Simulation of the tropospheric sulfur cycle in a global climate model

Emission, transport, chemistry and rainout of the sulfur species DMS, SO2 and sulfate are calculated on-line with the meteorology in a global atmospheric circulation model. The model simulates the main components of the hydrological cycle, including the liquid water content of clouds, and hence it allows an explicit treatment of cloud transformation processes and precipitation scavenging. The importance of the different oxidation pathways of DMS and SO2 is estimated. About 2/3 of the sulfate is produced within clouds, with H2O2 being the most efficient pathway (59%) and with a minor contribution due to oxidation with O3 (7%). Predicted atmospheric surface concentrations of SO2 and sulfate and the deposition fluxes are compared with the observations. Over most parts of the globe the agreement between simulated and observed annual averages is within a factor of 2. A significant underestimate of the simulated sulfate concentrations was found in high latitudes in winter. This bias may be attributed to a too slow oxidation in clouds. The calculated global mean turn-over times for DMS (2.2 d), SO2 (1.6 d) and sulfate (4.4 d) are within the range of previous estimates.