Marine boundary layer (MBL) clouds exert a strong cooling influence on Earth’s climate, yet their response to aerosol perturbations remains one of the largest uncertainties in climate projections. A key challenge is identifying which aerosol and meteorological factors control cloud droplet number concentration (<em>N</em><sub>d</sub>), especially when boundary-layer structure alters the transport of surface-emitted aerosol to cloud base. Here, we combine aircraft and ground-based measurements from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign with the aerosol activation parameterization to quantify controlling factors on <em>N</em><sub>d</sub> during different seasons. We found that summer and winter boundary layer decoupling modulates the vertical transport of aerosols within the MBL and thereby influences cloud condensation nuclei number concentration (<em>N</em><sub>CCN</sub>) at cloud base. In both seasons, decoupling occurs for roughly 32 % of the total time, during which ~60 % of accumulation-mode and ~75 % of Aitken-mode aerosols fail to reach the cloud layer. The out-of-cloud accumulation-mode particle number concentration (<em>N</em><sub>acc</sub>) is higher in summer than in winter, whereas the corresponding <em>N</em><sub>d</sub> is comparable between the two seasons. Sensitivity experiments show that <em>N</em><sub>acc</sub> is the dominant control on <em>N</em><sub>d</sub> in summer, whereas cloud droplet formation in winter is additionally sensitive to Aitken-mode aerosol number concentration (<em>N</em><sub>ait</sub>) and size. In contrast, seasonal variations in updraft velocity and aerosol hygroscopicity have weaker effects. These results help clarify how boundary-layer coupling and aerosol size distributions jointly regulate marine cloud droplet formation and provide observational constraints for representing aerosol-marine cloud interactions in climate models.