Clarifying the controlling factors of the seasonal variation of droplet concentrations in marine low-level clouds

Abstract. 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 (N_d), 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 N_d 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 (N_CCN) 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 (N_acc) is higher in summer than in winter, whereas the corresponding N_d is comparable between the two seasons. Sensitivity experiments show that N_acc is the dominant control on N_d in summer, whereas cloud droplet formation in winter is additionally sensitive to Aitken-mode aerosol number concentration (N_ait) 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.