A new estimate of the contribution of DMS to the CCN annual cycle
We use GLOMAP 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 (40º 41’S, 144º 41’E) show a clear seasonal cycle with a factor 2–3 higher concentration in summer than in winter, which has been attributed to seasonal changes in the dimethylsulfide (DMS) ocean-to-atmosphere flux.
We find that this cycle at Cape Grim and also throughout the 30–45º S latitude band is caused mostly by changes in the regional scale DMS ocean water concentration. However, we estimate that DMS emissions increase the simulated CCN concentrations from November to April by at most 46% (calculated at 0.23% supersaturation). Further south, the impact of DMS on CCN is apparent only from December to February and increases the CCN concentration at most by 18% at 45–60º S and by 40% at 60–75º 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 the non-linear response of free tropospheric (FT) CCN to changing sulfur emissions.
The main microphysical pathway of DMS influence on CCN number in the marine boundary layer is nucleation of DMS-derived H2SO4 in the FT and subsequent growth of the particles by condensation and coagulation before they are entrained into the MBL. Even with fairly low amounts of sulfur, nucleation of new H2SO4 particles occurs rapidly in the FT so dCCN/dDMS is not linear.
Korhonen, H., K. S. Carslaw, D. V. Spracklen, G. Mann, and M. Woodhouse (2008), Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote southern hemisphere oceans: A global model study, J. Geophys. Res., doi:10.1029/2007JD009718, in press.
http://www.agu.org/journals/pip/jd/2007JD009718-pip.pdf