Filling the CCN Gap: Contributions of Plankton Fragments, Sea Salt and DMS to the CCN Budget over the Southern Ocean

The contributions of sea-derived plankton fragments, sea salt (SS) and DMS emissions to Cape Grim (CG) long-term mean seasonal CCN concentrations of 100/cc, summer, and 40/cc, winter, are discussed. Our previous studies have shown that nucleation and SS aerosols contribute in a minor way to CCN concentrations over the Southern Ocean (SO). Nucleation in winter and SS in summer contribute negligibly to CG CCN concentrations. Recently obtained all seasons CG aerosol impactor 60-550 nm ion data indicate primary aerosol plankton fragments (PFs) contribute about equally - on a % basis - to CG winter and summer CCN. Winter transmission electron microscopy (TEM) particle analysis shows that PFs coated with highly-surface-active exopolymer (EP) gels contribute substantially to submicron particle number concentrations. DMS-derived SO2 is removed from the MBL before nucleation occurs by ozone oxidation in SS that is extended by PF alkalinity incorporated into supermicron SS aerosol and still further reduced by halogenated radicals (do not favor SO4= formation). DMS emissions contribute negligibly to CCN in the SO's MBL across all seasons. Cape Grim VH-TDMA data show that organic matter constitutes 20-40% of submicron MBL particles in summer. Organic content of Aitken mode particles is greater than in accumulation mode particles. PFs and viruses, predominantly emitted from the sea surface in EP-gel-bound aggregates, constitute a major portion of this organic matter. It has previously been shown that much of the EP gel material de-polymerizes in the CG region MBL leaving substantial numbers of 10-200 nm particles. These primary sea-derived particles, partially coated with residual surface active EP gel material, contribute to the SO's MBL CCN across all seasons. Plankton fragment derived CCN, efficiently emitted into the atmosphere by high winds, must be included in SO CCN budget analyses to fully account for observed CG CCN of 100/cc, summer, and 40/cc, winter. Consideration of PFs as a CCN source in GCMs may provide a better mechanism to address ocean-derived CCN influences on climate change.