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).

Mineral dust composition affects a multitude of processes in the atmosphere and adjacent compartments. Dust dry deposition was collected near source in northwest Africa, in Central Asia, and on Svalbard and at three locations of the African outflow regime. Samples were subjected to automated scanning electron microscopy with energy-dispersive X-ray analysis to obtain size and composition of 216,000 individual particles. Results show low temporal variation in estimated optical properties for each location, but considerable differences between the African, Central Asian, and Arctic regimes. No significant difference was found between the K-feldspar relative abundances, indicating comparable related ice-nucleation abilities. The mixing state between calcium and iron compounds was different for near source and transport regimes, potentially in part due to size sorting effects. As a result, in certain situations (high acid availability, limited time) atmospheric processing of the dust is expected to lead to less increased iron solubility for near-source dusts (in particular for Central Asian ones) than for transported ones (in particular of Sahelian origin).

The transport of South Asian (SA) anthropogenic haze to the northern Indian Ocean (NIO) is often observed during winter, however, its chemical and molecular constituents are not well explored. To better address this problem, shipborne measurements of size‐resolved aerosol sampling were carried out over the northern and equatorial Indian Ocean (EIO) for the quantification of aerosol composition (organic and inorganic) and molecular markers during the winter of 2018. The most striking outcome was the dominance of anthropogenic aerosols derived from biomass‐burning and combustion‐related emissions over the NIO during the SA outflow. Low‐level concentrations were noticed over the EIO where oceanic air mass was dominated. Organic carbon (OC), sea‐salts (Na⁺ and Cl⁻), calcium (Ca²⁺), and nitrates (NO₃⁻) were enriched in the coarse mode (>2.0 µm) while elemental carbon (EC), water‐soluble OC, sulfates (SO₄²⁻), and potassium (K⁺) were abundant in fine mode (<2.0 µm) aerosols. Water‐soluble nitrogen aerosols showed a bimodal size distribution with a significant peak in fine mode. The molecular composition of organic aerosols (OAs) in PM_1.1 was characterized by high abundances of sugars and lignin/resin products followed by hydroxy/polyacids and plasticizers. These results underscore that biomass‐burning emissions (BBEs), plastic combustions, and secondary anthropogenic aerosols are the leading sources of OAs in the SA outflow as further corroborated by the linear‐regression analysis between organic and inorganic marker compounds. The mass absorption efficiency (MAE) of organic (brown carbon; BrC) aerosols (MAE_{OC‐BrC‐365nm}) was two times higher in SA outflow and largely contributed from BBEs as confirmed by the positive relationship with levoglucosan.