Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote Southern Hemisphere oceans: A global model study

[1] We use a global chemical transport model with size-resolved aerosol microphysics to investigate the sources of cloud condensation nuclei (CCN) in the Southern Hemisphere remote marine boundary layer (MBL). Long-term observations of CCN number at Cape Grim (40 degrees 41'S, 144 degrees 41'E) show a clear seasonal cycle with a 2-3 times higher concentration in summer than in winter, which has been attributed to seasonal changes in the dimethyl sulfide (DMS) ocean-to-atmosphere flux. We find that this cycle at Cape Grim and also throughout the 30 degrees-45 degrees S latitude band is caused mostly by changes in the regional-scale DMS ocean water concentration. In this latitude band, DMS emissions increase the simulated CCN concentrations from November to April, with a maximum effect of 46% in January (calculated at 0.23% supersaturation). Farther south, the impact of DMS on CCN is apparent only from December to February and increases the CCN concentration at most by 18% at 45 degrees-60 degrees S and by 40% at 60 degrees-75 degrees S. These model-derived contributions of DMS to Southern Ocean summertime CCN are smaller than the 80% derived from correlations between satellite-observed chlorophyll and column CCN, which we explain in terms of nonlinear behavior of CCN from the free troposphere (FT). We show that the main microphysical pathway of DMS influence on CCN number is nucleation of DMS-derived H2SO4 in the FT and subsequent growth of formed particles by condensation and coagulation during entrainment into the MBL. Our simulations suggest that > 90% of the increase in MBL CCN when DMS is added to the model is formed in this way. The growth of ultrafine sea spray particles to CCN sizes due to condensation of DMS-derived H2SO4 in the MBL affects the simulated CCN concentrations by less than 6%. Overall, entrainment of nucleated sulfate aerosol into the MBL from the FT accounts for 43-65% of the summer zonal mean CCN concentrations but only 7-20% of the winter CCN over the Southern Hemisphere oceans.