Aircraft‐derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Abstract. New particle formation (NPF) in marine boundary layers plays a critical role in cloud condensation nuclei (CCN) budgets and aerosol–cloud interactions, yet the vertical distribution of NPF sources remains poorly constrained. We identified the vertical location of NPF events by deriving turbulent fluxes of 3–10 nm particles from aircraft measurements during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign. To overcome stationarity limitations of traditional eddy covariance methods, we applied continuous wavelet transform analysis to data collected during June–July 2017 and January–February 2018 flights over the Azores. Our flux‐based analysis revealed two distinct NPF scenarios with fundamentally different vertical structures and spatial extents. The first scenario featured nucleation in the entrainment zone, where free tropospheric air entrains into the boundary layer. The second scenario showed nucleation in the decoupled layer, a stratified region between the well‐mixed surface layer and cloud-topped upper boundary layer. Both cases exhibited strong downward particle fluxes driven by similar mechanisms: air masses from different layers and mixing, which diluted aerosols to very low particle surface area, creating favorable nucleation conditions. NPF occurred in 15 % of flights, challenging prevailing theoretical expectations that NPF should rarely occur in marine boundary layers due to high condensation and coagulation sinks from sea spray aerosols. Aircraft‐derived aerosol fluxes provide essential observational constraints on the vertical distribution and source strength of new particle formation in marine environments, enabling improved representation of these processes in climate models.