Cloud condensation Nuclei over the Southern Ocean: wind dependence and seasonal cycles

Abstract. Multi-decadal observations of aerosol microphysical properties from regionally representative sites can be used to challenge regional or global numerical models that simulate atmospheric aerosol. Presented here is an analysis of multi-decadal observations at Cape Grim (Australia) that characterise production and removal of the background marine aerosol in Southern Ocean marine boundary layer (MBL) on both short-term weather-related and underlying seasonal scales. A trimodal aerosol distribution comprises Aitken nuclei ( 350 nm) modes, with the Aitken mode dominating number concentration. While the integrated particle number in the MBL over the clean Southern Ocean is only weakly dependent on wind speed the different modes in the aerosol size distribution vary in their relationship with windspeed. The balance between a positive wind dependence in the coarse mode and negative dependence in the accumulation/CCN mode leads to a relatively flat wind dependence in summer and moderately strong positive wind dependence in winter. The change-over in wind dependence of these two modes occurs in a very small size range at the mode intersection, indicative of differences in the balance of production and removal in the coarse and accumulation/CCN modes. While a marine biological source of reduced sulfur appears to dominate CCN concentration over the summer months (December to February) other components contribute to CCN over the full annual cycle. Wind-generated coarse mode sea-salt is an important CCN component year round and is the second most important contributor to CCN from autumn through to mid-spring (March to November). A portion of the non-seasonal dependent contributor to CCN can clearly be attributed to wind generated sea-salt with the remaining part potentially being attributed to long range transported material. Under conditions of greater supersaturation, as expected in more convective cyclonic systems and their associated fronts, Aitken mode particles become increasingly important as CCN.