Importance of submicron surface-active organic aerosols for pristine Arctic clouds

Recent results from summer measurement campaigns over the partly ice covered central Arctic Ocean show that thehigh Arctic aerosol has a larger organic fraction than previously thought.We use a Lagrangian parcel model to infer theproperties of the unexplained organic aerosol fraction that is necessary for reproducing the observed concentrations ofcloud 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 dropletformation. The presence of such an Aitken mode population increases the high Arctic indirect aerosol effect (addedcooling) relative to just a marine source of CCN from oxidation products of dimethyl sulfide (DMS) released from phytoplankton.