Coordinated vertical tandem-profiling of a Saharan dust intrusion over Central Europe on 20 June 2024 based on balloon-borne soundings from two different sites.

When favourable synoptic conditions prevail, desert dust is transported from North Africa to Central Europe. Between June 19 and 21, 2024, air from North and Northwest Africa spread from Algeria across the south-coast of France with predicted dust load > 1200 mg m^-2 over an area limited by the Rhone Valley, extending to the coasts of Belgium and the Netherlands. The intrusion reached as far as the Skagerrak and the Kattegat and stretched across parts of Poland and the Czech Republic to the Aegean Sea and Greece, and it entirely covered Italy. On June 20, 2024, time-coordinated balloon-borne vertical soundings were carried out over Germany from two locations: 1) at 13:18 CEST from Oberpfaffenhofen (OPH - near Munich) and 2) at 14:15 CEST from Spielberg (SPb - near Frankfurt/Main, in the framework of “TPChange”, DFG TRR301) with the aim to analyse the same (intermediately transported) air mass. The SPb balloon payload included (a. o.) a radiosonde (RS41 SGP by VAISALA), a set of dual-stage impactors to perform particle sampling for offline physico-chemical analyses, and optical particle counters (OPC) such as the Portable Optical Particle Spectrometer (POPS). The OPH payload consisted of an OPC-N3 (by Alphasense) and the RS41 SGP.

Qualitative agreement was obtained from the independent profiles: from 1.5 km to 4.8 km height, a layer of increased particle number concentration (N) with 100 to 1000 cm^-3 stands out from the background (N < 20 cm^-3) in the vertical profile for particles with a diameter (D_p) from 0.14 µm to 2.6 µm (POPS-detected sized range). While below ~ 4.5 km (OPH) and ~ 4.8 km (SPb), the relative humidity (RH) remains below 87 %, the region of particle enhancement is effectively capped by a cloud layer (RH exceeding 100 %) of about 200 m vertical thickness above ~ 4.5 km (OPH) and ~ 4.8 km (SPb), respectively. Aloft, N drops abruptly and temporarily reaches background values < 20 cm^-3. The impactor sample taken throughout passage of the particle layer showed considerable presence of mineral dust (generally > 75 % of all particles collected), the largest of which have estimated D_p of 10 µm and smallest D_p were estimated with 0.1 µm. Admixtures of sea salt (particle fraction D_p > 500nm) and sulphates (fraction D_p < 500 nm) were also identified. We will present more specific microphysical properties of the mineral dust aerosol, including morphology and chemical composition, and discuss these in the context of the atmospheric conditions at both measurement sites.