Using uncrewed aerial systems at the sub-arctic site Pallas to study aerosol–cloud interactions

Low-level clouds play a crucial role in the Arctic climate system, for example by contributing to surface warming. Although many efforts have been made to investigate low-level clouds, there is still a significant in-situ data gap within the atmospheric boundary layer (ABL) and the lower troposphere. While long-term ground-based observatories provide valuable continuous measurements, they cannot resolve the vertical structure of aerosols and clouds.

To address this data gap, five uncrewed aerial systems (UAS) were deployed during two intensive measurement campaigns at the Pallas Atmosphere-Ecosystem Supersite in northern Finland in spring (4–12 April 2025) and autumn (16–30 September 2025). Fixed-wing, vertical take-off and landing (VTOL), and multirotor platforms were operated jointly by the Finnish Meteorological Institute (FMI) and the Technische Universität Braunschweig. In total, 246 measurement flights were conducted, reaching altitudes of up to 2 km above ground level and conducting over 80 hours of in-situ sampling.

The UAS were equipped with different sensors to measure aerosols, including two condensation particle counters with different cut-offs to measure the aerosol particle number concentration, a Partector 2 Pro to measure the size distribution between 10 and 300 nm and a POPS to measure the size distribution between 115 and 3370 nm. In addition meteorological parameters, and cloud droplet properties were also measured.  This enables a detailed characterization of the vertical distribution of aerosols and their interaction with the ABL and low-level clouds. These measurements were compared to long-term observations from the nearby ground-based observatory Sammaltunturi. This study demonstrates the value of combining ground-based measurements with UAS profiling when investigating aerosol-cloud interactions.

Preliminary results indicate pronounced seasonal differences. Spring conditions were dominated by new particle formation events associated with long-range air mass transport from the central Arctic. In contrast, autumn measurements were strongly influenced by low-level cloud formation and local aerosol sources. Overall, this campaign demonstrates the added value of UAS observations in improving the understanding of aerosol-cloud interactions in the sub-Arctic and enhancing the interpretability of existing ground-based datasets.