Marine boundary layer (MBL) clouds play a vital role in Earth’s radiation budget. However, the response of MBL clouds to aerosol remains a major source of uncertainty in climate models, largely because of a limited understanding of aerosol properties, aerosol vertical transportation, and activation under different meteorological conditions. Here, we analyze aircraft measurements conducted during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign (Jun-Aug 2017 and Dec 2017 - Feb 2018). We found that meteorological conditions significantly modulate the vertical distribution of aerosols and affect the cloud condensation nuclei concentration (N_CCN) within the MBL. In winter, a greater decoupling between the ground and cloud base hinders vertical aerosol transport, leading to a substantial reduction in N_CCN at the cloud base. Specifically, approximately 50% of the accumulation mode and 33% of Aitken mode aerosols fail to reach the cloud layer. Consequently, ground-based aerosol observations significantly overestimate cloud-base N_CCN during decoupled periods. N_d shows a distinct seasonal variability, with higher values in summer than in winter. During summer, N_d is primarily governed by the high number concentration of accumulation mode aerosols (N_acc, 80–1000 nm). Conversely, in winter, the Aitken mode (30–80 nm) aerosols contribute ~30% of N_d due to low N_acc.
 

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