Tropospheric sulfur cycle in the Canadian general circulation model

Emission, transport, chemistry, and scavenging of the gaseous sulfur species dimethyl sulfide and sulfur dioxide (SO2) and sulfate aerosols are calculated on-line with the meteorology in the general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA). Additionally, prognostic equations for cloud water and cloud ice have been introduced. The sensitivity of this sulfur cycle to differences in GCM physics and dynamics has been studied by comparing the results to those obtained with the ECHAM GCM which has a very similar sulfur cycle and cloud scheme, but a different turbulent diffusion and convection scheme, The differences in the global mean burdens of SO2 and sulfate are less than 2%. Simulated surface SO2 concentrations with CCCMA in winter as well as the seasonal cycle are in better agreement with observations at several sites than those simulated with ECHAM because of stronger boundary layer mixing in CCCMA. The simulated surface SO42- with CCCMA, however, is often higher than observed and in ECHAM. Additionally, sensitivity experiments showed that the global sulfur budgets are most sensitive to changes in the cloud cover parameterization and less sensitive to changes in pH calculation and oxidation of SO2 in convective clouds. The results of the sensitivity experiments give evidence for the importance of all of these effects on the geographical and vertical distribution of sulfur and cloud liquid water.