Sensitivity of cloud condensation nuclei to regional changes in dimethyl-sulphide emissions
The atmospheric oxidation of dimethyl-sulphide (DMS) derived from marine phytoplankton is a significant source of marine sulphate aerosol. DMS has been proposed to regulate climate via changes in cloud properties, though recent studies have shown that present-day global cloud con- densation nuclei (CCN) concentrations have only a weak de- pendence on the total emission flux of DMS. Here, we use a global aerosol microphysics model to examine how effi- cientlyCCNare produced whenDMSemissions are changed in different marine regions.We find that global CCN produc- tion per unit mass of sulphur emitted varies by more than a factor of 20 depending on where the change in oceanic DMS emission flux is applied. The variation in CCN pro- duction efficiency depends upon where CCN production processes (DMS oxidation, SO2 oxidation, nucleation and growth) are most efficient and removal processes (deposi- tion) least efficient. The analysis shows that the production of aerosol sulphate through aqueous-phase oxidation of SO2 limits the amount ofH2SO4 available for nucleation and con- densational growth and therefore suppresses CCN formation, leading to the weak response of CCN to changes in DMS emission. Our results showthat past and future changes in the spatial distribution of DMS emissions (through changes in the phytoplankton population or wind speed patterns) could exert a stronger control on climate than net increases in bio- logical productivity.