Enhancing dust aerosols monitoring capabilities across North Africa and the Middle East using the A-Train satellite constellation

Abstract. North Africa and the Middle East encompass the most active dust sources on the planet. Due to the limited availability of ground-based aerosol observations across the deserts, spaceborne retrievals represent the most reliable source of information for monitoring dust particles over these vast areas. In the current study, we present a synergistic approach incorporating aerosol retrievals acquired by active (CALIOP) and passive (POLDER-3, MODIS) instruments mounted on satellites of the A-Train constellation. Our main objective is to dynamically (in terms of space and time) estimate the dust lidar ratio (LR) throughout the CALIPSO operational period (2006–2023) by collocating columnar aerosol optical depth observations (POLDER-3/GRASP, MIDAS) and vertically resolved dust aerosol profiles obtained by CALIPSO. According to our findings, the agreement among the satellite-based retrievals improves, when the default and constant CALIPSO dust LR (44 sr) is adjusted. Specifically, increasing and decreasing dust LR tendencies are recorded in North Africa and in the Middle East, respectively, whereas a narrow transition zone of neutral declinations appears between the two regions. Furthermore, compared to the standard dust LR, higher values are recorded east of the Caspian Sea, while negative departures are found in Iran and southern Pakistan. The proposed dust LR adjustments are maximized over/near dust sources and during the dry period of the year, indicating a dependence on dust activity. The evaluation against AERONET observations clearly demonstrates that the refined dust LRs improve CALIOP’s performance when mineral particles are probed, but further justification is needed. The integration of the refined dust LRs significantly modifies the standard CALIPSO dust optical depth (DOD) climatological patterns, particularly in the Western Sahara, the Bodélé depression (northern Chad Basin), the Libyan Desert and Saudi Arabia. Positive and negative shifts in the regional DOD timeseries are found in North Africa and in the Middle East, respectively, with no notable modifications on the inter-annual and intra-annual trends in either region. A key priority of the analysis carried out is to improve the efficiency of CALIPSO dust retrievals towards advancing their utility in a wide range of scientific applications. The advent of the EarthCARE satellite mission, along with the incorporation of new aerosol models into the forthcoming CALIPSO Version 5 aerosol retrieval algorithm, will serve as a reference for our calculations. In this context, our findings also highlight that a synergy of multisensor aerosol products with modelling tools can enhance the spatiotemporal representation of aerosol properties, such as the lidar ratio, further improving retrieval utility.