In the framework of the ESA-NASA Joint Aeolus Tropical Atlantic Campaign (JATAC), the ASKOS experiment was implemented during the summer and autumn of 2021 and 2022. ASKOS comprised roughly 9 weeks of measurements in the Saharan dust outflow towards the North Atlantic, with operations conducted from the Cabo Verde Islands. Through its unprecedented dataset of synergistic measurements in the region, ASKOS will allow for the calibration and validation of the aerosol/cloud product from Aeolus and the preparation of the terrain for EarthCARE cal/val activities. Moreover, ASKOS marks a turning point in our ability to study Saharan dust properties and the processes affecting its atmospheric transport, as well as the link to other components of the Earth’s system, such as the effect of dust particles on cloud formation over the Eastern Atlantic and the effect of large and giant particles on radiation. This is possible through the synergy of diverse observations acquired during the experiment, which include intense 24/7 ground-based aerosol, cloud, wind, and radiation remote sensing measurements, and UAV-based aerosol in situ measurements within the Saharan air layer, up to 5.3 km altitude, offering particle size-distributions up to 40 μm as well as sample collection for mineralogical analysis. We provide an outline of the novel measurements along with the main scientific objectives of ASKOS. The campaign data will be publicly available by September of 2023 through the EVDC portal (ESA Validation Data Center).

The eVe dual-laser/dual-telescope lidar system is introduced here, focusing on the optical and mechanical parts of the system's emission and receiver units. The compact design of the linear–circular emission unit along with the linear–circular analyser in the receiver unit allows eVe to simultaneously reproduce the operation of the ALADIN lidar on board Aeolus as well as to operate it as a traditional ground-based polarisation lidar system with linear emission. As such, the eVe lidar aims to provide (a) ground reference measurements for the validation of the Aeolus L2A aerosol products and (b) the conditions for which linear polarisation lidar systems can be considered for Aeolus L2A validation, by identifying any possible biases arising from the different polarisation state in the emission between ALADIN and these systems, and the detection of only the co-polar component of the returned signal from ALADIN for the L2A products' retrieval. In addition, a brief description is given concerning the polarisation calibration techniques that are applied in the system, as well as the developed software for the analysis of the collected signals and the retrieval of the optical products. More specifically, the system's dual configuration enables the retrieval of the optical properties of particle backscatter and extinction coefficients originating from the two different polarisation states of the emission and the linear and circular depolarisation ratios, as well as the direct calculation of the Aeolus-like backscatter coefficient, i.e. the backscatter coefficient that Aeolus would measure from the ground. Two cases, one with slightly depolarising particles and one with moderately depolarising particles, were selected from the first conducted measurements of eVe in Athens in September 2020, in order to demonstrate the system's capabilities. In the slightly depolarising scene, the Aeolus-like backscatter coefficient agrees well with the actual backscatter coefficient, which is also true when non-depolarising particles are present. The agreement however fades out for strongly depolarising scenes, where an underestimation of ∼18 % of the Aeolus like backscatter coefficient is observed when moderately depolarising particles are probed.

Since 2018, the Aeolus satellite of the European Space Agency (ESA) has acquired wind HLOS (horizontal line-of-sight) profiles throughout the troposphere and up to the lower stratosphere, filling a critical gap in the Global Observing System (GOS). Aeolus, carrying ALADIN (Atmospheric LAser Doppler INstrument), the first UV HSRL (High Spectral Resolution Lidar) Doppler lidar ever placed in space, provides also vertically resolved optical properties of particulates (aerosols and clouds). The present study focuses on the assessment of Aeolus L2A particulate backscatter coefficient (baseline 2A11), retrieved by the Standard Correct Algorithm (SCA), in the eastern Mediterranean, a region hosting a variety of aerosol species. Ground-based retrievals acquired by lidar instruments operating in Athens (central Greece), Thessaloniki (northern Greece) and Antikythera (southwestern Greece) serve as reference. All lidar stations provide routine measurements to the PANACEA (PANhellenic infrastructure for Atmospheric Composition and climatE chAnge) network. A set of ancillary data, including sun-photometric observations (AERONET), reanalysis products (CAMS and MERRA-2), satellite observations (MSG-SEVIRI and MODIS Aqua) and backward-trajectories modelling (FLEXPART), is utilized towards an optimum characterization of the probed atmospheric conditions under the absence of a classification scheme in Aeolus SCA profiles. First, emphasis is given on the assessment of Aeolus SCA backscatter coefficient under specific aerosol scenarios over Antikythera island. Due to the misdetection of the cross-polar component of the backscattered lidar signal, Aeolus underestimates the aerosol backscatter coefficient by up to 33 % when non-spherical mineral particles are recorded (10 July 2019). A good performance is revealed on 3 July 2019, when horizontally homogeneous loads of fine spherical particles are confined below 4 km. For other two cases (8 July 2020 and 5 August 2020), due to noise issues, the SCA performance degrades in terms of depicting the stratification of aerosol layers composed of particles of different origin. According to the statistical assessment analysis of 43 identified cases, a poor-to-moderate performance is revealed for the unfiltered (aerosols plus clouds) SCA profiles, which improves substantially when cloud-contaminated profiles are excluded from the collocated sample. This improvement is evident at both Aeolus vertical scales (regular scales have 24 bins and mid-bin scales have 23 bins), and it is justified by the drastic reduction in the bias (from 0.45 to 0.27 Mm⁻¹ sr⁻¹ for SCA and from 0.69 to 0.37 Mm⁻¹ sr⁻¹ for SCA mid-bin) and root mean square error (from 2.00 to 1.65 Mm⁻¹ sr⁻¹ for SCA and from 1.88 to 1.00 Mm⁻¹ sr⁻¹ for SCA mid-bin) scores. In the vertical, the SCA performance degrades at the lowermost bins due to either the contamination from surface signals or the increased noise levels for the aerosol retrievals. Among the three PANACEA stations, the best agreement is found at the remote site of Antikythera with respect to the urban sites of Athens and Thessaloniki. Finally, all key Cal/Val (calibration and validation) aspects necessary for future relevant studies, the recommendations for a possible Aeolus follow-on mission and an overview of the ongoing related activities are thoroughly discussed.

