Ice nuclei in the sea surface microlayer

The formation of ice particles in clouds is facilitated by the presence of ice nucleating particles (INP). Recent ship based measurements in the Arctic, Atlantic and North Pacific Oceans show enhanced concentrations of INP in the sea surface microlayer (SML) compared to subsurface water sampled at the same locations. Heating tests indicate that the INP present in the microlayer are biological in origin, however samples filtered through 0.2 μm pore diameter filters retained activity. Together, these findings suggest that the INP are biogenic material derived from the bacteria and phytoplankton communities. STXM/NEXAFS (Scanning Transmission X-ray Microscopy coupled with Near Edge Absorption Fine Structure Spectroscopy) analysis of Arctic SML samples found polysaccharide-rich and proteinaceous material consistent with diatom cell wall fragments and diatom exudates. We further demonstrate that exudates from the diatom Thalassiosira pseudonana nucleate ice heterogeneously similarly to Arctic SML water and that they have the potential to act as atmospheric INP. These results indicate that sea spray particles containing biogenic material found worldwide can be an important source of INP for ice cloud formation.