Cloud optical properties in the trade winds over the easternCaribbean Sea have been shown to be sensitive to cloud condensationnuclei (CCN) concentrations. The objective of the current study wasto investigate the CCN properties in the marine boundary layer (MBL)in the tropical western North Atlantic, in order to assess the respective roles ofinorganic sulfate, organic species, long-range transported mineral dust and sea-saltparticles.Measurements were carried out in June–July 2013, on the east coastof Barbados, and included CCN number concentrations, particle numbersize distributions and offline analysis of sampledparticulate matter (PM) and sampled accumulation mode particles foran investigation of composition and mixing state with transmissionelectron microscopy (TEM) in combination with energy-dispersiveX-ray spectroscopy (EDX).During most of the campaign, significant mass concentrations oflong-range transported mineral dust was present in the PM, andinfluence from local island sources can be ruled out. The CCN andparticle number concentrations were similar to what can be expectedin pristine marine environments. The hygroscopicity parameter was inferred, and values in the range 0.2–0.5 were foundduring most of the campaign, with similar values for the Aitken andthe accumulation mode. The accumulation mode particles studied withTEM were dominated by non-refractory material, and concentrations ofmineral dust, sea salt and soot were too small to influence the CCNproperties. It is highly likely that the CCN were dominated bya mixture of sulfate species and organic compounds.
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