Recent results from summer measurement campaigns over the partly ice covered central Arctic Ocean show that the high Arctic aerosol has a larger organic fraction than previously thought. We use a Lagrangian parcel model to infer the properties of the unexplained organic aerosol fraction that is necessary for reproducing the observed concentrations of cloud condensation nuclei (CCN). With increasing distance from the open ocean a highly surface-active Aitken mode, associated with particles found in the open lead surface microlayer, becomes increasingly important for cloud droplet formation. The presence of such an Aitken mode population increases the high Arctic indirect aerosol effect (added cooling) relative to just a marine source of CCN from oxidation products of dimethyl sulfide (DMS) released from phytoplankton.
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