Quantifying dust concentration and mineralogy in the atmosphere, by combining remote sensing and airborne (UAV) in-situ, during the ASKOS campaign

Airborne dust plays a significant role in influencing weather phenomena, climate dynamics, and human health. Accurate quantification of dust concentration and its vertical distribution in the atmosphere, as well as accurate monitoring of dust microphysical properties including chemical composition, is essential for understanding dust impacts on radiation, cloud formation, weather and climate, and for developing corresponding mitigation strategies. Various observational methods have been developed to measure atmospheric dust properties, utilizing remote sensing (e.g. lidar and sun-photometers) and in-situ techniques. These techniques use different assumptions and their combination is a challenging task (e.g. Tsekeri et al., 2017). Herein, we try to harmonize their outputs for dust concentration profiles, using an extensive dataset gathered during the ASKOS campaign in the Cabo Verde Islands, a region uniquely positioned to observe dust outbreaks.

The ASKOS campaign (Marinou et al., 2023) was the ground-based component of the JATAC campaign organized by ESA and NASA in the islands of Cabo Verde (Fehr et al., 2023) during 2021 and 2022. Its main aim was to provide data for the calibration and validation of the Aeolus mission, with a focus on aerosol products. Over the course of these two years, a combination of aircraft, UAV, and ground-based remote sensing measurements was conducted.

The instruments deployed during ASKOS included a multiwavelength Raman-polarization lidar (Polly^XT), an AERONET sun-photometer, and in-situ sampling performed using optical particle counters (OPCs) and impactors onboard UAVs (Kezoudi et al., 2023). The UAV in-situ measurements were acquired from ground level up to 5 km above sea level, collocated with the remote sensing data. The OPCs provided measurements of particle size distribution, and the impactors of particle mineralogy.

The dust concentration profiles were calculated from lidar data using the POLIPHON method (Mamouri & Ansmann, 2014). This method utilizes extinction-to-mass conversion factors derived from AERONET data for various aerosol types (including dust), and calculates the mass concentration profiles from the extinction coefficient profiles provided by the lidar.

Dust mass concentration at different altitudes was also derived by using in-situ observations of the number size distribution of the particles. First, the corresponding volume size distribution and the total volume of the particles, are calculated. The dust mass concentration is calculated based on the percentage of dust particles in the volume (provided by the impactor chemical composition observations), using a mean density for dust particles, equal to 2.6 g/cm³.  For cases for which no in-situ observations are available, the volume size distribution of AERONET is utilized, though providing column-effective values.

The initial results indicate that the integrated mass concentration across the dust layer, as determined by both techniques, lies within the uncertainty ranges of the respective methods. Also, the analysis from the impactors provides information on the mineralogical composition of the dust particles that are transported from the Sahara.

Our work will assess each technique’s validity and identify the conditions under which the remote sensing method can be used independently.

This research was supported by the REVEAL project (GA 7222) funded by the Hellenic Foundation for Research & Innovation and by the PANGEA4CalVal project (GA 101079201) EU-funded.