Evaluation of the anthropogenic sulfate direct radiative forcings in a general circulation model

The direct radiative forcing by sulfate aerosols is still uncertain. The estimation in global aerosol models, for example, can have an error originated from an uncertainty in the simulated sulfate burden. To reduce this type of error in our global model, SPRINTARS, we first upgraded our sulfur module, taking into account for two major understandings: (1) previous models show that SO2 liquid-phase reaction is major process in the sulfate production and (2) the liquid-phase reaction rate and a timescale of equilibrium between gas-aerosol phases through Henry's law are generally faster than a timestep used in a global model calculation. The new module can solve the SO2 liquid-phase reaction with an analytical solution in finer sub-cycle timestep keeping equilibrium between gas-aerosol phases. Additionally, we modified SO2 dry deposition process, which is also important for the sulfur cycle. As a result, the predicted sulfate mass concentrations near the surface in this study were in much better agreement with in-situ measurements than those in the original SPRINTARS. Its vertical distributions in this study were also much closer to the flight measurements. At the same time, these improvements also led to be smaller differences in the sulfate burdens and the vertical distributions between SPRINTARS and other AeroCom models showed in Schulz et al. [2006] and Textor et al. [2006]. Furthermore, predicted aerosol optical thickness (AOT) in this study was also closer to satellite-observed AOT. The global annual mean radiative forcings due to anthropogenic sulfate aerosol direct effect was estimated to be -0.3 W m-2 (-0.2 W m-2 in the original SPRINTARS). The magnitude of this difference in the aerosol direct radiative forcing corresponds to a half of magnitude of the uncertainty shown by the IPCC-AR4 assessment. That means these modifications in this study have a large impact on the estimation of the aerosol radiative forcings.