The BACSAM II aircraft campaign, conducted with the polar research aircraft Polar 6 and the towed body T-Bird, took place from 3 April to 2 May 2024. It was based in Longyearbyen, Svalbard. In total, 11 low-level research flights and two transit flights were conducted, covering the region around Svalbard and north of Greenland. T-Bird data are available for 5 low-level research flights.The data sets present aerosol number concentrations in the size range of 10-300 nm measured with the Partector 2 in the T-Bird towed about 70 m below the Polar 6. The concentrations are provided for ambient conditions as well as reduced to standard temperature and pressure (STP). Additional parameters include particle diameter, GPS position, flow rate, and internal relative humidity, temperature and pressure.