Networks for continuous ground based measurements of aerosol particles have been established over the last decade. However, most of the measurement sites are located on continental areas and are not representative of marine regions. Roughly, 70% of Earth’s surface is covered by water and thus, the knowledge of climate-relevant parameters such as aerosols, clouds and radiation also in marine regions is of high importance to understand the global climate. Although the marine boundary layer structure over the oceans is considered simpler compared to continental boundary layers, decoupled layers with several levels often characterize the stratification. This system is not well understood up to now,for cloudy as well for cloud-free situations. The ACORES campaign (Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols) has been performed in July 2017 in the Azores, in the Atlantic Ocean. The overarching goals of the project was the detailed study of the stratification of the marine boundary layer in terms of clouds and aerosol properties. Besides helicopter-borne vertical measurements, this study comprises also ground-based measurements at sea level (ARM site ENA) and in the free troposphere (UAc site OMP - Pico Mountain Observatory, 2225 m). Aerosol Particle numbersize distribution measurements between 10 nm and 10 µm at both sites (ENA and OMP) have been performed using a Mobility Particle Sizer System (MPSS) and an Aerodynamic Particle Sizer (APS) over 3 weeks in July 2017. In addition, a mini cloud condensation nuclei counter (CCNc) has been operated in flow scanning modeat the OMP site. Furthermore, measurements of aerosol scattering and absorption coefficients (or black carbon, BC), as well as meteorological data, are available. A number of ground-based aerosol and meteorological, as well as remote sensing data completed the measurements at ENA (https://www.arm.gov/capabilities/observatories/ena). During this campaign, we measured for the first time on Pico Mountain, particle number size distributions down to 10 nm and CCN concentrations. The data have been investigated for special features, such as new particle formation or long range transport of polluted air masses. The summer 2017 was relatively clean compared to earlier years and the number concentration of larger particles (> 100 nm) has been relatively low, as confirmed by low BC concentrations. Only two periods with slightly increased BC values have been observed, but values were still below 40 ng m-3. The aerosol number concentrations have been relatively low over the whole period, while small plumes, obviously caused by local anthropogenic pollution, often influenced the ENA site. During nighttime, Pico Mountain was five times inside a cloud, i.e. the relative humidity was 100% and particles larger than 50 nm, as well as CCN, were completely removed, while smaller particles were still detected as interstitial aerosol. On one dayout of three weeks, new particle formation on Pico was observed while there was no clear event visible at ENA. On the majority of measurement days, the particle number size distribution at sea level consisted of two modes. Instead, the particles in the FT showed typically only one mode, but, contradictory to previous studies, this mode occured in the Aitken mode range (< 100 nm) but also above 100 nm, i.e., in the accumulation mode, originated probably from long-range transport.