During the ACLOUD campaign in May/ June 2017, 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 size distribution between 0.25 µm and 40 µm, two flags indicating the used inlet system and a flag for cloud residual sampling. The time resolution of the instrument is 6 seconds. Each data point is given at the end of the corresponding measurement interval.