Dust orientation has been an ongoing investigation in recent years. Its potential proof will be a paradigm shift for dust remote sensing, invalidating the currently used simplifications of randomly oriented particles. Vertically resolved measurements of dust orientation can be acquired with a polarization lidar designed to target the off-diagonal elements of the backscatter matrix which are nonzero only when the particles are oriented. Building on previous studies, we constructed a lidar system emitting linearly and elliptically polarized light at 1064 nm and detecting the linear and circular polarization of the backscattered light. Its measurements provide direct flags of dust orientation, as well as more detailed information of the particle microphysics. The system also has the capability to acquire measurements at varying viewing angles. Moreover, in order to achieve good signal-to-noise ratio in short measurement times, the system is equipped with two laser sources emitting in an interleaved fashion and two telescopes for detecting the backscattered light from both lasers. Herein we provide a description of the optical and mechanical parts of this new lidar system, the scientific and technical objectives of its design, and the calibration methodologies tailored for the measurements of oriented dust particles. We also provide the first, preliminary measurements of the system during a dust-free day. The work presented does not include the detection of oriented dust (or other oriented particles), and therefore the instrument has not been tested fully in this objective.

The eVe lidar is a scanning system that can perform combined linear/circular polarization and Raman measurements at 355 nm and consists of the European Space Agency’s (ESA) reference aerosol lidar system. eVe was deployed in the ASKOS campaign, which was held in Cabo Verde during the summer/autumn of 2021 and 2022, for the validation of the aerosol products of the ESA’s Aeolus mission. During the campaign, eVe performed routine linear/circular depolarization measurements during Aeolus overpasses as well as dual-field-of-view measurements for the investigation of multiple scattering effects on dust layers. Herein, we present an overview of the acquired measurements and findings.

Aeolus was an ESA Earth Explorer satellite mission launched in 2018 with a lifetime of almost five years. The mission carried the Atmospheric Laser Doppler Instrument (ALADIN), a Doppler wind lidar for providing wind profiles on global scale and also vertically resolved optical properties of particles (aerosols and clouds) using the high spectral resolution lidar technique. To validate the particles' optical properties obtained from Aeolus as Level 2A products, the eVe lidar, ESA's reference system for the calibration and validation of Aeolus mission, has been deployed at the ASKOS campaign in the framework of the Joint Aeolus Tropical Atlantic Campaign (JATAC). ASKOS is the ground-based component of JATAC where ground-based remote sensing and in-situ instrumentation for aerosols, clouds, winds and radiation observations has been deployed at Cado Verde during summer 2021 and 2022 for the validation of the Aeolus products. The eVe lidar is a combined linear/circular polarization and Raman lidar specifically designed to mimic the operation of Aeolus and provide ground-based reference measurements of the optical properties for aerosols and thin clouds. The eVe lidar measurements can be used for the validation of the Aeolus L2A products while taking into consideration the ALADIN's limitation of misdetection of the cross-polar component of the backscattered signal. As such, in this validation study the cloud-free Aeolus L2A profiles obtained from the Standard Correct Algorithm (SCA), the Maximum Likelihood Estimation (MLE), and the AEL–PRO algorithms of Baseline 16 are compared against the corresponding cloud-free Aeolus-like profiles from eVe lidar, which are calculated using the retrieved particle circular depolarization ratio profile from eVe in order to account for the lack of the detection of the circular cross-polar backscatter component from Aeolus. The analysis focuses on the 14 collocated measurements between eVe and Aeolus during the nearest Aeolus overpass from the ASKOS site passing within a radius of 100 km from the site and within a temporal window of ± 1.5 h from the overpass time. The validation results reveal good performance for the co-polar particle backscatter coefficient, being the most accurate L2A product from Aeolus with absolute systematic and random errors up to 0.37 and 2 Mm⁻¹ sr⁻¹, respectively. The particle extinction coefficient also performs good with absolute systematic and random errors up to 53 and 183 Mm⁻¹, respectively. The co-polar lidar ratio seems to be the noisiest L2A product with extreme error values and variability. The observed discrepancies between eVe and Aeolus L2A profiles increase at lower altitudes where higher atmospheric loads (molecules and aerosols) are encountered leading to increased noise levels in the Aeolus retrievals due to enhanced laser beam attenuation, and greater atmospheric variability (e.g. Planetary Boundary Layer inhomogeneities) are typically encountered. Overall, this study underlines the strengths of the optimal estimation algorithms (MLE and AEL–PRO) with consistent performance and reduced uncertainties, while the standard inversion algorithm (SCA), which was originally developed, could be further improved particularly in the retrieval of the particle extinction coefficient and lidar ratio. In addition, the SCAmid-bin resolution profiles outperform the corresponding SCAnormal-bin as expected, since mid-bin resolution is obtained when averaging the values from two consecutive SCAnormal height bins.