During the HALO-(AC)³ campaign in March/ April 2022, in-situ particle number concentration and size distribution of ambient aerosol particles and cloud residual particles were measured using a Grimm Sky OPC (model 1.129 Sky-OPC). This data set provides the particle size distribution divided in 31 size bins between 0.25 µm and 40 µm. Particle number concentrations are pressure-corrected to standard temperature and pressure (STP). The time resolution of the instrument is 6 seconds. Each data point is given at the end of the corresponding measurement interval.