an iterative approach, the CALIOP-POLDER optical depth departures are minimized in order to extract a representative LR. For intercomparison purposes, LR is also calculated independently via the GRASP algorithm. The extensive volume of CALIOP-POLDER collocated samples will realize a mapping of LRs depicting possible spatial patterns. The integration of EMIT mineralogical retrievals will enhance the obtained findings. Our methodology will be expanded to all the major deserts of the planet, each characterized by different soil and optical properties, which are both determinant for LRs. Moreover, similar analyses will be enhanced in the future by utilizing products from the PACE and the EarthCARE satellite missions. Finally, it is anticipated that the optimized CALIOP/CALIPSO dust products will improve the assessment of the dust-induced perturbations of the Earth-Atmosphere radiation budget.&#160; &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;Acknowledgements: The current research has been supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the &#8220;2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers&#8221; (Project Acronym:&#160; ATLANTAS, Project number:&#160; 544).