Simulation of global sulfate distribution and the influence of effective cloud drop radii with a coupled photochemistry-sulfur cycle model

A sulfur cycle model is coupled to a global chemistry-climate model. The simulated surface sulfate concentrations are generally within a factor of 2 of observed concentrations, and display a realistic seasonality for most background locations. However, the model tends to underestimate sulfate and overestimate surface SO2 at relatively polluted locations. A possible explanation for this is that additional oxidation reactions not considered in the model, may be important. Calculated tropospheric sulfate column abundances exceed those of previous studies, which is predominantly associated with a less efficient nucleation scavenging in wet convective updrafts. Through the simultaneous calculation of the sulfur cycle and tropospheric photochemistry, simulated H2O2 and SO2 concentrations are strongly linked, especially in polluted areas. The coupled model simulates a stronger oxidant limitation and, consequently, a smaller contribution to sulfate formation by H2O2 oxidation of SO2 when compared to sulfur cycle models that use monthly averaged oxidant distributions as input. In the polluted NH, the differences in calculated sulfate columns are largest in winter and relatively small in summer. Therefore, the coupling between the sulfur cycle and the oxidant chemistry is expected to have a minor impact on the calculation of the indirect and direct radiative forcing by sulfate. An empirical relation between sulfate concentration and cloud drop number concentration, derived from cloud measurements at Grean Dun Fell (UK), is applied to the simulated cloud and sulfate fields to derive distributions of effective could drop radii. Additionally, a relation between wind speed and cloud drop number concentration is applied over marine regions to account for the effect of seasalt aerosol on cloud formation when sulfate concentrations are relatively low. Calculated droplet radii are systematically underestimated by about 10-20% in the NH compared to satellite derived values, but they agree relatively well in the SH.