1Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
2National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
3Met Office Hadley Centre, FitzRoy Road, Exeter, Devon, EX1 3PB, UK
*now at: Laboratoire de Météorologie Dynamique, Centre National de la Recherche Scientifique/Université Pierre et Marie Curie, 4 place Jussieu, 75252, Paris, France
Abstract. 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 condensation nuclei (CCN) concentrations have only a weak dependence on the total emission flux of DMS. Here, we use a global aerosol microphysics model to examine how efficiently CCN are produced when DMS emissions are changed in different marine regions. We find that global CCN production 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 production efficiency depends upon where CCN production processes (DMS oxidation, SO2 oxidation, nucleation and growth) are most efficient and removal processes (deposition) least efficient. The analysis shows that the production of aerosol sulphate through aqueous-phase oxidation of SO2 limits the amount of H2SO4 available for nucleation and condensational growth and therefore suppresses CCN formation, leading to the weak response of CCN to changes in DMS emission. Our results show that 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 biological productivity.