During the airborne measurement campaign "Boundary layer and Aerosol and Cloud Study in the Arctic, based on aircraft and T-Bird Measurements II" (BACSAM II), aerosol particle number size distributions (PNSDs) were measured utilizing a Mobility Particle Size Spectrometer (MPSS, custom-built by the Leibniz Institute for Tropospheric Research; Wiedensohler et al., 2012). The flights were carried out with the research aircraft Polar 6 (Wesche et al., 2016) of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. The corresponding flight paths are available on PANGAEA for all research flights of the BACSAM II campaign (Jurányi et al., 2024).The MPSS measures the PNSDs with a time resolution of about 5 min based on the electric mobility diameter of the aerosol particles in the size range from 10 nm to 850 nm. PNSDs (dN/dlogDp) are provided in units of #/ccm in 40 size bins. For each of the size bins, the geometric mean diameter is given in the header of the data file for each column. Additionally, mean altitude and geographical position during the measurement interval are reported. Near-isokinetic sampling was established throughout the flights and all reported particle diameters refer to dry conditions (Jurányi et al., 2025). All data have been corrected to standard conditions (1013.0 hPa and 273.15 K). Measurements during taxing, take-off, and landing have been excluded from the dataset since, for these times, erroneous measurements due to potential sampling of aircraft exhaust cannot be ruled out.

Acknowledgement: We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project Number 268020496 – TRR 172, within the framework of the Transregional Collaborative Research Center "ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)³, as well as the support of the Alfred-Wegener-Institute, Helmholtz Centre for Polar and Marine Research through grant number AWI_PA_02172.