Simulation of the tropospheric sulfur cycle in a global model with a physically based cloud scheme

The treatment of the sulfur cycle in the CSIRO global climate model (GCM) is described. It is substantially based on the scheme developed previously for the European Center/Hamburg (ECHAM) model, but the treatment of wet scavenging has been completely rewritten to better reflect the different properties of liquid and frozen precipitation, and the treatment of these in the model's cloud microphysical scheme. The model is able to reproduce the observed finding that wet deposition of sulfur over Europe and North America is larger in summer than in winter, but the seasonal cycle of sulfate over Europe is not well simulated. The latter is improved when the amplitude of the seasonal cycle of European emissions is increased. Below-cloud scavenging makes an important contribution in our scheme: On omitting it, the global sulfate burden increases from 0.67 to 0.93 Tg S. On reverting to the less efficient scavenging treatment used in ECHAM, the global sulfate burden again increases from 0.67 to 0.93 Tg S, and excessive sulfate concentrations are obtained in Europe and North America. Some deficiencies in the simulation are investigated via further sensitivity tests. In particular, during the Arctic winter, the modeled sulfur dioxide (SO2) concentrations are too large, and the modeled sulfate concentrations are too small (as in most global sulfur-cycle models). Recent laboratory experiments suggest that SO2 oxidation in ice clouds is nonnegligible. We obtain a much improved Arctic simulation when a simple treatment of SO2 oxidation in ice clouds is included.