Sensitivity Of Cloud Condensation Nuclei Research Articles

Sensitivity of cloud condensation nuclei to regional changes in dimethyl-sulphide emissions

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.

Low sensitivity of cloud condensation nuclei to changes in the sea-air flux of dimethyl-sulphide

Abstract. The emission of dimethyl-sulphide (DMS) gas by phytoplankton and the subsequent formation of aerosol has long been suggested as an important climate regulation mechanism. The key aerosol quantity is the number concentration of cloud condensation nuclei (CCN), but until recently global models did not include the necessary aerosol physics to quantify CCN. Here we use a global aerosol microphysics model to calculate the sensitivity of CCN to changes in DMS emission using multiple present-day and future sea-surface DMS climatologies. Calculated annual fluxes of DMS to the atmosphere for the five model-derived and one observations based present day climatologies are in the range 15.1 to 32.3 Tg a−1 sulphur. The impact of DMS climatology on surface level CCN concentrations was calculated in terms of summer and winter hemispheric mean values of ΔCCN/ΔFluxDMS, which varied between −43 and +166 cm−3/(mg m−2 day−1 sulphur), with a mean of 63 cm−3/(mg m−2 day−1 sulphur). The range is due to CCN production in the atmosphere being strongly dependent on the spatial distribution of the emitted DMS. The relative sensitivity of CCN to DMS (i.e. fractional change in CCN divided by fractional change in DMS flux) depends on the abundance of non-DMS derived aerosol in each hemisphere. The relative sensitivity averaged over the five present day DMS climatologies is estimated to be 0.02 in the northern hemisphere (i.e. a 0.02% change in CCN for a 1% change in DMS) and 0.07 in the southern hemisphere where aerosol abundance is lower. In a globally warmed scenario in which the DMS flux increases by ~1% relative to present day we estimate a ~0.1% increase in global mean CCN at the surface. The largest CCN response occurs in the Southern Ocean, contributing to a Southern Hemisphere mean annual increase of less than 0.2%. We show that the changes in DMS flux and CCN concentration between the present day and global warming scenario are similar to interannual differences due to variability in windspeed. In summary, although DMS makes a significant contribution to global marine CCN concentrations, the sensitivity of CCN to potential future changes in DMS flux is very low. This finding, together with the predicted small changes in future seawater DMS concentrations, suggests that the role of DMS in climate regulation is very weak.