This dataset contains raw particle number concentration data measured during the year long MOSAiC expedition from October 2019 to September 2020. The measurement was performed in the Swiss aerosol container on the D-deck of Research Vessel Polarstern using an inlet with an upper cut-off of 1 µm (called also interstitial inlet). Data were collected by a Condensation Particle Counter (CPC) model 3025 (TSI Inc.) in 10 s resolution with a lower cut-off diameter of 3 nm. More detailed information about its position and the measurement setup can be found in (Beck et al., in prep.).Participation of the Swiss Container was co-financed by the Swiss Polar Institute and University of Helsinki. The particle concentration data were not processed, however periods where the instrument did not measure outdoor air were removed. We used 10 sec time resolution positional data from the following datasets: Rex, M (2020), Haas, C (2020), Kanzow, T (2020), Rex, M (2021a,b).